# Frequently asked questions

## General

- Why a new programming language?
- What is an extensible programming language?
- Are Seed7 programs portable?
- Which license does Seed7 use?
- Is Seed7 a descendant of Pascal?
- What kind of programs can be written in Seed7?
- How many lines of code are in the Seed7 project?
- On which operating systems does Seed7 run?
- How do I uncompress the *.tgz file from the release?
- How do I compile the Seed7 interpreter?
- I got errors when compiling Seed7. What should I do?
- How do I verify that the interpreter works correct?
- How can I use the Seed7 interpreter?
- Is it possible to compile Seed7 programs?

## Language

- What are the reserved words of Seed7?
- How is the syntax of Seed7 defined?
- Why does Seed7 not use the C statements like C++ and Java?
- Isn't the code unreadable if everybody invents new statements?
- Hasn't Lisp already user defined statements and operators?
- Why does Seed7 use static type checking?
- Is the program development slowed down with static type checking?
- How can static type checking work when types are first-class objects?
- Why does Seed7 not use type inference?
- Are there automatic casts to the right type?
- Can I use something and declare it later?
- Can functions be overloaded?
- Can I overload two functions which just differ in the result type?
- Can functions have variable parameter lists?
- Why is it necessary to initialize all variables?
- Is Unicode supported?
- Why is the div operator used for integer divisions?
- Why are & and <& defined for string concatenation?
- Is there a garbage collection?
- Is Seed7 object oriented?
- Is everything inherited from object?
- What is the difference between overloading and object orientation?
- What is an abstract data type?
- What is multiple dispatch?
- What is call-by-name?
- What container classes do exist?
- Are there primitive types?
- What is the difference between object and primitive types?
- When to use an object type and when a primitive type?
- How does the assignment work?
- Why are there two forms of assignment?
- What types of parameters does Seed7 have?
- Where are the constructors?
- Are there static methods / class methods?
- Are there generics / templates?
- Are there exceptions?
- What happens when an exception is not caught?

## Implementation

- Where does the interpreter look for include libraries?
- How is the directory of the predefined include libraries determined?
- How does the Seed7 compiler get information about C compiler and runtime?
- What should a binary Seed7 package install?
- Does the interpreter use bytecode?
- How does the analyze phase of the interpreter work?
- How does the compiler implement call-by-name parameters?
- What does action "XYZ_SOMETHING" mean?
- Why are there dollar signs at some places?
- Why does "seed7_05.s7i" contain a version number?
- Can I use an "abc.s7i" include file to boot to the abc language?

### Why a new programming language?

Because Seed7 has several features which are not found in other programming languages:

- The possibility to declare new statements (syntactical and semantically) in the same way as functions are declared (There are also user definable operators with priority and associativity).
- Declaration constructs for constant-, variable-, function-, parameter-, and other declarations are described in Seed7 (The user can change existing declaration constructs or invent new ones).
- Templates use no special syntax. They are just functions with `type` parameters or a `type` result.
- Seed7 has abstract data types. For example the types `array`, `hash`, `struct` and `set`. They are not hard coded in the compiler, but are abstract data types written in Seed7. User defined abstract data types are also possible.
- The object orientation of Seed7 allows multiple dispatch. That means that a function or method is connected to more than one type.
- Seed7 is a syntactically and semantically extensible language: Almost all of the Seed7 language (statements, operators, declaration constructs, and more) is defined in Seed7 in an include file (seed7_05.s7i).
- The application program contains an include statement and the s7 interpreter is booted with the language description when it starts. This way it is possible to define language variants or a totally different language.

### What is an extensible programming language?

An extensible programming language supports mechanisms to extend the programming language, compiler/interpreter and runtime environment. The programmer is allowed to define new language constructs such as statements, declaration constructs and operators syntactically and semantically. Most programming languages allow user defined variables, functions and types, but they also use constructs which are hard-coded in the compiler/interpreter. An extensible programming language tries to avoid such hard-coded constructs in normal programs.

Extensible programming was an area of active research in the 1960s, but in the 1970s the extensibility movement was displaced by the abstraction movement. Todays software history gives almost no hint that the extensible languages movement had ever occurred. In the historical movement an extensible programming language consisted of a base language providing elementary computing facilities, and a meta-language capable of modifying the base language. A program then consisted of meta-language modifications and code in the modified base language. A popular approach to do language extension was the use of macro definitions. The constructs of the base language were hard-coded.

The design and development of Seed7 is based on independent research, which was done without knowing that the historic extensible programming language movement existed. Although Seed7 has different roots it reaches many of the original extensible programming language goals. Seed7 differentiates between syntactic and semantic extensions. Syntactic extensions are described in Chapter 9 (Structured syntax definition) of the manual. The semantic extensions of Seed7 are done by declaring statements and operators as functions. To do this statically typed call-by-name parameters are used.

### Are Seed7 programs portable?

Yes. Seed7 spares no effort to support portable programming. Several driver libraries assure that the access to operating system resources such as files, directories, network, clock, keyboard, console and graphics are done in a portable way. Many functions are defined to avoid the need to use shell (and cmd.exe) commands.

- Different Unicode encodings (e.g.: UTF-8 or UTF-16) in system calls (e.g. fopen()/wopen()) are hidden from the programmer.
- Portable file functions:
  - Copying, moving and removing files (and directory trees).
  - File properties such as size, type, time and mode can be obtained and changed.
  - Reading the contents of a directory as array of strings or via the file interface.
  - A standard path representation removes all problems with drive letters and different path delimiters.
- Differences between Unix sockets and winsockets are hidden and in Seed7 a `socket` is a `file` (as in Unix).
- The library keybd.s7i defines the file KEYBOARD, which supports reading single key presses. The characters read from KEYBOARD are not echoed to the console and there is no need to press ENTER. There is also a portable way to check, if a key has been pressed.
- Reading keys and key combinations such as ctrl-F1 from a text console or a graphic window under different operating systems always delivers the same character code.
- An operating system independent type for times and dates, based on the proleptic Gregorian calendar, is provided in the library time.s7i.
- A text console which allows cursor positioning.
- A portable graphics library allows drawing, image operations, windows manipulation and bitmap fonts. Events to redraw a window and other annoyances are managed in the graphics library.
- Weaknesses of operating systems are hidden (E.g.: The windows function utime() does not work on directories, but Seed7 allows to modify directory access and modification times also under windows).

### Which license does Seed7 use?

Seed7 is "Free as in Freedom" and not only "Free as in Free Beer". The s7 interpreter and the example programs (extension .sd7) are under the GPL (General Public License, see also the file COPYING).

The Seed7 runtime library is under the LGPL (Lesser General Public License, see also the file LGPL). The Seed7 include files (extension .s7i) are a part of the Seed7 runtime library.

Seed7 allows the interpretation and compilation of programs with any license. There is no restriction on the license of your Seed7 programs.

For the development of the Seed7 compiler it will be necessary to move some source code from the s7 interpreter (under GPL) to the Seed7 runtime library (under LGPL). This will only be done to for the Seed7 runtime library and only as far as necessary to make no restriction on the license of compiled Seed7 programs.

If you send me patches (I would be very pleased), it is assumed that you accept license changes from GPL to LGPL for parts of code which need to be in the runtime library to support compilation of Seed7 programs.

### Is Seed7 a descendant of Pascal?

No, not really. The keywords and statements remind people of Pascal, but behind the surface there is much difference. Don't judge a book by its cover. Seed7 is neither limited to Pascal's features, nor is it implemented like Pascal. Notable differences are:

  Feature | Standard Pascal | Seed7
  --- | --- | ---
  syntax | hard-coded in the compiler | defined in a library
  statements | hard-coded in the compiler | defined in a library
  operators | hard-coded in the compiler | defined in a library
  array | hard-coded in the compiler | defined as abstract data type `array`
  record / struct | hard-coded in the compiler | defined as abstract data type
  hash table | not in the standard library | defined as abstract data type `hash`
  compiler target | machine code or P-code | C, compiled to machine code afterwards
  template | none | function with type parameters
  abstract data type | none | function with type result
  object orientation | none | interfaces and multiple dispatch

Except for LL(1) parsing, no technology used by classical Pascal compilers could be used to implement Seed7.

### What kind of programs can be written in Seed7?

Seed7 can be used in various application areas:

- Applications, like an Excel look-alike (Seed7 is a general purpose language and programs can be compiled to an executable).
- Scripts that deal with files and directories (The Seed7 Homepage and the Seed7 release are created with Seed7 scripts).
- Tools for networking (There is support for sockets, listeners, HTTP, FTP and HTML parsing). E.g.:
  - A ftp file transfer program
  - A ftp server
  - A web server
- Programs that deal with XML (There is support for XML parsing and the possibility to read XML into a DOM data structure).
- CGI scripts (A CGI support library is available and the Comanche web server can be used to test CGI scripts).
- As language to describe algorithms.
- Command line utilities. E.g.:
  - A make utility capable to process Unix and Windows makefiles
  - A tar archiving utility
  - A program to save a potentially damaged harddisk partition to an image file
- 2D games. E.g.:
  - Panic
  - Mahjong solitaire
  - Pairs
- Simulations. E.g.:
  - Planet Wator simulation with fish and sharks
  - Bacteria which fight against each other
- Functions to explore mathematics. E.g.:
  - Mandelbrot set
  - Bifurcation
  - Cellular automaton

### How many lines of code are in the Seed7 project?

The Seed7 package contains more than 100000 lines of C and more than 190000 lines of Seed7. For version 2013-03-17 the number of lines is:

  105128 |  | C source files (*.c)
  10331 |  | C header files (*.h)
  112009 |  | Seed7 source files (*.sd7)
  82442 |  | Seed7 library/include files (*.s7i)

C code (*.c and *.h files) can be divided into the following areas:

  0.3% |  | Interpreter main
  11.6% |  | Parser
  2.8% |  | Interpreter core
  24.7% |  | Primitive action functions
  7.4% |  | General helper functions
  48.5% |  | Runtime library
  4.7% |  | Compiler data library

Details about this files can be found in the file 'seed7/src/read_me.txt'.

### On which operating systems does Seed7 run?

Seed7 runs on the following operating systems:

- Linux is supported out of the box (because the development is done using Linux)
- Unix (I used Seed7 also under various Unix variants, so it is probably easy to port Seed7 to a Unix variant)
- BSD (there is a FreeBSD port)
- Windows is supported with several compilers:
  - MinGW GCC (the binary Windows release of Seed7 uses MinGW)
  - Cygwin GCC (the X11 graphics needs Cygwin/X)
  - MSVC cl.exe (cl.exe is the stand-alone compiler of MSVC)
  - BDS bcc32.exe (bcc32.exe is the stand-alone compiler of the BDS)
- DOS (uses DJGPP, sockets and graphics are currently not supported)
- Mac OS X

For other operating systems it might be necessary to write driver modules for screen (=text console), graphics, time or other aspects of Seed7. The package contains various older driver modules which are not up to date, but can be used as base to write such driver modules. For more detailed information look at the files 'seed7/read_me.txt' and 'seed7/src/read_me.txt'.

### How do I uncompress the *.tgz file from the release?

When you have a gnu `'tar'` program available you can just do

    $ tar -xvzf seed7_05_yyyymmdd.tgz

If your `'tar'` command does not accept the `'z'` option you need to uncompress the file first with `'gunzip'`:

    $ gunzip seed7_05_yyyymmdd.tgz
    $ tar -xvf seed7_05_yyyymmdd.tar

Sometimes the browser downloads a *.gz file instead of a *.tgz file. In that case you could also use `'gunzip'` as shown above. As an alternative you can also use `'zcat'`:

    $ zcat seed7_05_yyyymmdd.gz > seed7.tar
    $ tar -xvf seed7.tar

Under windows you can use the 7-Zip compression/decompression utility (there is no relationship to Seed7). 7-Zip is open source software and is available at: www.7-zip.org.

### How do I compile the Seed7 interpreter?

The way to compile the interpreter is dependent on the operating system and the development tools used. You need a stand-alone C compiler and a make utility to compile the interpreter. A C compiler which is only usable from an IDE is not so useful, since some Seed7 programs (e.g. The Seed7 compiler `s7c`) need to call the C compiler as well. To compile the interpreter under Linux just go to the `'src'` directory and type:

    make depend
    make

For other cases several makefiles are prepared for various combinations of operating system, make utility, C compiler and shell:

  makefile name | operating system | make prog | C compiler | shell
  --- | --- | --- | --- | ---
  mk_linux.mak | Linux/Unix/BSD | (g)make | gcc | sh
  mk_clang.mak | Linux/Unix/BSD | (g)make | clang | sh
  mk_cygw.mak | Windows (Cygwin) | (g)make | gcc | sh
  mk_msys.mak | Windows (MSYS) | mingw32-make | gcc | sh
  mk_mingw.mak | Windows (MinGW) | mingw32-make | gcc | cmd.exe
  mk_nmake.mak | Windows (MinGW) | nmake | gcc | cmd.exe
  mk_msvc.mak | Windows (MSVC) | nmake | cl | cmd.exe
  mk_bcc32.mak | Windows (bcc32) | make | bcc32 | cmd.exe
  mk_bccv5.mak | Windows (bcc32) | make | bcc32 V5.5 | cmd.exe
  mk_djgpp.mak | DOS | (g)make | gcc | cmd.exe
  mk_osx.mak | Mac OS X (Xcode) | (g)make | gcc | sh

In the optimal case you just copy one of this files to 'makefile' and do (with the make program from the table above):

    make depend
    make

When the interpreter is compiled successfully the executable and the libraries are placed in the `'bin'` directory. Additionally a symbolic link to the executable is placed in the `'prg'` directory (Under Windows symbolic links are not supported, so a copy of the executable is placed in the `'prg'` directory). The Seed7 compiler (`s7c`) is compiled with:

    make s7c

The compiler executable is copied to the `'bin'` directory. If you do several compilation attempts in succession you need to do

    make clean

before you start a new attempt. More details about the compilation process can be found in the file 'seed7/src/read_me.txt'.

### I got errors when compiling Seed7. What should I do?

In most cases errors indicate that some development package of your distribution is missing. If your operating system is Linux, BSD or Unix not all development packages with header files might be installed. In this case you get some errors after typing 'make depend'. Errors such as

      chkccomp.c:56:20: fatal error: stdlib.h: No such file or directory
      s7.c:30:20: fatal error: stdlib.h: No such file or directory

indicate that the development package of the C library is missing. I don't know the name of this package in your distribution (under Ubuntu it has the name libc6-dev), but you can search for C development libraries and header files.

Errors such as

      con_inf.c:54:18: error: term.h: No such file or directory
      kbd_inf.c:53:18: error: term.h: No such file or directory
      trm_inf.c:47:18: error: term.h: No such file or directory

indicate that the curses or ncurses development package is missing. I don't know the name of this package in your distribution (under Ubuntu it has the name libncurses5-dev), but you can search in your package manager for a curses/ncurses package which mentions that it contains the header files. To execute programs you need also to install the non-developer package of curses/ncurses (in most cases it will already be installed because it is needed by other packages).

Errors such as

      drw_x11.c:38:19: error: X11/X.h: No such file or directory
      drw_x11.c:39:22: error: X11/Xlib.h: No such file or directory
      drw_x11.c:40:23: error: X11/Xutil.h: No such file or directory
      drw_x11.c:45:24: error: X11/keysym.h: No such file or directory

indicate that the X11 development package is missing. Under Ubuntu this package has the name libx11-dev and is described as: X11 client-side library (development headers) Note that under X11 'client' means: The program which wants to draw. A X11 `'server'` is the place where the drawings are displayed. So you have to search for a X11 client developer package with headers. If you use X11 in some way (you don't do everything from the text console) the non-developer package of X11 will already be installed.

Errors such as

      gcc chkccomp.c -lm -o chkccomp
      chkccomp.c:28:21: fatal error: version.h: No such file or directory
      compilation terminated.
      mingw32-make: *** [version.h] Error 1

or

      del version.h
      process_begin: CreateProcess(NULL, del version.h, ...) failed.
      make (e=2): The system cannot find the file specified.
      mingw32-make: *** [clean] Error 2

indicate that your makefile contains commands for the cmd.exe (or command.com) windows console, but your 'make' program uses a Unix shell (/usr/bin/sh) to execute them. Either use a makefile which uses Unix shell commands (e.g. mk_msys.mak or mk_cygw.mak) or take care that the 'make' program uses cmd.exe (or command.com) to execute the commands.

Errors such as

      s7.c:28:21: error: version.h: No such file or directory

indicate that you forgot to run 'make depend' before running 'make'. Since such an attempt produces several unneeded files it is necessary now to run 'make clean', 'make depend' and 'make'.

When you got other errors I would like to know about. Please send a mail with detailed information (name and version) of your operating system, distribution, C compiler, the version of Seed7 you wanted to compile and the complete log of error messages to seed7-users@lists.sourceforge.net .

### How do I verify that the interpreter works correct?

A comprehensive test of the `'s7'` interpreter can be done in the `'prg'` directory with the command:

      ./s7 chk_all

Under windows using `./` might not work. Just omit the `./` and type:

      s7 chk_all

The `'chk_all'` program uses several check programs to do its work. First a check program is interpreted and the output is compared to a reference. Then the program is compiled and executed and this output is also checked. Finally the C code generated by the compiled compiler is checked against the C code generated by the interpreted compiler. If everything works correct the output is (after the usual information from the interpreter):

      compiling the compiler - okay
      chkint - okay
      chkflt - okay
      chkstr - okay
      chkprc - okay
      chkbig - okay
      chkbool - okay
      chkset - okay
      chkexc - okay

This verifies that interpreter and compiler work correct.

### How can I use the Seed7 interpreter?

The s7 interpreter is called with the command

      s7 [options] sourcefile [parameters]

Note that the `'options'` must be written before the `'sourcefile'`. If the `'sourcefile'` is not found `.sd7` is appended to the `'sourcefile'` and searched for that file.

The following options are recognized by s7:

- -? Write Seed7 interpreter usage.
- -a Analyze only and suppress the execution phase.
- -d x Set compile time trace level to x. Where x is a string consisting of the following characters:
  - a Trace primitive actions
  - c Do action check
  - d Trace dynamic calls
  - e Trace exceptions and handlers
  - h Trace heap size (in combination with 'a')
- -d Equivalent to -da
- -i Show the identifier table after the analyzing phase.
- -l Add a directory to the include library search path (e.g.: -l`../lib`).
- -q Compile quiet. Line and file information and compilation statistics are suppressed.
- -t x Set runtime trace level to x. Where x is a string consisting of the following characters:
  - a Trace primitive actions
  - c Do action check
  - d Trace dynamic calls
  - e Trace exceptions and handlers
  - h Trace heap size (in combination with 'a')
- -t Equivalent to -ta
- -v n Set verbosity level of analyse phase to n. Where n is one of the following characters:
  - 0 Compile quiet (equivalent to -q)
  - 1 Write just the header with version information (default)
  - 2 Write a list of include libraries
  - 3 Write line numbers, while analyzing
- -v Equivalent to -v2
- -x Execute even when the program contains errors.

In the program the `'parameters'` can be accessed via `argv(PROGRAM)`. The function `argv(PROGRAM)` delivers an array of strings. The number of parameters is `'length(argv(PROGRAM))'` and `'argv(PROGRAM)[1]'` returns the first parameter.

### Is it possible to compile Seed7 programs?

Generally Seed7 is designed to allow the compilation from Seed7 to C. The Seed7 compiler (s7c) is written in Seed7. It uses the analyze phase of the interpreter to convert a program to call-code and then generates a corresponding C program. This C program is compiled and linked afterwards. The Seed7 compiler can be called with:

      s7c [ options ] source

Possible options are

- -? Write Seed7 compiler usage.
- -O and -O2 Tell the C compiler to optimize.
- -b Specify the directory of the Seed7 runtime libraries (e.g.: -b`../bin`).
- -e Generate code which sends a signal, when an uncaught exception occurs. This option allows debuggers to handle uncaught Seed7 exceptions.
- -g Tell the C compiler to generate an executable with debug information. This way the debugger will refer to Seed7 source files and line numbers. To generate debug information which refers to the temporary C program the option -g-debug_c can be used.
- -l Add a directory to the include library search path (e.g.: -l`../lib`).
- -oc n Optimize constants with level n. E.g.: -oc3 Where n is a digit between 0 and 3:
  - 0 Do no optimizations with constants.
  - 1 Use literals and named constants to simplify expressions (default).
  - 2 Evaluate constant expressions to simplify expressions.
  - 3 Like -oc2 and additionally evaluate all constant expressions.
- -r Suppress the generation of range checks for strings and arrays.
- -te Generate code to trace exceptions. This option works in the same way as the interpreter option -te: Every exception will write a message to stdout and the user will be asked to continue (with enter) or to terminate (with * ).

### What are the reserved words of Seed7?

In Seed7 there are no reserved words. Instead there are keywords which are used at various places. Some keywords introduce statements or other constructs (such as declarations). E.g.: The keywords `if`, `while`, `repeat`, `for`, and some others introduce statements. Other keywords like `do`, `then`, `range`, `result`, etc. are used in the middle of statements (or other constructs). Finally there are also keywords like `div`, `rem`, `lpad`, `times`, etc. which are used as operator symbols.

Seed7 uses syntax declarations to specify the syntax of statements. A keyword is a name which is used somewhere in a syntax declaration. Syntax declarations reduce the possibilities to use a keyword out of context. E.g.: After the keyword `if` the parser expects always an expression. This makes `if` unusable as variable name. This way you get error messages when you try to use `if` or other keywords as variable name. That behavior is just the same as in other languages which have reserved words. It can be summarized that Seed7 reaches the goal of avoiding the misuse of keywords in other ways and not by reserving them altogether.

In a classic compiler (e.g. a Pascal compiler) there is a distinction between reserved words and identifiers. Pascal compilers and probably also Ada, C/C++, Java and C# compilers use an enumeration type to represent the reserved words. Since Seed7 allows user defined statements (which may introduce new keywords) it is not possible to hard code reserved words in the compiler as it is done in Pascal, Ada, C/C++, Java and many other compilers.

### How is the syntax of Seed7 defined?

The syntax of Seed7 is described with the Seed7 Structured Syntax Description (S7SSD). The S7SSD is similar to an Extended Backus-Naur Form (EBNF), but there are important differences. S7SSD does not distinguish between different nonterminal symbols. Instead it only knows one nonterminal symbol: () . S7SSD syntax rules do not define named nonterminal symbols (EBNF rules define named nonterminal symbols). S7SSD syntax rules are introduced with:

    $ syntax

S7SSD syntax rules define a pattern of terminal and nonterminal symbols separated by dots. A S7SSD syntax rule defines also a priority and associativity. The syntax of the + operator is:

    $ syntax expr: .(). + .()   is -> 7;

The syntax of statements and other constructs is defined as if they were also operators:

    $ syntax expr: .while.().do.().end.while   is -> 25;

S7SSD is a simple syntax description that can be used by humans and compilers respectively interpreters. The syntax of a Seed7 program is defined in the library "syntax.s7i". When a Seed7 program is interpreted or compiled the syntax definitions are read from "syntax.s7i".

### Why does Seed7 not use the C statements like C++ and Java?

The C statements have some weaknesses which are avoided with the Seed7 statements:

1. The C if-statement

       if (condition)
         statement;

   allows just one statement after the condition. By using the compound statement it is possible to have several statements after the condition

       if (condition) {
         statement1;
         statement2;
       }

   Adding or removing a statement in the second if-statement is always possible. In the first if-statement you must add braces if you add a statement otherwise you get an undesired effect. Adding statements to an if-statement is quite common.

   Since both forms are legal and adding a statement to the first form can lead to errors Seed7 closes this possible source of errors with its if-statement:

       if condition then
         statement
       end if;

2. The following switch statement is formally correct but probably wrong

       switch (number) {
         case 1:
         case 2:
           result = 5;
         case 3:
         case 4:
           result = 8;
           break;
         default:
           result = 0;
       }

   Forgetting break statements in a switch is another possible source of errors which is avoided with the case-statement of Seed7:

       case number of
         when {1, 2}:
           result = 5;
         when {3, 4}:
           result = 8;
         otherwise:
           result = 0;
       end case;

### Isn't the code unreadable if everybody invents new statements?

There are lots of possibilities to write unreadable code without using the extension features of Seed7. The programmer is (as always) responsible to write readable programs. The variable/type/function names and other things chosen by the programmer can always lead to obfuscated code.

Defining new statements and operators is a feature which should not be used in every program by every programmer. It is a feature which allows experienced programmers, to write libraries which use statement or operator syntax instead of function syntax, in areas where such a notation is already accepted practice.

Statements to access a database or operators for vector arithmetic would be such an example. Another example is a construct which can be used in the definition of text adventure games.

The possibility to define statements allows also a more precise language definition. The C++ for/while/if statements are described in the C++ manuals using BNF and an English description. Seed7 statements can be defined in Seed7. For example:

    $ syntax expr: .while.().do.().end.while is -> 25;

    const proc: while (ref func boolean: condition) do
                  (ref proc: statement)
                end while is func
      begin
        if condition then
          statement;
          while condition do
            statement;
          end while;
        end if;
      end func;

The syntax and semantic of a while-statement is described using an if-statement and recursion. For performance reasons the implementation will usually use a different approach to implement a while-loop, but this example shows the expressive power of Seed7.

### Hasn't Lisp already user defined statements and operators?

Defining the semantic of a new 'statement' in Lisp is a classic example. Normally such 'statements' still use the list notation with lots of parentheses. The read macros of Lisp could be used to define the syntax of a statement, but read macros make no type checks at compile time. Any type checking must be written by the programmer and is not mandated by Lisp. The type checks will be performed at runtime and might issue warnings at compile time (this is implementation dependent). In general: Lisp 'statement' declarations do not force compile time checks and look less elegant. Seed7 statement declarations force a type check at compile time.

While Lisp allows new and overloaded functions, the Lisp 'operators' are functions which use the prefix notation (with lots of parentheses). Again read macros could be used to support infix operators with priority and associativity. This read macros would have the same problems as above. Although Lisp fanatics would never admit it, infix operators with priority and associativity are not really supported by Lisp. If somebody tells you that everything can be done in Lisp, send him to the next advocacy group. In general: Seed7 supports user definable infix operators with priority and associativity. Such operators can be overloaded and the type checks are done at compile time. In Lisp all this would be a hack.

### Why does Seed7 use static type checking?

With static type checking all type checks are performed during compile time. Type errors, such as an attempt to divide an integer by a string, can be caught earlier (unless this unusual operation has been defined). The key point is that type errors are found without the need to execute the program. Some type errors can be hidden in rarely executed code paths. Static type checking can find such errors easily. With dynamic type checking extensive tests are necessary to find all type errors. Even tests with 100% code coverage are not enough since the combination of all places where values are created and all places where these values are used must be taken into account. That means that testing cannot guarantee to find all type errors that a static type checker can find. Additionally it would be necessary to repeat all tests every time the program is changed. Naturally there are doubts that enough tests are done and that the tests are adjusted and repeated for every change in the program. Therefore it can be said that compile time type checks increase the reliability of the program.

Seed7 makes sure that the object values always have the type of the object. This goal is reached with mechanisms like mandatory initialization, runtime checks and the impossibility to change arbitrary places in memory. When the generation of garbage values is avoided, it can be guaranteed that only legal values of the correct type are used as object values. This way runtime type checks are unnecessary and the program execution can be more efficient.

Type declarations can also serve as a form of documentation, because they can illustrate the intent of the programmer. Although static type checking is very helpful in finding type errors, it cannot replace a careful program design. Some operations, allowed by the static type system, can still be wrong because of different measurement units or other reasons. In the end there are also other possible sources of errors, such as range violations.

Interface types can be used when an object can have several types at runtime. In this case the interface type of the object can be determined at compile time and the type of the object value (implementation type) can vary at runtime. The static type checking can still check the interface type and the presence of interface functions. Additionally the compiler can also check that all functions granted by the interface type are defined for the implementation type.

### Is the program development slowed down with static type checking?

No, especially when the time spent to debug a program is taken into account. Except for artificial corner cases all type errors found by a "nitpicking" compiler correspond to runtime type errors that can happen in a dynamically typed language under some circumstances. That way the compile time type checks save the time necessary to find and debug those errors. The time that a compiler needs to find and flag type errors is so small that it can be ignored in this comparison.

Some people claim, that adding type information to a program is a time consuming process. This is only true when the type information is added afterwards, but it is wrong when type considerations take place during the program development. Every good programmer has some concepts about what values will be hold by variables or parameters and what values will be returned by functions. A good type system helps to formalize the type concepts which are already in the mind of the programmer. That way the ideas of the programmer are also documented.

When comparing compile time and runtime type checking it can be concluded that dynamic typed languages save some programming time by omitting type declarations, but this time must be paid back with massive interest rates to do the debugging.

### How can static type checking work when types are first-class objects?

This question refers to something which seems paradox: When Seed7 types are created at runtime how can they be checked at compile time. The simple answer is that a `type` created at runtime cannot be used to define something in the program that is currently running.

Seed7 declarations are not executed at runtime. Functions with type parameters and type result are executed at compile time. This is done in templates and abstract data types (both are executed at compile time). It is possible to have type variables and type expressions at runtime but is not possible to declare objects with such a variable type for the program which currently runs. Such type variables and type expressions are used in the Seed7 compiler.

### Why does Seed7 not use type inference?

Seed7 has a basic principle that would break if type inference would be used:

  For every expression (and sub expression) you know its type at compile time without knowing where this expression is used.

It is exactly the violation of this principle that makes type inference possible. As long as this principle holds you need to know the global and local declarations to find out the result type of an expression. With type inference it is necessary to take other expressions in the local function and even expressions in other functions into account. I do not say that this is not possible (for sure it is an interesting challenge to invent an algorithm to do this). But the reader of the program needs to use this algorithm also every time he/she reads the program. And that is very bad since a program is more often read than written.

### Are there automatic casts to the right type?

No, because Seed7 is strong typed. This means that for every expression (and sub expression) you know its type at compile time without knowing where this expression is used. Although this means that you have to convert types explicit (for example from `integer` to `float`) it has more advantages than disadvantages:

1. The overloading rules are much simpler.
2. An expression can be understood without its calling context.
3. Errors caused by unplanned automatic type conversions cannot happen.
4. Since you have to do type conversions explicit you are more aware of the run time overhead.

### Can I use something and declare it later?

No, everything must be declared before it is used. The possibility to declare new statements and new operators on one side and the static typing requirements with compile time checks of the parameters on the other side would make the job of analyzing expressions with undeclared functions very complex.

Forward declarations help, if something needs to be used before it can be declared fully.

### Can functions be overloaded?

Yes, functions, operators and statements can be overloaded. Additionally it is possible to define new operators and statements.

### Can I overload two functions which just differ in the result type?

No, it is not possible to overload functions (operators, statements) which have the same parameter types and just differ in the result type. This type of overloading has a big advantage:

  For every expression (and sub expression) you know its type at compile time without knowing where this expression is used.

Therefore it is not possible to overload something which has no parameters (like a variable or a literal). As a consequence it is sometimes necessary to cast a literal to get an unambiguous expression. This concept parallels the approach used in mathematics where it is also required to specify measurement units. Obviously there is a difference between 5 seconds, 5 square meters and 5 apples. In school it is usually considered wrong to just write 5 and let the teacher guess the measurement unit.

### Can functions have variable parameter lists?

No, because functions with variable parameter list as the C printf function have some problems:

- Normally type checking is only possible at run time.
- The recognition of overloaded functions becomes more complicated.

Instead Seed7 has array aggregates and allows functions with arrays as parameters. So you could declare a function

    const proc: print_list (in array integer: arr) is func
      local
        var integer: number is 0;
      begin
        for number range arr do
          writeln(number);
        end for;
      end func;

and call it with

    print_list([](1, 1, 2, 3, 5, 8, 13, 21, 34, 55));

### Why is it necessary to initialize all variables?

Forgetting to initialize a variable is a common source of errors. In some programming languages uninitialized variables have a random value which could lead to errors. To avoid errors caused by uninitialized variables in Seed7 each variable must be initialized when it is declared.

### Is Unicode supported?

Seed7 `characters` and `strings` support Unicode. Unicode values are encoded with UTF-32. This way it is not necessary to distinguish character-length from byte-length. All functions which exchange strings with the operating system automatically convert the strings to and from UTF-32. It is possible to read and write files with Latin-1, UTF-8 and UTF-16 encoding. Functions to deal with codepages and functions to convert between different Unicode encodings are also available.

Seed7 source code allows Unicode in char literals, string literals, block comments and line comments. Interpreter and compiler assume that a Seed7 program is written with UTF-8 encoding. Therefore a program editor with UTF-8 encoding should be used.

### Why is the div operator used for integer divisions?

In Pascal and Ada the keyword `div` is used as integer division operator. Other languages like C and its descendants use `/` for integer division. Using `div` has some advantages:

- It opens the opportunity to use `/` for a different purpose. The library rational.s7i defines `/` to create a rational number.
- An integer division truncates the result. In the common case the result is not equal to that of a floating point division (E.g.: `flt(4 div 3)` returns `1.0`, but `flt(4) / flt(3)` returns `1.333333`). This difference is emphasized by using different operator symbols.
- A negative result of a division can be rounded towards zero or towards minus infinite. Seed7 provides both possibilities with the two integer division operators `div` and `mdiv`.

### Why are & and <& defined for string concatenation?

The operators `&` and `<&` both concatenate strings, but they have different purposes.

- The `&` operator is intended for `string` concatenations in normal expressions. The `&` operator does not convert an `integer` (or some other value) to a `string`.

  The priority of `&` is defined to execute the concatenation before doing a comparison. E.g.:

      name & extension = check

  has the meaning

      (name & extension) = check

  So the `&` operator can be used like `+` `-` `*` (the expression is evaluated and its result can be compared).

- The `<&` operator is intended for `write` statements. It is overloaded for many types. As long as the first or the second parameter is a `string` it does convert the other parameter to a `string` (with the function `str`) and does the concatenation afterwards.

  The priority of `<&` is defined to allow also the output of `boolean` expressions. E.g.:

      name <& extension = check

  has the meaning

      name <& (extension = check)

  Note that `extension` and `check` could be e.g. `integers`. The result of 'extension = check' is converted to `string` with the function `str`. So

      writeln(name <& extension = check)

  would write (when `name` is `"asdf"` and `extension` is not equal to `check`):

      asdfFALSE

The `<&` operator can be defined for new types with `enable_io` respectively `enable_output`. The description of the Seed7 file system contains also a chapter about the conversion to strings and back.

### Is there a garbage collection?

There is no garbage collection process, that interrupts the normal processing, to free some memory. The automatic memory management of Seed7 uses different mechanisms. Memory usage can be categorized and for every category a specific strategy of automatic memory management is used:

- Memory used by local variables and parameters is automatically freed, when a function is left. The interpreter maintains a list of local values and frees them. The compiler inserts code, to free the memory used by local variables, in front of each return statement.
- Memory allocated for intermediate results is freed automatically in a stack like manner. Like an arithmetic expressions such as (1+2)*3+4 can be evaluated with the help of a stack (which stores the intermediate results 3 and 9). For structured values it is possible to maintain a stack of pointers to the values. The interpreter uses a temp flag, which is present in every interpreter object, to free memory. The compiler determines the point, where intermediate results can be be freed, at compile time. Functions, such as the assignment, can abstain from freeing the intermediate result and just assign it to the variable. This way it is not always necessary to copy arbitrary complex values. All this things can be decided by the compiler.
- Strings, arrays, hashes and other containers manage their memory. E.g.: When an element is removed from a hash table the memory used by the element is freed as well as the hash table internal data.
- A struct value can be referred by one struct variable and by several interface variables. Struct values use a reference counter to free the struct, when no reference to it exists.

### Is Seed7 object oriented?

Yes, but object orientation is organized different compared to other object oriented languages. In a nutshell: It is based on interfaces and allows multiple dispatch. Chapter 7 (Object orientation) of the manual contains a detailed description of the Seed7 object orientation.

An example of an object oriented type is `file`. A `file` describes references to values with some other type. A value of a `file` can have one of the following types: null_file, external_file, echo_file, line_file, etc. Each of this `file` value types acts differently to the same requests.

For the type `file` two kinds of functions are defined:

1. Functions which work for all files the same way.
2. Dynamic functions which are just an interface. At run time the corresponding function defined for the type of the value is used.

Compared to Java the type file can be seen as interface or abstract class, while the type of the file value can be seen as the class implementing the interface.

### Is everything inherited from object?

There can be several base types, each with their own hierarchy. In many object oriented languages the class `object` is used as element of all container classes. Abstract data types provide a better and type safe solution for containers and other uses of the root class `object`. Therefore a single rooted hierarchy is not needed.

### What is the difference between overloading and object orientation?

Overloading is resolved at compile time while object orientation uses dynamic dispatch which decides at runtime which method should be called. Overloading resolution uses static types to decide. Dynamic dispatch uses the implementation type, which is only known at runtime, to decide. Besides this difference overloading resolution and dynamic dispatch both use the same approach to do the work: The types and the access rights of all parameters are used in the decision process.

### What is an abstract data type?

An abstract data type defines, like every other type, a set of functions to handle data. An abstract data type leaves, like an interface type from OO, the details of the data representation open. The difference between the two is:

- An interface type is resolved to an implementation type at runtime.
- An abstract data type is resolved to a concrete type at compile time, when it is used.

Usually an abstract data type uses parameters to resolve to a concrete type. Examples of abstract data types are `arrays`, `structs` and `hashes`. An abstract `array` type needs the element type as parameter. E.g.:

    array string

This `array` has `string` elements and uses `integer` indices. An abstract `array`, were the index type is also specified as parameters is:

    array [char] string

This `array` has `string` elements and uses `char` indices. Arrays are present in many programming languages, but they are usually hard-coded into the compiler / interpreter. Seed7 does not follow this direction. Instead it introduces abstract data types as common concept behind `arrays`, `structs`, `hashes` and other types. Like templates abstract data types are implemented with functions that are executed at compile time. In contrast to templates abstract data types return a type as result.

### What is multiple dispatch?

Multiple dispatch means that a function or method is connected to more than one type. The decision which method is called at runtime is done based on more than one of its arguments. The classic object orientation is a special case where a method is connected to one class and the dispatch decision is done based on the type of the `'self'` or `'this'` parameter. The classic object orientation is a single dispatch system.

In a multiple dispatch system the methods cannot be grouped to one class and it makes no sense to have a `'self'` or `'this'` parameter. All parameters are taken into account when the dispatch decision is done. In the following example the interface type `Number` uses multiple dispatch:

    const type: Number is sub object interface;

    const func Number: (in Number param) + (in Number param) is DYNAMIC;

The `DYNAMIC` declaration creates an interface function for the `'+'` operator. The interface type `Number` can represent an `Integer` or a `Float`:

    const type: Integer is new struct
        var integer: val is 0;
      end struct;

    type_implements_interface(Integer, Number);

    const type: Float is new struct
        var float: val is 0.0;
      end struct;

    type_implements_interface(Float, Number);

The declarations of the converting `'+'` operators are:

    const func Float: (in Integer: a) + (in Float: b) is func
      result
        var Float: sum is Float.value;
      begin
        sum.val := flt(a.val) + b.val;
      end func;

    const func Float: (in Float: a) + (in Integer: b) is func
      result
        var Float: sum is Float.value;
      begin
        sum.val := a.val + flt(b.val);
      end func;

The declarations of the normal `'+'` operators (which do not convert) are:

    const func Integer: (in Integer: a) + (in Integer: b) is func
      result
        var Integer: sum is Integer.value;
      begin
        sum.val := a.val + b.val;
      end func;

    const func Float: (in Float: a) + (in Float: b) is func
      result
        var Float: sum is Float.value;
      begin
        sum.val := a.val + b.val;
      end func;

The decision which `'+'` operator should be called at runtime is based on the implementation type (`Integer` or a `Float`) of both arguments of the `'+'`.

### What is a call-by-name?

Call-by-name is an evaluation strategy for parameters. The actual call-by-name parameter is not evaluated before the function is called. When the function is executed the call-by-name parameter might be executed once, many times or not at all. Examples of call-by-name parameters are:

- The conditions of while-loops
- The statements in loop bodies
- The statements that are conditionally executed in an if-statement

As can be seen, call-by-name parameters are used all the time, without realizing it. A call-by-name parameter is a function without parameters. Function types such as `proc` or `func boolean` are used as type of formal call-by-name parameters. An expression with the correct type is allowed as actual call-by-name parameter. This actual parameter expression is not evaluated when the function is called. Instead the call-by-name expression is evaluated every time the formal call-by-name parameter is used. A `'conditional'` function (similar to the ?: operator from C) is defined with:

    const func integer: conditional (in boolean: condition,
        (in func integer: trueValue, in func integer: falseValue) is func
      result
        var integer: conditionalResult is 0;
      begin
        if condition then
          conditionalResult := trueValue;
        else
          conditionalResult := falseValue;
        end if;
      end func;

Seed7 does not require a special notation (like brackets) for actual call-by-name parameters, therefore the `'conditional'` function can be called with:

    conditional(a < 100.0, sqrt(a), a ** 2)

Depending on the condition `'a < 100.0'` only one of the expressions `'sqrt(a)'` and `'a ** 2'` is evaluated. This evaluation takes place when `'trueValue'` or `'falseValue'` is assigned to `'result'`.

### What container classes do exist?

Abstract data types are used to replace container classes. When using an abstract data type as container you have to specify the type of the element in the type declaration. Therefore abstract data types are always type safe. Typeless container classes with object elements do not exist. The only thing which comes near to this is the `ref_list` which is used in the reflection. A `ref_list` should not be misused as container class. Predefined abstract data types are:

array
  The type `'array baseType'` describes sequences of identical elements of a `'baseType'`
hash
  The type `'hash [keyType] baseType'` describes hash tables with elements of `'baseType'` which can be accessed using an index of `'keyType'`
set
  The type `'set of baseType'` describes a set of elements of a `'baseType'`
struct
  The type `'struct ... end struct'` describes all structured types.

Usage examples of abstract data types are:

    array string
    array [boolean] string
    hash [string] boolean
    hash [string] array array string
    set of char
    set of integer

### Are there primitive types?

As in C++, Java, C# and other hybrid object oriented languages there are predefined primitive types in Seed7. These are `integer`, `char`, `boolean`, `string`, `float`, `rational`, `time`, `duration` and others. Additionally to the predefined primitive types there is also the possibility to declare new primitive types.

### What is the difference between object and primitive types?

Variables with object types contain references to object values. This means that after

    a := b

the variable 'a' refers to the same object as variable 'b'. Therefore changes of the object value that 'a' refers to, will effect variable 'b' as well (and vice versa) because both variables refer to the same object.

For primitive types a different logic is used. Variables with primitive types contain the value itself. This means that after

    a := b

both variables are still distinct and changing one variable has no effect on the other.

If 'a' and 'b' are declared to have type `'aType'` which contains the integer field 'property' you can do the following:

    b.property := 1;
    a := b;
    b.property := 2;

Everything boils down to the question: What value does `'a.property'` have now.

- When `'aType'` is an object type a.property has the value 2 because 'a' and 'b' both refer to the same object.
- When `'aType'` is a primitive type a.property has still the value 1 because 'a' and 'b' are distinct objects.

### When to use an object type and when a primitive type?

You should declare a new primitive type if you don't need the object oriented paradigm that a variable (and a constant) is just a reference to the object. Another indication is: If you don't need two concepts of what is equal (An == operator and an equal method).

### How does the assignment work?

For object types just the reference to the object value is copied. For primitive types the value itself is copied. Since values can be very big (think of arrays of structs with string elements) value copies can be time consuming.

In pure object oriented languages the effect of independent objects after the assignment is reached in a different way: Every change to an object creates a new object and therefore the time consuming copy takes place with every change. Because usually changes to an object are more frequent than assignments this approach can be even more time consuming than the approach using value copies for the assignment.

### Why are there two forms of assignment?

Seed7 has an approach for the assignment where practical arguments count more than the classic object oriented principles. In Seed7 every type has its own logic for the assignment where sometimes a value copy and sometimes a reference copy is the right thing to do. Exactly speaking there are many forms of assignment since every type can define its own assignment. If a value copy works like a deep or a shallow copy can also be defined depending on the type.

For example: For `integer`, `char` and `string` variables a value copy is what most people expect. For files you don't expect the whole file to be copied with an assignment, therefore a reference copy seems appropriate.

And by the way: Although it is always stated that in object oriented languages everything is done with methods, this is just not true. Besides statements and operators in C++ and Java which are special even Smalltalk treats the assignment and the comparison special. Seed7 does not have such special treatment for the assignment and the comparison operators.

### What types of parameters does Seed7 have?

There are value and reference parameters. The formal parameter can be constant or variable. The combination of these features allows four types of parameters:

  parameter | call by | access right
  --- | --- | ---
  val | value | const
  ref | reference | const
  in var | value | var
  inout | reference | var

Additionally every type defines an `'in'` parameter which is either a `'val'` or a `'ref'` parameter. Types with little memory requirements like `integer`, `char` and `boolean` use a `'val'` parameter as `'in'`. Other types like `string`, `array` and `struct` use a `'ref'` parameter as `'in'`. Usually it is not necessary to care if an `'in'` parameter is by value or by reference.

### Where are the constructors?

Seed7 does not need constructors, but you can define normal functions which create a new value in a similar way as constructors do it.

Seed7 uses a special create statement ( ::= ) to initialize objects. Explicit calls of the create statement are not needed.

The lifetime of an object goes like this:

1. Memory is reserved for the new object (stack or heap memory make no difference here).
2. The content of the new memory is undefined (It may contain garbage), therefore a create statement is necessary instead of an assignment.
3. The create statements copies the right expression to the left expression taking into account that the left expression is undefined.
4. If the object is variable other values can be assigned using the assign statement ( := ). The assignment can assume that the left expression contains a legal value. This allows that for strings (and some other types which are just references to a memory area) the memory containing the old string value (and not the memory of the object itself) can be freed when necessary.
5. At the end of the lifetime of an object the destroy statement is executed. For strings (and some other types which are just references to a memory area) the memory containing the string value (and not the memory of the object itself) is freed.
6. The memory of the object is freed.

The first three steps are usually hidden in the declaration statement.

### Are there static methods / class methods?

Seed7 allows defining functions (procedures and statements) without corresponding class. When this is not desired Seed7 uses a special parameter, the `'attr'` (attribute) parameter, to archive the functionality of static methods (elsewhere named class methods) in a more general way. How a static method is declared is shown in the following example:

    const func integer: convert_to (attr integer, in char: ch) is func
      result
        var integer: converted is 0;
      begin
        converted := ord(ch);
      end func;

The function `'convert_to'` can be called as

    number := convert_to(integer, 'a');

Since the result of a function is not used to determine an overloaded function, this is sometimes the only way to use the same function name for different purposes as in:

    ch   := convert_to(char,    1);
    stri := convert_to(string,  1);
    ok   := convert_to(boolean, 1);
    num  := convert_to(typeof(num), 1);

Attribute parameters allow a function to be attached to a certain type. But this concept is much more flexible than static methods (or class methods). A function can also have several `'attr'` parameters and `'attr'` parameters can be at any parameter position (not just the first parameter). Furthermore the type can be the result of a function as for example typeof(num).

### Are there generics / templates?

The generics (templates) of Ada, C++ and Java use special syntax. In Seed7 you get this functionality for free without special syntax or other magic.

Generally all Seed7 functions can be executed at compile time or at runtime. The time of the function execution depends on the place of the call. Declarations are just a form of statement and statements are a form of expression. A Seed7 program consists of a sequence of declarations (expressions), which are executed one by one at compile time. This expressions can also invoke user defined functions.

A function body can contain declaration statements. When such a function is executed at compile time, it defines things that are part of the program. It is an error to execute such a function at runtime.

Seed7 uses the word template to describe a function which is executed at compile time and declares some things while executing (at compile time). Naturally a template function can have parameters. Especially types as parameters are useful with template functions. That way a template function can declare objects with the type value of a parameter.

It is necessary to call template functions explicit. They are not invoked implicit as the C++ template functions. The explicit calls of template functions make it obvious what it is going on. This way the program is easier to read.

### Are there exceptions?

Yes, Seed7 has exceptions which are similar to Ada exceptions. Chapter 14.2 (Exceptions) of the manual contains a detailed description of the Seed7 exceptions.

### What happens when an exception is not caught?

When an EXCEPTION is not caught the program is terminated and the s7 interpreter writes a stack trace:

      *** Uncaught EXCEPTION NUMERIC_ERROR raised with
      {integer: <SYMBOLOBJECT> *NULL_ENTITY_OBJECT* div fuel_max }

      Stack:
      in (val integer: dividend) div (val integer: divisor) at integer.s7i(95)
      in init_display at lander.sd7(840)
      in setup at lander.sd7(909)
      in main at lander.sd7(1541)

This stack trace shows that a `div` operation causes a `NUMERIC_ERROR` (probably a division by zero) in line 840 of the file `lander.sd7`. A short examination in `lander.sd7` shows that an assignment to `'fuel_max'` was commented out to show how stack traces work.

A compiled program creates a much shorter crash message:

      *** Uncaught EXCEPTION NUMERIC_ERROR raised at tmp_lander.c(764)

In this case the mentioned file name and line number refers to the temporary C file or the Seed7 runtime library. To get useful information there are two possibilities:

1. Start the program in the interpreter instead.
2. Compile the program with the options `-g` `-e` and start it from a debugger.

When s7c is called with the option `-g` it instructs the C compiler to generate debugging information. This way a debugger like gdb can run the program and provide information. The option `-e` tells the compiler to generate code which sends a signal, when an uncaught exception occurs. This option allows debuggers to handle uncaught Seed7 exceptions. Note that `-e` sends the signal SIGFPE. This is done even when the exception is not related to floating point operations.

Chapter 14.4 (Stack trace) of the manual contains a detailed description how to debug compiled Seed7 programs.

### Where does the interpreter look for include libraries?

Include libraries with absolute path (an absolute path starts with a forward slash) are only searched at the specified place. All other include libraries are searched in several directories. This is done according to a list of library directories (a library search path). The directories of the list are checked one after another for the requested include file. As soon as the include file is found the search is stopped and the file is included. The following directories are in the list of library directories:

1. The directory of the interpreted program. E.g.: When the program `"/home/abc/test/pairs.sd7"` is interpreted the directory `"/home/abc/test"` is in the list of library directories.
2. This point is only in effect when the interpreter was not compiled from source (The C preprocessor macro PATHS_RELATIVE_TO_EXECUTABLE is defined): Interpreter and compiler from the binary release use `"../lib"` relative to the directory of the `'s7'` interpreter executable. E.g.: When the interpreter is in the directory `"/home/abc/seed7/bin"` the directory `"/home/abc/seed7/lib"` is in the list of library directories.
3. The directories that are specified at the commandline with the option `-l`.
4. The directory containing the predefined Seed7 include libraries. This directory is hard-coded in the interpreter (an absolute path like `"/directory_where_Seed7_was_installed/seed7/lib"`). The hard-coded library directory is determined when the interpreter is compiled. When the interpreter was not compiled from source (binary release) the path `"../lib"` relative to the current working directory is used.
5. The directory specified with the `SEED7_LIBRARY` environment variable.
6. Directories specified in the source file with the `library` pragma. E.g.: The line: `$ library "/home/abc/seed7/lib"` adds the directory `"/home/abc/seed7/lib"` to the list of library directories.

Seed7 interpreter and compiler (s7c) use the same list of library directories (the same library search path). When Seed7 is compiled from source code both (interpreter and compiler) will find the Seed7 include files automatically. Interpreter and compiler from the binary release will only find library include files when the path `"../lib"` relative to the `'s7'` or `'s7c'` executable leads to the library directory. Additionally it is possible to set the environment variable `SEED7_LIBRARY` to the absolute path `"/directory_where_Seed7_was_installed/seed7/lib"`.

### How is the directory of the predefined include libraries determined?

The directory of the predefined include libraries is hard-coded in the interpreter. This information is determined when the Seed7 interpreter is compiled. The command `'make depend'` writes a line, which defines the C preprocessor variable SEED7_LIBRARY, to the file `"seed7/src/version.h"`. E.g.: The file `"version.h"` contains the line:

    #define SEED7_LIBRARY "/home/abc/seed7/lib"

The preprocessor macro SEED7_LIBRARY is used by the function `init_lib_path()`, which is defined in `"seed7/src/infile.c"`.

When the interpreter of a binary release is compiled, a slightly modified makefile is used. The command `'make depend'` writes the following preprocessor macro definitions to the file `"version.h"`:

    #define PATHS_RELATIVE_TO_EXECUTABLE
    #define SEED7_LIBRARY "../lib"

The preprocessor macro PATHS_RELATIVE_TO_EXECUTABLE is used by the function `init_lib_path()`, to add a path relative to the interpreter or compiler executable, to the library search list (lib_path).

Interpreter and compiler use the same strategy to determine the directory with predefined include libraries.

### How does the Seed7 compiler get information about C compiler and runtime?

The Seed7 compiler needs detailed information about the C compiler and its runtime library. This information is created when the Seed7 interpreter is compiled. The command `'make depend'` writes C preprocessor macros to `"seed7/src/version.h"`. E.g.:

    #define SEED7_LIB "seed7_05.a"
    #define CONSOLE_LIB "s7_con.a"
    #define DRAW_LIB "s7_draw.a"
    #define COMP_DATA_LIB "s7_data.a"
    #define COMPILER_LIB "s7_comp.a"
    #define S7_LIB_DIR "/home/abc/seed7/bin"

The command `'make depend'` compiles and executes the program `"chkccomp.c"`, which also writes preprocessor macros to `"version.h"`. E.g.:

    #define RSHIFT_DOES_SIGN_EXTEND
    #define TWOS_COMPLEMENT_INTTYPE
    #define LITTLE_ENDIAN_INTTYPE

The preprocessor macros used by `"version.h"` are described in `"seed7/src/read_me.txt"`. The function `'cmdConfigValue()'`, which is defined in `"cmd_rtl.c"`, allows access to several values from `"version.h"`. A Seed7 program, like the Seed7 compiler, can obtain this information with the function `'configValue'`. The Seed7 function `'configValue'` calls the C function `'cmdConfigValue()'`, which returns the requested config value. Configuration values are returned as strings. E.g.: `configValue("S7_LIB_DIR")` returns `"/home/abc/seed7/bin"`. For macros which are either defined or undefined `'configValue'` returns `"TRUE"` or `"FALSE"` respectively. E.g.: `configValue("TWOS_COMPLEMENT_INTTYPE")` returns `"TRUE"`.

The Seed7 compiler uses the runtime libraries SEED7_LIB, CONSOLE_LIB, DRAW_LIB, COMP_DATA_LIB and COMPILER_LIB in the directory S7_LIB_DIR when it links object files to an executable. Config values like RSHIFT_DOES_SIGN_EXTEND, TWOS_COMPLEMENT_INTTYPE and LITTLE_ENDIAN_INTTYPE are used to control the kind of C code produced by the Seed7 compiler. The function `'configValue'` can also obtain config values that do not come from `"version.h"`, but are defined in `"seed7/src/common.h"`. E.g.:

    #define WITH_STRI_CAPACITY
    #define ALLOW_STRITYPE_SLICES

This configuration values describe data structures and implementation strategies used by the Seed7 runtime library. They do not depend on the C compiler and its runtime library, but they may change between releases of Seed7.

### What should a binary Seed7 package install?

A binary Seed7 package needs to install four groups of files:

- The executables of the interpreter (s7 from `"seed7/bin"`) and the compiler (s7c from `"seed7/bin"` or `"seed7/prg"`).
- The Seed7 include libraries (files from `"seed7/lib"` with the extension `.s7i`).
- The static Seed7 object libraries (the files `seed7_05.a`, `s7_con.a`, `s7_draw.a`, `s7_data.a` and `s7_comp.a` from `"seed7/bin"`).
- Documentation files (the files `COPYING` and `LGPL` and all files from `"seed7/doc"`).

The table below shows the suggested directories for Linux/Unix/BSD:

  Directory | Macro | Group of files
  --- | --- | ---
  /usr/bin | - | Executables (s7 + s7c)
  /usr/lib/seed7/lib | SEED7_LIBRARY | Seed7 include libraries
  /usr/lib/seed7/bin | S7_LIB_DIR | Static libraries

The macros must be defined, when the interpreter is compiled. This can be done by calling `'make depend'` with:

    make S7_LIB_DIR=/usr/lib/seed7/bin SEED7_LIBRARY=/usr/lib/seed7/lib depend

Afterwards the interpreter can be compiled with `'make'` and the Seed7 compiler can be compiled with `'make s7c'`. This three make commands can be combined to

    make S7_LIB_DIR=/usr/lib/seed7/bin SEED7_LIBRARY=/usr/lib/seed7/lib depend s7 s7c

Alternatively the Seed7 compiler can be compiled as post-install step. This requires that `"seed7/prg/s7c.sd7"` is also installed. The actual compilation of s7c is done with:

    s7 s7c -O2 s7c

It is also possible to compile the Seed7 compiler in the build directory. In this case it is necessary to specify the direcories `SEED7_LIBRARY` and `S7_LIB_DIR` with the options `-l` and `-b`:

    ./s7 -l ../lib s7c -l ../lib -b ../bin -O2 s7c

Compiling s7c with a make command should be preferred.

### Does the interpreter use bytecode?

No, the analyze phase of the Seed7 interpreter produces call-code which consists of values and function calls. This call-code is just handled in memory and never written to a file. After the analyze phase the call-code is interpreted.

### How does the analyze phase of the interpreter work?

The analyzer reads successive expressions. The expressions are read with a table-driven LL(1) recursive descent parser. The parser is controlled by Seed7 syntax definitions. The parser calls a scanner, which skips whitespace and reads identifiers and literals. Each parsed expression is searched in the internal database of defined objects. This search process is called matching. The matching resolves overloaded functions and generates call-code for the parsed expression. Call-code uses a data structure which is similar to S-Expressions. The analyzer executes the call-code of the parsed and matched expressions. Normally parsed and matched expressions represents declaration statements. Executing a declaration statement adds new defined objects to the internal database.

### How does the compiler implement call-by-name parameters?

Every function with call-by-name parameters is searched for recursive calls. When no recursive call of the function is present it can be implemented with code inlining. In this case every call of the function is inlined and the actual call-by-name parameters replace all occurrences of the formal call-by-name parameter in the function body.

When a function cannot be implemented with code inlining (recursive calls occur) pointers to a closure structure are used as formal call-by-name parameters. This closure structure contains a function pointer and a structure which represents the environment of the closure. When a formal call-by-name parameter is used the function of the closure structure is called with a pointer to the closure environment as parameter.

When a function with call-by-name parameters is called the following things are done: For every actual call-by-name parameter a closure structure with the function pointer and the closure environment structure is generated. An actual function representing the closure code is also generated. Before a function with a call-by-name parameter is called a closure structure variable is initialized. This includes initializing the function pointer and the environment data of the closure structure variable. Finally a pointer to the closure structure variable is used as actual call-by-name parameter.

### What does action "XYZ_SOMETHING" mean?

Actions are used to call a corresponding C function in the interpreter. For example:

The action "INT_ADD" corresponds to the function 'int_add' in the file seed7/src/intlib.c .

Chapter 13 (Primitive actions) of the manual contains a detailed description of the primitive actions. In the interpreter all action functions get the parameters as list. The action functions take the parameters they need from the list, perform the action and deliver a result.

### Why are there dollar signs at some places?

The $ is used to force the analyzer to use a hard coded expression recognition instead of the configurable one. This mechanism is used to boot the Seed7 language:

At the beginning of the seed7_05.s7i file nothing is declared. This means that no statements, no functions, no operators, no types and no variables are predefined. To boot the Seed7 language the file syntax.s7i is included. The file syntax.s7i contains only $ commands. First the type `type` is defined. Declarations of other types, system variables and syntax descriptions of operators and statements follow. After finishing the inclusion of syntax.s7i the file seed7_05.s7i contains some $ declarations until the `'const'` declaration statement is established. From that point onward almost no $ statements are needed.

### Why does "seed7_05.s7i" contain a version number?

The number 05 is actually a 'branch info'. As if C had headers like

      <stdlib_c78.h> /* For K&R C programs */
      <stdlib_c89.h> /* For ANSI C */
      <stdlib_c99.h> /* For C99 */

and your program must include one of these three headers as first include file (Other include files have no version/branch info in the name). That way nobody is forced to upgrade an old program (to get no warnings or to make it compile). You can leave your old K&R program from 1980 as is. When you decide to rewrite your K&R program to use prototypes, you change the <stdlib...> include file as well.

Programming languages change over long time periods. This results in different language standards. Seed7 tries to address this problem from the beginning. Since most of the Seed7's constructs (statements, operators, types, ... ) are defined in seed7_05.s7i this is the right place to do it.

### Can I use an "abc.s7i" include file to boot to the abc language?

Theoretically yes. In practice there would be several problems. For example:

- All primitive actions are defined such that they fit to Seed7.
- Some concepts like goto's, return's and break's are not supported.
- Some things like comments and $ pragmas are hard coded.

But basically booting various languages was one of the goals of the extensible programming language Seed7 and the s7 interpreter.

In practice it turned out to be a better approach to steal concepts from other programming languages and to integrate them in Seed7 than to split the development in different branches.

The capability to boot a language can be used to allow slightly different future versions of Seed7 to coexist with the current version. This is also the reason why the file seed7_05.s7i contains a version number (05).