GNAT/ASIS User's Guide

Contents

About This Guide
   What This Guide Contains
   What You Should Know Before Reading This Guide
   Related Information

Introduction
   What Is ASIS?
   ASIS Scope: What Kind of Tools Can Be built with ASIS?

 1. Getting Started
    1.1 The Problem
    1.2 An ASIS application which Solves the Problem
    1.3 Required Sequence of Calls
    1.4 Building the Executable for an ASIS application
    1.5 Generating Trees for Input
    1.6 Running the ASIS Application

 2. ASIS Overview
    2.1 Main ASIS Abstractions
    2.2 ASIS Package Hierarchy
    2.3 Structural and Semantic Queries
    2.4 ASIS Error Handling Policy
    2.5 Dynamic Validity Checking of the ASIS Queries
    2.6 ASIS Iterator
    2.7 How to Navigate through the Asis Specification

 3. ASIS Context
    3.1 An ASIS Context and Tree Files
    3.2 Creating Tree Files for Use by ASIS
       3.2.1 Creating Trees for Data Decomposition Annex
    3.3 Different ways to define an ASIS Context for GNAT
    3.4 Consistency Problem
        3.4.1 Inconsistent versions of ASIS and GNAT
        3.4.2 Consistency of a set of tree and source files
    3.5 Processing Several Contexts at a Time

 4. ASIS Interpreter asistant
    4.1 Introduction
    4.2 asistant commands
    4.3 asistant variables
    4.4 Browsing an ASIS tree
    4.5 Example

 5. ASIS Application Templates

 6. ASIS Tutorial

 7. How to Build Efficient ASIS Applications
    7.1 Tree Swapping as a Possible Cause of Poor Application Performance
    7.2 Queries That Can Cause Tree Swapping
    7.3 How to Avoid Unnecessary Tree Swapping
    7.4 Using gnatmake to create tree files

 8. Processing an Ada Library by an ASIS-Based Tool

 9. Compiling, Building and Linking Applications with ASIS-for-GNAT

10. ASIS-for-GNAT Warnings

11. File Naming Conventions and Application's Name Space

======================================================================


About This Guide
----------------

This guide has two aims. The first one is to introduce you to the Ada Semantic
Interface Specification (ASIS) and show you how you can build various useful
tools on top of ASIS. The second is to describe the ASIS implementation
for the GNAT Ada 95 compiler.


What This Guide Contains
------------------------

This guide contains the following chapters:
- "Introduction" contains the general definition of ASIS and gives some
  examples of tools which can be built on top of ASIS;
- section "Getting Started" contains a short guided tour through the
  development and use of ASIS-for-GNAT-based tools;
- section "ASIS Overview" gives the general overview of the ASIS definition to
  help an ASIS newcomer to navigate through ASIS (readers already
  familiar with ASIS can just skip this section);
- section "ASIS Context" explains what is the meaning of ASIS Context in the
  case of ASIS-for-GNAT and what should be done to prepare a set of Ada
  components to be processed by an ASIS application;
- section "ASIS Tutorials" gives a short overview of some examples included in
  the ASIS-for-GNAT distribution which can be used as a hands-on tutorial to
  get some initial experience with ASIS;
- section "How to Build Efficient ASIS Applications" describes the problem of
  tree swapping as a possible source of a poor productivity of ASIS
  applications and explains how to avoid unnecessary tree swappings;
- section "Processing an Ada Library by an ASIS-Based Tool" touches the
  specific issues of processing Ada programs which use the pre-compiled Ada
  libraries by an ASIS tool;
- section "Compiling, Building and Linking Applications with ASIS-for-GNAT"
  explains how to compile an ASIS application with ASIS-for-GNAT and how to
  create an executable for it;
- section "File Naming Conventions and Application's Name Space" explains
  which names can and cannot be used as names of an ASIS application
  components.


What You Should Know Before Reading This Guide
----------------------------------------------

This User's Guide assumes that you are familiar with Ada 95 and that you have
some basic experience in Ada programming with GNAT.

This User's Guide also assumes that you have ASIS-for-GNAT properly installed
for your GNAT compiler, and that you are familiar with the structure of the
ASIS-for-GNAT distribution (if not, see the top README file from the
distribution, section 2)

This guide does not assume that you have any knowledge or experience in ASIS.
If you indeed do not, you will learn some basic things about ASIS when reading
this Guide, doing exercises, playing with examples and referring to the ASIS
definition if needed.


Related Information
-------------------

To get to know more about GNAT, refer to the GNAT User's Guide.

Refer to ASIS-for-GNAT Installation Guide to learn how to install the ASIS
implementation for your GNAT compiler.

The ASIS 95 definition now exists as ISO/IEC International Standard 15291.
To get more information about ASIS, visit the ASIS Working Group Web Pages
(http://www.acm.org/sigada/wg/asiswg).

To read this Guide you will hardly need GNAT or ASIS-for-GNAT Reference
Manual.


Introduction
------------


What Is ASIS?
------------

The Ada Semantic Interface Specification (ASIS) is an interface between an
Ada environment (as defined by ISO/IEC 8652:1995) and any tool requiring
information from it. An Ada environment includes valuable semantic and
syntactic information. ASIS is an open and published callable interface which
gives CASE tool and application developers access to this information. ASIS
has been designed to be independent of underlying Ada environment
implementations, thus supporting portability of software engineering tools
while relieving tool developers from needing to understand the complexities
of an Ada environment's proprietary internal representation.

Technically, ASIS is a hierarchy of the Ada package specifications. These
packages define a set of Ada private types which implement basic notions
needed to describe an Ada program. Operations for these types, called ASIS
queries, give you statically determinable information about Ada compilation
units in your environment.

You may use ASIS as a third-part Ada library to implement a number of useful
program analysis tools.


ASIS Scope: What Kind of Tools Can Be built with ASIS?
-----------------------------------------------------

The following ASIS properties define the ASIS scope:

- ASIS is a read-only interface;
- ASIS provides only statically-determinable information about Ada programs;
- ASIS provides all the syntax and the basic semantic information from/about
  Ada programs. If some semantic property of a program cannot be directly
  queried by means of ASIS queries, an ASIS application can compute the needed
  piece of information itself from the information available through ASIS
  queries;
- Even though containing some implementation dependencies, ASIS provides
  information from/about Ada program in high-level terms which are
  well-conformed with RM 95 and which are Ada/ASIS-implementation-independent
  in their very nature.

Examples of tools that benefit from the ASIS interface include, but are not
limited by: automated code monitors, browsers, call tree tools, code
reformators, coding standards compliance tools, correctness verifiers,
debuggers, dependency tree analysis tools, design tools, document generators,
metrics tools, quality assessment tools, reverse engineering tools,
re-engineering tools, style checkers, test tools, timing estimators, and
translators.


1. Getting Started
------------------

In this section we go through the ASIS application development and usage cycle
in a very simplified way: we take a sample problem to be solved with an ASIS
application, then we present the code of the ASIS application which gives the
solution for our problem, then we show how to compile it and build the
executable for it with ASIS-for-GNAT and how to prepare an ASIS Context to be
processed by the program, and finally we show the output produced by our
program when it is applied to itself.

1.1 The Problem
---------------

Suppose our goal is to process some set of Ada compilation units, and for
every unit to print its full expanded Ada name, whether this unit is a spec, a
body or a subunit, and whether this unit is a user-defined unit, a predefined
unit as defined by RM 95 or an implementation-specific unit (such as a part of
a Run-Time Library).

1.2 An ASIS application which Solves the Problem
------------------------------------------------

with Ada.Wide_Text_IO;        use Ada.Wide_Text_IO;
with Ada.Characters.Handling; use Ada.Characters.Handling;

--  ASIS-specific context clauses:
with Asis;
with Asis.Implementation;
with Asis.Ada_Environments;
with Asis.Compilation_Units;
with Asis.Exceptions;
with Asis.Errors;

procedure Example1 is
   My_Context : Asis.Context;
   --  ASIS Context is an abstraction of an Ada environment, it defines
   --  a set of ASIS Compilation Units available through ASIS queries
begin
   --  first, by initializing an ASIS implementation, we make it ready-to-work
   Asis.Implementation.Initialize;

   --  then we define our Context by making association with "physical"
   --  environment:
   Asis.Ada_Environments.Associate (My_Context, "My Asis Context", "-CA");
   --  See ASIS-for-GNAT Reference Manual for the description of the
   --  parameters of the Associate query, see also section 3 of this Manual
   --  for the description of different kinds of ASIS Context in case of
   --  ASIS-for-GNAT

   --  by opening a Context we make it ready for processing by ASIS queries
   Asis.Ada_Environments.Open (My_Context);

   Processing_Units: declare
      Next_Unit : Asis.Compilation_Unit;
      --  ASIS Compilation_Unit is the abstraction to represent Ada
      --  compilation units as described in RM 95

      All_Units : Asis.Compilation_Unit_List :=
      --  ASIS list are one-dimensional unconstrained arrays. Therefore, when
      --  declaring an object of an ASIS list type, we have to provide either
      --  a constraint or explicit initialization expression:

         Asis.Compilation_Units.Compilation_Units (My_Context);
      --  Compilation_Units query gives you a list of all the units
      --  contained in an ASIS Context
   begin
      Put_Line ("A Context contains the following compilation units:");
      New_Line;
      for I in All_Units'Range loop
         Next_Unit := All_Units (I);
         Put ("   ");

         --  to get a unit name, we just need a Unit_Full_Name query.
         --  ASIS uses Wide_String as a string type, therefore we convert the
         --  result into String to use Ada.Text_IO
         Put (Asis.Compilation_Units.Unit_Full_Name (Next_Unit));

         --  to get more info about a unit, we ask about its class and
         --  about its origin

         case Asis.Compilation_Units.Unit_Kind (Next_Unit) is
            when Asis.A_Library_Unit_Body =>
               Put (" (body)");
            when Asis.A_Subunit =>
               Put (" (subunit)");
            when others =>
               Put (" (spec)");
         end case;

         case Asis.Compilation_Units.Unit_Origin (Next_Unit) is
            when Asis.An_Application_Unit =>
               Put_Line (" - user-defined unit");
            when Asis.An_Implementation_Unit =>
               Put_Line (" - implementation-specific unit");
            when Asis.A_Predefined_Unit =>
               Put_Line (" - Ada predefined unit");
            when Asis.Not_An_Origin =>
               Put_Line (" - unit does not actually exist in a Context");
         end case;

      end loop;
   end Processing_Units;

   --  Cleaning up: we have to close out Context, to break its association
   --  with the external environment and to finalize our ASIS implementation
   --  to release all the resources used:
   Asis.Ada_Environments.Close (My_Context);
   Asis.Ada_Environments.Dissociate (My_Context);
   Asis.Implementation.Finalize;

exception
   when Asis.Exceptions.ASIS_Inappropriate_Context |
        Asis.Exceptions.ASIS_Inappropriate_Compilation_Unit |
        Asis.Exceptions.ASIS_Failed =>

      --  we check not for all the ASIS-defined exceptions, but only those of
      --  them which can actually be raised in our ASIS application.
      --
      --  If an ASIS exception is raised, we output the ASIS error status and
      --  the ASIS diagnosis string:

      Put_Line ("ASIS exception is raised:");
      Put_Line ("ASIS diagnosis is:");
      Put_Line (Asis.Implementation.Diagnosis);
      Put      ("ASIS error status is: ");
      Put_Line
        (Asis.Errors.Error_Kinds'Wide_Image (Asis.Implementation.Status));
end Example1;


1.3 Required Sequence of Calls
--------------------------------

An ASIS application must use the following sequence of calls:

1. Asis.Implementation.Initialize (...);

  This call initializes the ASIS implementation internal data structures
  and prepares the ASIS implementation for work. For most of the ASIS
  queries, it is erroneous to call them if an ASIS implementation is not
  initialized.

2. Asis.Ada_Environments.Associate (...);

  This call is the only means to define a value of a variable of the
  ASIS limited private type Context. This value is some specific
  association of the ASIS Context with the "external world". The way
  of making this association and the meaning of the corresponding
  parameters of the Associate query are implementation-specific,
  but as soon as this association has been made and a Context variable
  is opened, the ASIS Context designated by this variable may be
  considered to be a set of ASIS Compilation Units available through
  the ASIS queries.

3. Asis.Ada_Environments.Open (...);

  Opening an ASIS Context variable makes the corresponding Context
  accessible for all ASIS queries.

  After opening the Context, an ASIS application can start obtaining
  ASIS Compilation Units from it, further analyze Compilation Units
  by decomposing them into ASIS Elements etc.

  ASIS relies on the content of a Context being frozen as long
  as the Context remains open.
  It is erroneous to change through some non-ASIS program any data
  structures used by an ASIS implementation to define and implement
  this Context while the Context is open.

4. Asis.Ada_Environments.Close (...);

  After closing the Context it is impossible to retrieve any information
  from it. All the values of the ASIS objects of Compilation_Unit,
  Element and Line types obtained when this Context was open become
  obsolete, and it is erroneous to use them after the Context was closed.
  The content of this Context need not be frozen while
  the Context remains closed. Note that a closed Context keeps its
  association with the "external world" and it may be opened again with
  the same association. Note also that the content (that is, the
  corresponding set of ASIS Compilation Units) of the Context may be
  different from what was in the Context before, because the "external
  world" may have changed while the Context remained closed.

5. Asis.Ada_Environments.Dissociate (...);

  This query breaks the association between the corresponding ASIS
  Context and the "external world", and the corresponding Context
  variable becomes undefined.

6. Asis.Implementation.Finalize (...);

  This releases all the resources used by an ASIS implementation.

An Application can perform these steps in a loop. It may initialize and
finalize an ASIS implementation several times, it may associate and dissociate
the same Context several times while an ASIS implementation remains
initialized, and it may open and close the same Context several times while
the Context keeps its association with the "external world".

An application can have several ASIS Contexts opened at a time (the upper
limit is implementation-specific), and for each open Context, an application
can process several Compilation Units obtained from this Context at a time
(the upper limit is also implementation-specific). ASIS-for-GNAT does not
impose any special limitations on the number of ASIS Contexts and on the
number of the ASIS Compilation Units processed at a time, as long as an ASIS
application is within the general resource limitations of the underlying
system.


1.4 Building the Executable for an ASIS application
---------------------------------------------------

The rest of this section assumes that you have ASIS-for-GNAT properly
installed as an Ada library.

To get the executable for the ASIS application from subsection 1.2 (assuming
that it is located in your current directory as the Ada source file named
example1.adb), you have to call gnatmake as:

   gnatmake example1[.adb] -largs -lasis

For more details concerning compiling ASIS applications and building
executables for them with ASIS-for-GNAT see section 6.


1.5 Generating Trees for Input
    --------------------------

To get information from an Ada environment being processed, ASIS-for-GNAT
processes so-called tree files. A tree file is generated by GNAT, and it
contains some kind of a snapshot of compiler's internal data structures. For
more details see section 3.1 of this Guide

To create a tree file for a unit contained in some source file, you should
compile this file with '-gnatc -gnatt' compiler options. If we want to apply
the application described in section 1.2 to itself, we have to compile the
source of this application with a command

   gcc -c -gnatc -gnatt example1.adb

and as a result, we will get the tree file named example1.adt in the current
directory.

For more explanation how to generate and how to deal with tree files see
sections 3 and 5.


1.6 Running an ASIS Application
    ----------------------------

To complete our example, let's execute our ASIS application. If you have
followed all the steps described in Section 1, now you should have in your
current directory the executable for example1 and the tree file example1.atd.
(Note that a tree file contains information about a unit it was created for
and about all the units upon which this unit depends semantically). If we run
our application, it will process an ASIS Context defined by one tree file
example1.adt (for more details about defining an ASIS context see section 3 of
this Guide and ASIS Reference Manual). The result will be:

   A Context contains the following compilation units:

      Standard (spec) - Ada predefined unit
      Example1 (body) - user-defined unit
      Ada.Text_IO (spec) - Ada predefined unit
      Ada (spec) - Ada predefined unit
      Ada.IO_Exceptions (spec) - Ada predefined unit
      Ada.Streams (spec) - Ada predefined unit
      System (spec) - Ada predefined unit
      System.File_Control_Block (spec) - Ada predefined unit
      System.Parameters (spec) - Ada predefined unit
      Ada.Characters.Handling (spec) - Ada predefined unit
      Ada.Characters (spec) - Ada predefined unit
      Asis (spec) - user-defined unit
      A4G.A_Types (spec) - user-defined unit
      A4G (spec) - user-defined unit
      Ada.Characters.Latin_1 (spec) - Ada predefined unit
      A4G.Int_Knds (spec) - user-defined unit
      Types (spec) - user-defined unit
      Unchecked_Deallocation (spec) - Ada predefined unit
      Asis.Implementation (spec) - user-defined unit
      Asis.Errors (spec) - user-defined unit
      Asis.Ada_Environments (spec) - user-defined unit
      Asis.Compilation_Units (spec) - user-defined unit
      Asis.Ada_Environments.Containers (spec) - user-defined unit
      Asis.Exceptions (spec) - user-defined unit

In the current implementation, ASIS implementation components are considered
user-defined, not implementation-specific, units. Note also, that some
components of the GNAT Run-Time Library may be implicitly "withed" by some Ada
units, and therefore they may be presented by a tree file, that is why you can
see System.File_Control_Block in the list above.


 2. ASIS Overview
    -------------

This chapter contains a short overview of the ASIS definition as given in
the ISO/IEC ASIS Standard. This overview is aimed at helping an ASIS
newcomer to find needed information in the ASIS definition and to navigate
himself in it.

For more details look into the ASIS definition itself. To get some initial
experience with ASIS, go through the ASIS Tutorials (see section 4).

2.1 Main ASIS Abstractions
    ----------------------

ASIS is based on the three main abstractions used to describe Ada programs:

Context - an ASIS Context is a logical handle to an Ada environment, as
          defined in RM 95 Chapter 10. To avoid the language-lawyer
          difficulties when trying to understand the formal relation
          between an ASIS Context and an Ada environment, an ASIS
          application developer may view an ASIS Context as a way
          to define a set of compilation units available through the
          ASIS queries.

Compilation Unit - an ASIS Compilation Unit is a logical handle to an Ada
          compilation unit. It reflects practically one-to-one all the
          properties of compilation units defined by RM 95, and it also
          reflects some properties of "physical objects" treated by an
          underlying Ada implementation as compilation units (such as time of
          last update, the name of some object treated as containing the
          source text for a unit). An ASIS Compilation Unit provides the
          black-box view of a compilation unit, considering a unit as a whole.
          It may be decomposed and analyzed as a white-box by means of ASIS
          Elements.

Element - an ASIS Element is a logical handle to syntax components of ASIS
          Compilation Units (both explicit and implicit).

Some ASIS components use additional abstractions needed for specific pieces of
functionality provided by these components:

Container - an ASIS Container (defined and used by the
          Asis.Ada_Environments.Containers package) provides means for
          structuring the content of an ASIS Context by grouping ASIS
          Compilation units into Containers.

Line    - an ASIS Line (defined and used by the Asis.Text package) is the
          abstraction of a line in an Ada source text. An ASIS Line has a
          length, a string image and a number.

Span    - an ASIS Span (defined and used by the Asis.Text package) defines the
          location of an Element, a Compilation Unit or a whole compilation in
          the corresponding source text.

Id      - An ASIS Id (defined and used by the Asis.Ids package) provides a
          way to store some "image" of an ASIS Element outside an ASIS
          application. An application may create an Id value from an Element
          value and store it in a file. After that, the same or another
          application may read this Id value in and try to convert it back
          into the corresponding Element value.


2.2 ASIS Package Hierarchy
    ----------------------

ASIS is defined as a hierarchy of Ada package specifications. Below is the
short description of this hierarchy.

Asis - this is the top package of the hierarchy. It defines the main ASIS
       abstractions - Context, Compilation_Unit and Element - as Ada private
       types. It also contains a set of enumeration types that define the
       classification hierarchy for ASIS Elements (which closely reflects the
       Ada syntax defined in RM 95) and classification of ASIS Compilation
       Units. This package does not contain any queries;

Asis.Implementation - contains subprograms that control an ASIS
       implementation: initializing and finalizing it, retrieving and
       resetting the diagnosis information. Its child package
       Asis.Implementation.Permissions contains boolean queries which tells
       you how ASIS implementation-specific features are implemented in your
       ASIS implementation;

Asis.Ada_Environments - contains queries that deal with an ASIS Context:
       associating and dissociating, opening and closing a Context;

Asis.Compilation_Units - contains queries that work with ASIS Compilation
       Units: obtaining units from a Context, getting semantic dependencies
       between Units and black-box Unit properties;

Asis.Compilation_Units.Relations - contains queries that return integrated
       semantic dependencies among ASIS Compilation Units, e.g. all the Units
       needed by a given Unit to be included in a partition;

Asis.Elements - contains queries working on Elements and implementing general
       Element properties: gateway queries from ASIS Compilation Units to
       ASIS Elements, queries defining the position of an Element in the
       Element classification hierarchy, queries which define for a given
       Element its Enclosing Compilation Unit and its Enclosing Element.
       It also contains queries that work on pragmas;

Asis.Declarations, Asis.Definitions, Asis.Statements, Asis.Expressions and
ASIS.Clauses - each of these packages contains queries working on Elements of
       the corresponding kind - that is, representing Ada
       declarations, definitions, statements, expressions and clauses
       respectively;

Asis.Text - contains queries returning information about the source
       representation of ASIS Compilation Units and ASIS Elements;

Asis.Exceptions - defines ASIS exceptions;

Asis.Errors - defines possible ASIS error statuses.


2.3 Structural and Semantic Queries
    -------------------------------

Queries working on Elements and returning Elements or Element Lists are divided
into structural and semantic queries.

Each structural query (except Enclosing_Element) implements one step of the
parent-to-child decomposition of an Ada program according to the ASIS Element
classification hierarchy. Asis.Elements.Enclosing_Element query implements the
reverse child-to-parent step. (For implicit Elements obtained as results of
semantic queries, Enclosing Element may not correspond to what could be
expected from the Ada syntax and semantics as defined in RM 95, in this case
the documentation of a semantic query also defines the effect of
Enclosing_Element applied to its result).

A semantic query for a given Element returns the Element representing
some semantic property of the first - e.g. type declaration for an
expression as expression's type, a defining identifier as a definition for a
simple name etc.

For example, if we have Element El representing an assignment statement:

    X := A + B;

then we can get the structural components of this assignment statements by
applying the appropriate structural queries:

   El_Var  := Asis.Statements.Assignment_Variable_Name (El); --  X
   El_Expr := Asis.Statements.Assignment_Expression    (El); --  A + B

And then we can analyze semantic properties of the variable name represented by
El_Var and of the expression represented by El_Expr by means of appropriate
semantic queries:

   El_Var_Def   := Asis.Expressions.Corresponding_Name_Definition (El_Var);
   El_Expt_Type := Asis.Expressions.Corresponding_Expression_Type (El_Expr);

As the result, we should have got El_Var_Def being of A_Defining_Identifier
kind and representing the defining occurrence of X, and El_Expt_Type being of
An_Ordinary_Type_Declaration and representing the declaration of the type of
the expression   A + B.

If we apply Asis.Elements.Enclosing_Element to El_Var or to El_Expr, we will
get back to the Element representing the assignment statement.

An important thing about classifying queries working on Elements as structural
and semantic is that all the structural queries cannot go outside one ASIS
Compilation Unit, but for semantic queries it is quite usual that the
argument of a query is in one ASIS Compilation Unit, but the result of this
query is in another ASIS Compilation Unit.


2.4 ASIS Error Handling Policy
    --------------------------

Only ASIS-defined exceptions (and the Ada predefined Storage_Error exception)
are allowed to propagate outside the ASIS queries. ASIS exceptions are defined
in the Asis.Exceptions package.

When an ASIS exception is raised, ASIS sets the Error Status (the possible
ASIS error conditions are defined as the values of the
Asis.Errors.Error_Kinds type) and forms the Diagnosis string. An application
can query the current value of the ASIS Error Status by
Asis.Implementation.Status query, and the current content of the Diagnosis
string by Asis.Implementation.Diagnosis query. An application can reset the
Error Status and Diagnosis by Asis.Implementation.Set_Status procedure.


2.5 Dynamic Validity Checking of the ASIS Queries
    ---------------------------------------------

ASIS has just one type (Element) for all kinds of Ada syntax constructs,
and just one type (Compilation_Unit) for all kinds of Ada compilation units.
However, many of the queries working on Elements and Compilation Units can be
applied only to specific kinds of Elements and Compilation units
respectively. (For example, it does not make sense and is illegal to query
Assignment_Variable_Name for an Element of An_Ordinary_Type_Declaration kind).

ASIS is a dynamic validity checking interface. If a query working on
Elements has a list of appropriate Element kinds in its documentation,
this means that this query can work only on Elements of the kinds from
this list. Such a query should raise Asis.Exceptions.ASIS_Inappropriate_Element
exception with Asis.Errors.Value_Error error status set when called for any
Element with a kind not from the list of the appropriate Element kinds.

If a query working on Compilation Units has a list of appropriate unit kinds
in its documentation, then this query can work only on Compilation Units of
the kinds from this list. Such a query should raise
Asis.Exceptions.ASIS_Inappropriate_Compilation_Unit with
Asis.Errors.Value_Error error status set when called for any Compilation_Unit
with a kind not from the list of the appropriate unit kinds.

If a query has a list of expected Element kinds or expected Compilation Unit
kinds in its documentation, this query does not raise any exception when
called with any argument, but it produces a meaningful result only when called
with an argument with the kind from this list. For example, if
Asis.Elements.Statement_Kind query is called for an argument of A_Declaration
kind, it just returns Not_A_Statement, but without raising any exception.


2.6 ASIS Iterator
    -------------

ASIS provides a powerful mechanism to traverse an Ada code, the generic
procedure Asis.Iterator.Traverse_Element. This procedure makes top-down
left-to-right (or depth-first) traversal of the ASIS tree (that is, of
the syntax structure of the Ada code represented by the hierarchy of ASIS
Elements). In the course of this traversal, it applies to each Element the
formal Pre_Operation procedure when visiting this element for the first
time, and the formal Post_Operation procedure when leaving this Element.
By providing his own instantiations for Pre_- and Post_Operation, the
user gains the ability to automatically process all ASIS Elements found
in a given ASIS tree.

For example, suppose we have an assignment statement:

    X := F (Y);

When called for an Element representing this statement, an instantiation
of Traverse_Element does the following (below Pre_Op and Post_Op stand for
actual procedures provided for formal Pre_Operation and Post_Operation, and
numbers indicate the sequence of calls to Pre_Op and Post_Op during
traversal):

             (1 - Pre_Op)  X := F (Y) (10 - Post_Op)
                              |
                              |
              ---------------------------------------
             |                                       |
(2 - Pre_Op) X (3 - Post_Op)                         |
                                                     |
                                      (4 - Pre_Op) F(Y) (9 - Post_Op)
                                                     |
                                                     |
                                 ------------------------------
                                 |                             |
                    (5- Pre_Op)  F (6 - Post_Op)  (7 - Pre_Op) Y (8 - Post_Op)


To see in more detail how Traverse_Element may be used for fast-and-easy
development of a number of useful ASiS applications, see ASIS tutorials
provided as a part of ASIS-for-GNAT distribution (Section 4).


2.7 How to Navigate through the Asis Specification
    ----------------------------------------------

The following hints and tips may be useful when looking for some specific
information in the ASIS definition:

(1) Use the short overview of the ASIS packages given in section 2.2 to limit
    your browsing to a smaller set of ASIS packages (e.g. if you are
    interested in what can be done with Compilation_Units - look only in
    Asis.Compilation_Units, if you are looking for queries which can be used
    to decompose and analyze declarations, limit your search to
    Asis.Declarations).

(2) Inside Asis packages working with particular kinds of Elements
    (Asis.Declarations, Asis.Definitions, Asis.Statements, Asis.Expressions
    and ASIS.Clauses) queries are ordered according ordering of the
    description of the corresponding constructions in RM 95 (e.g., package
    Asis.Statements starts from a query retrieving labels and ends with the
    query decomposing a code statement).

(3) The names of all the semantic queries (and only ones) start from
    Corresponding_... or Implicit_...

(4) Use comment sentinels given in the specification of the ASIS packages. A
    sentinel of the form "--|ER" (from "Element Reference") introduces a new
    element kind, and it is followed by a group of sentinels of the form
    "--|CR" (from "Child Reference") which list queries yielding the child
    Elements for the Element just introduced.


3. ASIS Context
   ------------

3.1 ASIS Context and Tree Files
    ------------------------------

From an ASIS application viewpoint we may view an ASIS Context as a set of
ASIS Compilation Units accessible through the ASIS queries. The common ASIS
implementation technique is to base an implementation of an ASIS Context on
some persistent data structures created by the underlying Ada compiler when
compiling Ada compilation units maintained by this compiler. An ASIS Context
can only contain compilable (that is, legal) compilation units.

In case of ASIS-for-GNAT, an ASIS implementation is based on tree output
files, or, simply, tree files. When called with the special option (-gnatt),
GNAT creates and outputs a tree file in case if no error was detected during
the compilation. The tree file is a kind of the snapshot of the compiler
internal data structures (basically, of the Abstract Syntax Tree (AST)) in
the very end of the successful compilation. ASIS then inputs tree files and
recreates in its internal data structures just the same picture as
the compiler had in the end of the corresponding successful compilation.

An important consequence of the GNAT source-based compilation model is that
AST contains full information not only about the unit being compiled, but also
about all the units upon which this unit depends semantically. Therefore,
having read a tree file, ASIS can in general provide information about more
than one unit. By processing a tree file information can be provided about the
unit for which this tree was created and about all the units upon which it
depends semantically. However, to process several units, ASIS sometimes has to
change the tree being processed (in particular, it is the case when an
application switches between units which do not semantically depend on each
other). Therefore, in the course of an ASIS application, ASIS may read
different tree files and it may read the same tree file more then once.

The name of a tree file is obtained from the name of the source file being
compiled by replacing its suffix with '.adt'. For example, the tree file for
foo.adb is named foo.adt.


3.2 Creating Tree Files for Use by ASIS
    -----------------------------------

Neither GNAT nor gnatmake will create tree files automatically when you are
working with your Ada program. It is the responsibility of a user of an ASIS
application to create a set of tree files which would correctly reflect the
set of the Ada components to be processed by ASIS/ASIS applications, as well as
to maintain the consistency of the trees and the related source files.

To create a tree file for a certain source file, the corresponding source file
must be compiled with -gnatc -gnatt flags:

    gcc -c -gnatc -gnatt foo.adb

will produce foo.adt, provided that foo.adb contains the source of a legal Ada
compilation unit. -gnatt generates a tree file, and -gnatc turns off tree
expansion. ASIS needs tree files created without tree expansion, whereas to
create an object file, GNAT needs expanded AST. Therefore it is impossible to
produce tree files together with producing object files.

The following things are important to remeber when generating and dealing
with tree files:

 (1) ASIS-for-GNAT is distributed for a particular version of
     GNAT. All the trees to be processed by an ASIS application should be
     generated by this specific version of the compiler.

 (2) When creating a tree file for ASIS, use two options -gnatc and -gnatt
     separately, do not combine them into a single -gnatct or -gnattc option
     or it will result in a generation of a bogus object file.

 (3) The tree file is not created if an error has been detected during the
     compilation.

 (4) Opposite to object files, a tree file may be generated for any legal Ada
     compilation unit, including a library package declaration requiring a body
     and a subunit.

 (5) A set of tree files processed by ASIS (ASIS application) may be
     inconsistent, for example two tree files may have been created with the
     different versions of the source of the same unit. This will lead to
     inconsistencies in the corresponding ASIS Context. See section 3.4 for
     more details.

 (6) Do not move tree, object and source files among directories in the
     underlying file system! It may confuse ASIS, and it may detect
     inconsistency between tree and source files when opening a Context or you
     may get wrong results when asking about the source or object file for a
     given ASIS Compilation Unit.

 (7) When calling gcc or gnatmake to create tree files, all file and
     directory names containing relative path information should start from
     "./" or "../" (".\" and "..\" respectively in Windows NT/95). That is, to
     create a tree file for the source file foo.adb located in the inner
     directory named "inner", you should call gcc as:

     >gcc -c -gnatc -gnatt .\inner\foo.adb

     but not as

     >gcc -c -gnatc -gnatt inner\foo.ads

     otherwise ASIS will get confused.


 (8) When reading in a tree file, ASIS checks that this tree file was created
     with '-gnatc' option, and it does not accept trees created without
     '-gnatc'.

 (9) Tree and ALI files. If called to create a tree, GNAT does not destroy
     an ALI file if the ALI file already exists for the unit being compiled
     and if this ALI file is up-to-date. Moreover, GNAT may use some
     information from the existing ALI file to put it into the tree file. So
     if you would like to have both object and tree files for your program,
     first create object files and then - tree files.

(10) There is only one extension for tree files - .adt, whereas the standard
     GNAT name convention for the Ada source files uses two different
     extensions for a spec (.ads) and for a body (.adb). This means that if
     you first compile the body for the tree:

     >gcc -c -gnatc -gnatt foo.adb

     and then - compile the corresponding spec for the tree:

     >gcc -c -gnatc -gnatt foo.ads

     then the tree file foo.adt will be created twice - first for the
     body, and then - for the spec, the tree for the spec will override
     the tree for the body, and the information about the body will be lost
     for ASIS. If you first create the tree for a spec, and then for a body,
     the second tree will also override the first one, but no information will
     be lost for ASIS, because the tree for a body contains full
     information about the corresponding spec.

     To avoid losing information when creating trees for a set of Ada sources,
     use the following rules:

     - if a set of Ada components to process makes up a complete partition,
       use gnatmake (see section 6.4 for more details);

     - otherwise first create trees for specs, and then - for bodies:

       >gcc -c -gnatc -gnatt *.ads
       >gcc -c -gnatc -gnatt *.adb

(11) Reading tree files is a time-consuming operation. Try to minimize the
     number of tree files to be processed by your application and to avoid
     unnecessary tree swappings. (See sections 5.3 (1) and 5.4 for some tips).

Note that between opening and closing a Context, an ASIS application should
not change its working directory (or restore it before making an ASIS call),
otherwise application behavior may be erroneous.

3.2.1 Creating Trees for Data Decomposition Annex
      -------------------------------------------

Using the ASIS Data Decomposition Annex (DDA) does not require anything special
to be done by an ASIS user, except one thing. The implementation of the ASIS
DDA is based on some special annotation added by the compiler to the trees
used by ASIS. An ASIS user should be aware of the fact, that trees created for
subunits does not have this special annotation, therefore ASIS DDA queries do
not work correctly on trees created for subunits (and these queries may not
work correctly if a set of tree files making up a Context contain a tree
created for a subunit).

So, when working with ASIS DDA, a user should avoid creating separate trees
for subunits. Actually, it is not a limitation - to create a tree for a
subunit, a user should also have the source of the parent body around. If in
this situation a user creates the tree for the parent body, it will contain
the full information (including DDA-specific annotation) for all the subunits
which are around. From the other side, a tree created for a single subunit has
to contain information about the parent body, so it is about of the same size
as the tree for the parent body.

The best way to create trees when using ASIS DDA is to use gnatmake - it will
never create separate trees for subunits.

3.3 Different ways to define an ASIS Context in ASIS-for-GNAT
    ---------------------------------------------------------

The Asis.Ada_Environments.Associate query which defines a Context has the
following profile:

    procedure Associate
                 (The_Context : in out Asis.Context;
                  Name        : in     Wide_String;
                  Parameters  : in     Wide_String := Default_Parameters);

In ASIS-for-GNAT Name does not have any special meaning, and all the
properties of a Context being associated are set by the Parameters string.

When making an association of an ASIS Context in ASIS-for-GNAT, you may
specify the following things in the Parameters string of the
Asis.Ada_Environments.Associate query:

- the way of defining a set of tree files making up the Context (-C options);

- the way of dealing with tree files when opening the Context and when
  processing ASIS queries (-F options);

- the way of processing the source files during the consistency check when
  opening the Context (-S options):

- the search path for tree files making up the Context (-T options);

- the search path for source files used for calling GNAT to create a tree
  file on the fly (-I options);

Also the association parameters may (and in some cases - have to) contain the
names of tree files or directories making up search paths for tree and/or
source files. Below is the overview of the Context association parameters in
ASIS-for-GNAT, for full details refer to the ASIS-for-GNAT Reference Manual.

Note that the set of options for the Context association is not frozen, we
are open for discussing ASIS application developers' needs, and we can change
or extend an existing set of options in future.

The way to define a set of tree files making up a Context; the following
options are possible:

-C1 - "one tree" Context, defines a Context made up by a single tree file,
      this tree file name should be given explicitly in the Parameters string

-CN - "N-trees" Context, defines a Context made up by a set of tree files, the
      names of the tree files making up the Context should be given
      explicitly in the Parameters string

-CP - "partition" Context, this option is not implemented yet. The idea is to
      define a Context representing a complete partition, as defined in
      RM 95, 10.2;

-CA - "all trees" Context, defines a Context made up by all the tree files in
      the tree search path given in the same Parameters string, if this
      option is set together with -FM option, ASIS can also create new tree
      files on the fly when processing queries yielding ASIS Compilation
      units.

The way of dealing with tree files when opening the Context and when
processing ASIS queries; the following options are possible:

-FS -  all the trees considered as making up a given Context are created on
       the fly, whether or not the corresponding tree file already exists;
       once created, a tree file may then be reused while the Context remains
       open. This option can be set only with -CA option;

-FT - only pre-created trees are used, no tree file can be created by ASIS;

-FM - mixed approach: if a needed tree does not exist, the attempt to create
      it on the fly is made. This option can only be set with -CA option.

The way of processing the source files during the consistency check when
opening the Context; the following options are possible:

-SA - source files for all the Compilation Units belonging to the Context
      (except the predefined Standard package) are taken into account for
      consistency check when opening the Context (see 3.4 concerning the
      consistency problem in ASIS-for-GNAT);

-SE - only existing source files for all the Compilation Units belonging to
      the Context are taken into account for consistency check when opening
      the Context (see 3.4 concerning the consistency problem in
      ASIS-for-GNAT);

-SN - none of the source files from the underlying file system are taken into
      account when checking the consistency of the set of tree files making up
      a Context.


The default options are -CA, -FT and -SA.

Note, that for -C1 Context, a parameter string should contain exactly one name
of a tree file. Moreover, for such a Context if during the opening of the
Context this tree file could not be successfully read in because of any reason,
Asis_Failed is raised.

Using -I option for defining an ASIS Context is similar to using -I option
when calling GNAT, -T option is used in the same way, but for tree files,
for full details concerning using -T and -I options refer to the ASIS-for-GNAT
Reference Manual. Note, that -T option is used only to locate existing tree
files, and it has no effect for -FS Contexts. On the other side, -I option is
used only to construct a set of arguments when ASIS calls GNAT to create a
tree file "on the fly", it has no effect for -FT Contexts, and it cannot be
used to tell ASIS where it should look for source files for ASIS Compilation
Units.


3.4 Consistency Problem
    -------------------

There are two different kinds of consistency problems existing for
ASIS-for-GNAT, and both of them can show up when opening an ASIS Context.

First, it may be a tree file created by another version of GNAT (see the top
README file about the coordination between the GNAT and ASIS-for-GNAT
versions). This means that there is an ASIS-for-GNAT installation problem.

Second, it may be that the tree files are inconsistent with the existing
source files or with each other.


3.4.1 Inconsistent versions of ASIS and GNAT
      --------------------------------------

When ASIS-for-GNAT reads a tree file created by the version of the compiler
for which a given version of ASIS-for-GNAT is not supposed to be used, ASIS
treats the situation as the ASIS-for-GNAT installation problem
and raises PROGRAM_ERROR with the corresponding exception message. In this
case, PROGRAM_ERROR is not caught by any ASIS query and propagates outside
ASIS. Note that this is not a violation of the requirement stated in the ASIS
definition that only ASIS-defined exceptions are allowed to propagate outside
ASIS queries, because in this case you do not have ASIS-for-GNAT properly
installed and therefore you do not have a valid ASIS implementation. Note also
that the real cause may be some old tree file you have forgotten to remove when
reinstalling ASIS-for-GNAT. This is also considered an installation error.

Be careful when using "when others" exception handler in your ASIS
application: do not use it just to catch non-ASIS exceptions and to suppress
them without any analysis.


3.4.2 Consistency of a set of tree and source files
      ---------------------------------------------

When processing a set of more then one tree file making up the same Context,
ASIS may face a consistency problem. A set of tree files is inconsistent if it
contains two trees representing the same compilation unit and these trees
were created with different versions of the source of this unit. A tree file
is inconsistent with a source of a unit represented by this tree if the source
file currently available for the unit differs from the source used to create
the tree file.

When opening a Context (Asis.Ada_Environmens.Open query), ASIS does the
following checks for all the tree files making up the Context:

- if -SA option is set for the Context, ASIS checks that for every Compilation
  Unit represented by a tree, the source file is available and it is the same
  as the source file used to create the tree (a tree file contains the
  reference for all the source files used to create this tree file;

- if -SE option is set for the Context, then if for a Compilation Unit
  represented by a tree a source file is available, ASIS checks that this
  source is the same as the source used to create the tree. If for a
  Compilation Unit belonging to a Context a source file is not available, ASIS
  checks that all the tree files containing this unit were created with the
  same version of the source of this unit.

- if -SN option is set for the Context, ASIS checks that all the trees were
  created from the same versions of the sources involved.

If any of these checks fail, Asis_Failed is raised as a result of opening a
Context. If the Context has been successfully opened, it ensures that ASIS
will process only consistent set of tree and object files until the Context
is closed (provided that this set will not be changed by some non-ASIS
actions).


3.5 Processing Several Contexts at a Time
    -------------------------------------

If your application processes more then one open Context at a time, and if
at least one of the Contexts is defined with -FS or -FM option, be aware of
the fact that all the tree files created by ASIS on the fly
are placed in the current directory. Therefore, to be on the safe side when
processing several opened Contexts at a time, an ASIS application should
have at most one Context defined with -FS or -FM option. If it has such a
Context, all the other Context should not use tree files located in the
current directory.


4.  ASIS Interpreter asistant
    -------------------------

4.1 Introduction
    ------------

asistant is an interactive interface to ASIS queries. It allows a user to play
around with ASIS without building his own ASIS applications. It provides a
simple command language which allows to define variables of ASIS types and to
assign them values by calling ASIS queries.

asistant may be very useful during learning ASIS: it allows to try different
ASIS queries and to see immediately what are the results. asistant does not
crash in case of any error in calling ASIS queries (such as calling a query
for an inappropriate Element) - instead it reports an error and gives a user
the possibility to try again.

asistant may also be useful as a debug and "ASIS visualization" tool in a
real-life ASIS application project: if an ASIS programmer has some problems in
finding out which query should be used in a given situation or why a given
query does not work correctly with a given piece of Ada code, he may use
asistant to reconstruct the situation which causes problems in his ASIS
application and to do some experiments with ASIS queries.

Though primarily  an interactive tool, asistant also can interpret sequences of
asistant commands written to a file (called a script file below). asistant can
also store in a file the log of an interactive section which can then be
reused as a script file.

The full documentation of asistant may be found in the asistant Users' Guide
(file asistant.ug in the asistant source directory). Here only a very short
overview of asistant usage is presented.

The executable for asistant is created in the asistant source directory as a
part of the standard procedure of installing ASIS-for-GNAT as an Ada library.
Put this executable somewhere on your path, and then type 'asistant' to call
asistant in an interactive mode. As a result, asistant will output a brief
information about itself and then the asistant prompt '>' will appear:

ASIStant - ASIS Tester And iNTerpreter, v1.2
(C) 1997-1999, Free Software Foundation, Inc.
  Asis Version: ASIS 2.0.R

>

Now a user can input asistant commands (asistant supports in its command
language just the same form of comments as Ada does, names in asistant are
not case-sensitive):

>Initialize ("") -- the ASIS Initialize query is called with an empty string
	      	  --  as a parameter

>set (Cont) --  the non-initialized variable Cont of the ASIS Context type
	    --  is created

>Associate (Cont, "", "") --  the ASIS Associate query with two empty strings
                          --  as parameters is called for Cont

>Open (Cont)  --  the ASIS Open query is called for Cont

>set (C_U, Compilation_Unit_Body ("Test", Cont)) -- the variable C_U of the
  --  ASIS Compilation_Unit type is created and initialized by the result of
  --  the call to the ASIS query Compilation_Unit_Body. As a result, C_U
  --  will represent an compilation unit named "Test" and contained in the
  --  ASIS Context named Cont

>set (Unit, Unit_Declaration (C_U))  --  the variable Unit of the ASIS
  --  Element type is created and initialized by the result of calling
  --  the ASIS Unit_Declaration query

>print (Unit) --  as a result of this command, the ASIS debug image of the
              --  current value of Unit will be printed:

Element Debug_Image:
A_PROCEDURE_BODY_DECLARATION
located in Test (body, Unit_Id = 2, Context_Id = 1)
text position : 1 : 1 - 9 : 7
   Nodes:
      Node            : 1363 - N_SUBPROGRAM_BODY
      R_Node          : 1363 - N_SUBPROGRAM_BODY
      Node_Field_1    : 0 - N_EMPTY
   Rel_Sloc           : -10
   obtained from the tree .\test.atb (Tree_Id = 1)

--  suppose now, that we do make an error - we call an ASIS query for
--  inappropriated element:

>set (Elem, Assignment_Expression (Unit))

--  ASIS will raise an exception, asistant will output the ASIS debug
--  information:

Exception is raised by ASIS query ASSIGNMENT_EXPRESSION.
Status : VALUE_ERROR
Diagnosis :
Inappropriate Element Kind in Asis.Statements.Assignment_Expression

--  it does not change any of the existing variables and it prompts a user
--  again:

> ...


4.2 asistant commands
    -----------------
The list of the asistant commands given in this section is incomplete and
it is not supposed to be used as a reference manual for these commands. Its
purpose is only to give some general feeling of what can be done with
asistant:

Help [(name)] - outputs the profile of the ASIS query 'name', when calling
                with no argument, generates a general asistant help;

Set (name)    - creates a (non-initialized) variable 'name' of the ASIS
                Context type;

Set (name, expr) - evaluates the expression 'expr' (it may be any legal
                asistant expression, a call to some ASIS query is the most
		common case in practice) and creates the variable 'name' of
		the type and with the value of 'expr';

Print (expr)  - evaluate the expression 'expr' and outputs its value;

Run ("filename") - launches the script from a file "filename", reading further
                commands from it;

Pause         - paused the current script and turns asistant into interactive
                mode;

Run           - resumes a previously paused script;

Browse        - switches asistant into step-by-step ASIS tree browsing;

Log ("filename") - opens a file "filename" for session logging

Log           - closes the current log file

Quit [(exit-status)] - quits asistant



4.3 asistant variables
    ------------------

asistant variables have Ada-style (simple) names. Variables can be of any ASIS
type and of conventional integer, boolean and string type. All the variables
are created and assigned dynamically by the asistant Set command, there is no
predefined variables.

There is no type checking in asistant: each call to a Set command may be
considered as creating the first argument from scratch and initializing it by
the value provided by the second argument.


4.4 Browsing an ASIS tree
    ---------------------
Browser is invoked by calling the asistant service function BROWSE. BROWSE
disables the asistant command interpreter and enables the command interpreter
of Browser. The Browser 'Q' command switches back into the asistant
environment by enabling asistant command interpreter and disabling the Browser
interpreter.

BROWSE has a single parameter of Element type, and it starts browsing the ASIS
tree starting from its argument Element. BROWSE returns the result of Element
type, an Element on which the process of tree browsing was stopped. So, if a
user types"

> set (e0, Browse (e1))

he will start ASIS tree browsing from e1, and when he finishes the
browsing, e0 will represent the last Element being visited during the
browsing.

If a user types

> Browse (e1)

he will be able to browse the ASIS tree, but the last element of the
browsing will be discarded.

Browser displays the ASIS Element it currently points at and expects one of
the following keystrokes:

  U - one step up the ASIS tree (equivalent to calling the ASIS
      Enclosing_Element query);

  D - one step down the ASIS tree, to the left-most component of the
      current Element

  N - go to the right sibling (to the next element in the ASIS tree
      hierarchy)

  P - go to the left sibling (to the previous element in the ASIS
      tree hierarchy)

  \(D|d)(T|t) - change the form of displaying the current Element:
      'D' turns ON displaying the debug image, 'd' turns it OFF. 'T' turns ON
      displaying the text image, 't' turns it OFF.

  <SPACE><query> - call the <query> for the current Element (see 5.4.);

  Q - back to the asistant environment, the Browser command interpreter is
      disabled and the asistant command interpreter is enabled with the
      current Element returned as a result of the call to BROWSE;

Browser immediately interprets the keystroke and displays the new current
Element. If the message "Cannot go in this direction." appears, this means
that traversal in this direction from current node is impossible (that is,
the current node is either a terminal Element and it is not possible to go
down, or it is the leftmost or the rightmost component of some element, and
it is not possible to go left or right, or it is the top Element in its
enclosing unit structure and it is not possible to go up).

It is possible to issue some ordinary ASIS queries from inside the Browser (for
example, semantic queries). The legal queries are those that accept one
parameter of type Element and return Element as a result.

When the user presses <SPACE>, he is asked to enter the query name. If the
query is legal, the current Element is replaced by the result of the call to
the given query with the current Element as a parameter.


4.5 Example
    -------

Suppose we have an ASIS compilation unit Demo in the source file demo.adb:

    procedure Demo is
       function F (I : Integer) return Integer;

       function F (I : Integer) return Integer is
       begin
          return (I + 1);
       end F;

       N : Integer;

    begin
    	N := F (3);
    end Demo;

And suppose that the tree for this source is created in the current directory.
Below is a sequence of asistant commands which does some work with this unit.
asistant comments are used to explain what is doing:

initialize ("")

--  creating and opening a Context made up by all the tree files
--  in the current directory;

Set (Cont)
Associate (Cont, "", "")
Open (Cont)

-- getting the ASIS Compilation_Unit (body) named "Demo" from this Context;
Set (CU, Compilation_Unit_Body ("Demo", Cont))

--  going into the unit structure and getting to the expression
--  in the right part of the assignment statements in the unit body:
Set (Unit, Unit_Declaration (CU))
Set (Stmts, Body_Statements (Unit, False))
Set (Stmt, Stmts (1))
Set (Expr, Assignment_Expression (Stmt))

- outputting the debug image and the text image of this expression:
Print (Expr)
Print (Element_Image (Expr))

--  this expression is of A_Function_Call kind, so it's possible to ask
--  for the declaration of the called function:
Set (Corr_Caled_Fun, Corresponding_Called_Function (Expr))

--  the debug and the text image of the declaration of the called function is
--  outputted:
Print (Corr_Caled_Fun)
Print (Element_Image (Corr_Caled_Fun))

-- the asistant session is closed:
Quit


5. ASIS Application Templates
   --------------------------

The subdirectory 'templates' of the ASIS distribution contains a set of Ada
source components which may be considered as 'templates' for developing simple
ASIS applications. The general idea is that one can easily build an ASIS
application by adding the code performing some specific ASIS analysis in
well-defined places in these templates.

See the solutions provided for ASIS tutorial as the examples of the use of the
templates.

For more information see the README file in the 'templates' subdirectory.


6. ASIS Tutorial
   -------------

The subdirectory 'tutorial' of the ASIS distribution contains a simple
hands-on ASIS tutorial which may be useful in getting the quick start with
ASIS. The tutorial contains a set of simple tasks based on the asistant tool
and on a set of the ASIS Application Templates provided as a part of the ASIS
distribution. The complete solutions are provided for all the tasks, so the
tutorial may also be considered as a set of ASIS examples.

At the moment the documentation of the tutorial exists as a set of README
files in the 'tutorial' subdirectory and its subdirectories. This
documentation is supposed to be moved into this Guide later.


7. How to Build Efficient ASIS Applications
   ----------------------------------------

7.1 Tree Swapping as a Possible Cause of Poor Application Performance
    -----------------------------------------------------------------

If an ASIS Context is made up by more then one tree, then ASIS may switch
between different trees during an ASIS application run. Switching between
trees means that ASIS reads trees over and over again, and this may slow
down an application considerably.

Basically, there are two causes for tree swapping:

(1) Processing of semantically independent units. Suppose in Context Cont we
    have units P and Q which do not depend on each other, and Cont does not
    contain any third unit depending on both P and Q. This means, that P and Q
    cannot be represented by the same tree. To get some information about P,
    ASIS needs tree p.adt to be accessed, and to get some information about Q,
    ASIS needs q.adt. Therefore, if an applications retrieves some information
    from P, and then starts processing of Q, ASIS has to read q.adt.

(2) The possibility for the same unit to be presented in more then one tree. A
    unit may be presented by the tree created for itself, and it also is
    presented by all the trees created for unit which semantically depend
    upon a given unit. Suppose we have a library procedure Proc depending on a
    library package Pack, and in the set of trees making up our Context we
    have trees pack.adt and proc.adt. Suppose we have got some Element
    representing some component of Pack, when pack.adt was accessed by ASIS,
    and suppose that because of some other actions undertaken by an
    application ASIS changed the tree being accessed to proc.adt. Suppose
    that now the application wants to do something with the Element
    representing some component of Pack and obtained from pack.adt. Even
    though the unit Pack is represented by the currently accessed tree
    proc.adt, ASIS has to switch back to pack.adt, because all the references
    into the tree structure kept as a part of the value of this Element are
    valid only for pack.adt.


7.2 Queries That Can Cause Tree Swapping
    --------------------------------------

In ASIS-for-GNAT, tree swapping can currently take place only when
processing queries defined in:

    Asis.Elements
    Asis.Declarations
    Asis.Definitions
    Asis.Statements
    Asis.Clauses
    Asis.Expressions
    Asis.Text

except the queries that return enumeration or boolean results. For any
instantiation of Asis.Iterator.Traverse_Element, the traversal itself can
cause at most one tree read to get the tree appropriate for processing the
Element to be traversed, but procedures provided as actuals for Pre_Operation
and Post_Opertion may cause additional tree swappings.


7.3 How to Avoid Unnecessary Tree Swapping
    --------------------------------------

To speed up your application, try to avoid unnecessary tree swapping. The
following advises may help you in this:

(1) Try to minimize a set of tree files processed by your application. In
    particular, try to avoid having separate trees created for subunits.

    Minimizing of a set of tree files processed by the application also cuts
    down the time needed for opening a Context. Try to use gnatmake to create
    a suitable set of tree files covering an Ada program for processing by
    an ASIS application.

(2) Choose the right way of Context definition for your application. For
    example, use "one tree" Context (-C1) for applications that are limited
    to processing single units (such as a pretty printer or gnatstub). By
    processing the tree file created for this unit, ASIS can get all the
    syntax and semantic information about this unit. Using "one tree" Context
    definition, an application has only one tree file to read during
    opening a Context, and no other tree file will be read during the
    application run. A "N-trees" Context is a natural extension of "one tree"
    Context for applications which know in advance what units shall be
    processed, but opening a Context becomes longer, and ASIS may switch among
    different tree files during an application run. Use "all trees" Context
    only for applications which are not targeted at processing a specific
    unit or a specific set of units, but are supposed to process all the
    available units, or in case when an application has to process a big
    system consisting of a large number of units. When using an
    application based on "all trees" context, use the approach for creating
    tree files described above to minimize a set of tree files to be
    processed.

(3) In your application, try to avoid switching between processing units or
    sets of units with no dependencies among them - such a switching will
    certainly cause tree swapping.

(4) If you are going to analyze some library unit having both spec and body,
    start from obtaining an Element from the body of this unit. This will set
    the tree created for the body as the tree accessed by ASIS, and this tree
    will be enough for processing both the spec and the body of this unit
    without tree swapping.

(5) To see "tree swapping profile" of your application use -dt debug flag when
    initializing ASIS ( Asis.Implementation.Initialize ("-dt") ). The
    information you will get from the application run may give you some hints
    how to avoid tree swapping.


7.4 Using gnatmake to create tree files
    -----------------------------------

To create a suitable set of tree files, you may use gnatmake. GNAT creates the
ALI files for every successful compilation, whether or not the code has been
generated. Therefore, it is possible to run gnatmake with -gnatc and -gnatt
parameters, and this will create the set of tree files representing all the
compilation units needed by a unit to which gnatmake is applied to be included
in a partition. Below we will use gnatmake to create a set of tree
files for a complete Ada program (partition). You may adapt this approach to
an incomplete program or to a partition without a main subprogram, applying
gnatmake to some of its components.

Using gnatmake for creating tree files has another advantage - this will
keep tree files consistent among themselves and with the sources.

There are two different ways to use gnatmake to create a set of tree files.

First, suppose you have object, ALI and tree files for your program in the same
directory, and main_subprogram.adb contains the body of the  main subprogram.
If you run gnatmake as

   gnatmake -f -c ... main_subprogram.adb -cargs -gnatc -gnatt

or simply as

   gnatmake -f -c -gnatc -gnatt ... main_subprogram.adb

this will create the trees representing the full program for which
main_subprogram is the main procedure. The trees will be created from scratch,
that is, if some tree files already exist, they will be recreated. This is
because gnatmake is called with -f option (which means "force recompilation").
Usng gnatmake without -f option for creating tree files is not reliable
if your tree files are in the same directory with object files, because
object and tree files "share" the same set of ALI files, and in case of
object file existing and being consistent with the ALI and source
files, the source will not be recompiled for creating a tree file if -f
option was not set.

A different approach is to keep the tree files and the associated ALI files
in a separate directory, and to use this directory only for keeping the tree
files and maintaining their consistency with source files (that is, object
files and ALI files corresponding to them should be in another directory).
In this case, by calling gnatmake as

   gnatmake -c ... main_subprogram.adb -cargs -gnatc -gnatt

or simply as

   gnatmake -c -gnatc -gnatt ... main_subprogram.adb

(that is, without forcing recompilation) you will still get the full and
consistent set of tree files representing your programs, but in this case the
existing tree files will be reused.

See the next section for specific details related to Ada compilation units
belonging to precompiled Ada libraries.


8. Processing an Ada Library by an ASIS-based Tool
   -----------------------------------------------

In the cases when an Ada program to be processed by some ASIS-based tool makes
use of some Ada library, it is necessary to be aware of the following features
of using Ada libraries in case of GNAT:

- an Ada library is a collection of precompiled Ada components. The sources
  of the Ada components belonging to the library are also presented as a
  part of a library, but if a user program uses some components from a
  library, these components are not recompiled when calling gnatmake (in a
  usual way) for this program (for example, you never recompile Ada.Text_IO
  when you call gnatmake for any program which uses Ada.Text_IO;

- according to the GNAT source-based compilation model, specs of library
  components are processed when a user unit which uses these components is
  compiled, but bodies of library components are not compiled. As a result,
  if you call gnatmake to create a set of tree files covering a given program,
  and if this program uses something from some Ada library, then the set of
  tree files created by such a call will contain only specs, but not bodies
  for library components;

- any GNAT installation contains the GNAT Run-Time Library (RTL) as a
  precompiled Ada library. In some cases, a GNAT installation may contain some
  other libraries (such as Win32Ada Binding in case of Windows95/NT GNAT
  porting);

- in ASIS-for-GNAT, there is no reliable way to define whether or not a given
  Compilation Unit belongs to some precompiled Ada library other then
  GNAT RTL (some euristics may be added to Asis.Extensions). ASIS-for-GNAT
  classifies (by means of Asis.Compilation_Units.Unit_Origin query) a unit as
  A_Predefined_Unit, if it is from RTL and if it is mentioned in RM95 A(2) as
  an Ada 95 predefined unit, and a unit is classified as
  An_Implementation_Unit if is belongs to RTL, but is not mentioned in RM 95
  A(2). Components of Ada libraries other then RTL are always classified as
  An_Application_Unit;

- there is a possibility to recompile the components of the Ada libraries used
  by a given program. To do this, you have to call gnatmake for this program
  with '-a' gnatmake option. Therefore, if you create a set of tree files for
  you program by calling gnatmake with '-a' option, the resulting set of tree
  files will contain all the units needed by this program to make up a
  complete partition.

Therefore, there are two possibilities for ASIS-based tools and their users in
case if processing (or avoiding processing) of Ada libraries is important for
the functionality of the tool:

1. If the tool does not want to process components of Ada libraries, then
   a set of tree files for this tool may be created by calling gnatmake
   without '-a' option (this is the usual way of using gnatmake). When the
   tool faces a Compilation_Which which represents a spec of some library
   unit, and for which Asis.Compilation_Units.Is_Body_Required gives True,
   but Asis.Compilation_Units.Corresponding_Body yields a result of
   A_Nonexistent_Body kind, then the tool may conclude that this library unit
   belongs to some precompiled Ada library;

2. If a tool wants to process all the Ada compilation units making up a
   program, then a set of tree files for this program should be created by
   calling gnatmake with '-a' option;

3. Asis.Compilation_units.Unit_Origin may be used to filter out RTL components.


9. Compiling, Binding and Linking Applications with ASIS-for-GNAT
   --------------------------------------------------------------

If you have installed ASIS-for-GNAT as an Ada library and added the directory
containing all source, ALI and library files of this library to the values
of the ADA_INCLUDE_PATH and ADA_OBJECTS_PATH environment variables (which
is a recommended way to install ASIS-for-GNAT), you do not need any
ASIS-specific options for the GNAT compiler (that is, for gcc calls)
and for gnatbind when working with your ASIS applications. However for gnatlink
you have to provide an additional parameter "-lasis":

   gnatlink my_application -lasis

When using gnatmake, you also have to provide this linker parameter whenever
a call to gnatmake invokes gnatlink:

   gnatmake ... my_application -largs -lasis

You do not need these linker parameters if a call to gnatmake is not creating
the executable:

   gnatmake -c ... my_application

If you have installed ASIS-for-GNAT without building an ASIS library, then you
have to do the following when working your ASIS application code:

- when compiling, you have to put catalogs with ASIS-for-GNAT implementation
  sources (asis-[version#]-src/asis and asis-[version#]-src/gnat) in the
  search path for the source files; you may do it either by -I gcc options or
  by adding these directories in the ADA_INCLUDE_PATH environment variable;

- when binding, you have to put the directory where all the object and ALI
  files for the ASIS-for-GNAT components were created
  (asis-[version#]-src/obj, if you followed the manual installation procedure
  described in README/ASIS Installation Guide) in the search path for
  gnatbind, you can do it either by -aO gnatbind option or by adding this
  directory in the ADA_OBJECTS_PATH environment variable;

If you have added directories with ASIS-for-GNAT source, object and ALI files
to the values of the GNAT-specific environment variables, you do not
have to provide any ASIS-specific parameter when using gnatmake for your
ASIS application.


10. ASIS-for-GNAT Warnings
    ----------------------

The ASIS definition specifies the situations when a certain ASIS-defined
exception should be raised, and ASIS-for-GNAT follows these rules.

ASIS-for-GNAT also generates warnings if it considers some situation arising
during the ASIS query processing to be potentially wrong, and if the
ASIS definition does not require to raise an exception in this case. Usually
this is the case for actual or potential problems happening in an
implementation-specific parts of the ASIS functionality, such as providing
implementation-specific parameters to the queries Initialize, Finalizes and
Associate or opening a Context.

There are three warning modes in ASIS-for-GNAT:

- default        - warning messages are generated into stderr;
- suppress       - warning messages are suppressed;
- treat as error - a warning is treated as an error by ASIS-for-GNAT: instead
                   of sending a message to stderr, ASIS-for-GNAT raises
                   Asis_Failed and converts the warning message into the ASIS
                   Diagnosis string. ASIS Error Status depends on the cause of
                   the warning.

The ASIS-for-GNAT warning mode may be set when initializing the ASIS
implementation. The "-ws" parameter of Asis.Implementation.Initialize query
suppresses warnings, "-we" parameter of this query sets treating all the
warnings as errors. When set, the warning mode remains the same for all
Contexts processed until ASIS-for-GNAT has finalized.


11. File Naming Conventions and Application's Name Space
   ----------------------------------------------------

Any ASIS application being developed with ASIS-for-GNAT depends on the ASIS
interface components and, transitively on other ASIS-for-GNAT implementation
components. Therefore, the name space available for application's compilation
unit names in the very beginning of the application development already
contains some names, which cannot be used as the names of application's
components.

ASIS-for-GNAT includes the full specification of the ISO/IEC ASIS Standard.

The following children and grandchildren of the top Asis package are added in
ASiS-for-GNAT

- Asis.Extensions hierarchy (the source file names start from asis-extensions
  - defines some useful ASIS extensions, see ASIS Reference Manual for more
  details;

- Asis.Set_Get (the source files asis-set_get.ad[bs] respectively) - contains
  the access and update routines for the implementation of the main ASIS
  abstractions defined in Asis;

- Asis.Text.Set_Get (the source files asis-text-set_get.ad[bs] respectively -
  contains the access and update routines for the implementation of the ASiS
  abstractions defined in Asis.Text;

All other ASIS-for-GNAT Ada implementation components belong to the
hierarchy headed by the package named A4G (which comes from Asis-for-GNAT)
and have names starting from "A4G.".

ASIS-for-GNAT also incorporates the following GNAT components as a part of the ASIS
implementation:

   Alloc
   Atree
   Casing
   Csets
   Debug
   Einfo
   Elists
   Fname
   Gnatvsn
   Hostparm
   Krunch
   Lib
     Lib-List
     Lib-Sort
   Namet
   Nlists
   Opt
   Output
   Repinfo
   Scans
   Sinfo
   Sinput
   Snames
   Stand
   Stringt
   Table
   Tree_In
   Tree_Io
   Types
   Uintp
   Uname
   Urealp
   Widechar

Therefore, in your ASIS application you can use for your Ada components any
names except package names defined by ASIS as the names of the ASIS interface
packages, Asis.Extensions, Asis.Set_Get, Asis.Text.Set_Get, and any name from
the hierarchy headed by "A4G" and any name from the list of the GNAT component
names given above.

All Ada source files making up the ASIS implementation for GNAT (including
the GNAT components being a part of ASIS-for-GNAT) follow the GNAT file name
conventions without any name krunching.