21. Bigloo A ``practical Scheme compiler'' User manual for version 3.2a January 2009 -- Eval and code interpretation

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Eval and code interpretation

Chapters

This chapter describes the Bigloo evaluator.

### 21.1 Eval compliance

Bigloo includes an interpreter. Unfortunately, the language accepted by the interpreter is a proper subset of that accepted by the compiler. The main differences are:

- No foreign objects can be handled by interpreter.
- Classes of the object system cannot be declared within interpreted code.
- The interpreter ignores modules, and has a unique global environment.

Compiled code and interpreted code can be mixed together. That is, interpreted code is allowed to call compiled code and vice versa. This connection can be use to circumvent the missing features of the interpreter (see Section see Module Declaration, for a description of how to connect compiled and interpreted code).
By default the evaluator assumes that operators from the standard library (e.g., `+`, `car`) are immutable. Hence, it optimizes these operators calls. This optimization can be disables using the `bigloo-eval-strict-module` parameter described in the chapter describing the parameters (see see Parameters).

### 21.2 Eval standard functions

eval exp [env] | procedure

This form evaluates `exp`. The second argument is optional. It can be the evaluation of one of these three function forms:

    (scheme-report-environment 5)
    (null-environment 5)
    (interaction-environment)

scheme-report-environment version | procedure

null-environment `version` | procedure

interaction-environment `version` | procedure

These three procedures have the definitions given in the R5RS so see info-file `r5rs.info', 6.5 Eval, for more details.

byte-code-compile exp [env (default-environment)] | bigloo procedure

byte-code-run byte-code | bigloo procedure

The function `byte-code-compile` compiles a Scheme expression into a sequence of byte codes that is implemented as a string. The function `byte-code-run` execute such a sequence.

repl | bigloo procedure

This invokes the `read-eval-print` loop. Several `repl` can be embedded.
The `repl` function can be used to implement custom Bigloo interpreters. For instance, one may write:

    (module repl)
    (repl)

When compiled, this will deliver an executable containingthe sole Bigloo interpreter.

quit | bigloo procedure

This exits from the currently running `repl`. If the current `repl` is the first one then this function ends the interpreter.

set-prompter! proc | bigloo procedure

The argument `proc` has to be a procedure of one argument and invoking this function sets the `repl` prompter. That is, to display its prompt, `repl` invokes `proc` giving it the nesting level of the current loop as its argument.

get-prompter | bigloo procedure

Returns the current `repl` prompter.

set-repl-printer! proc | bigloo procedure

The argument `proc` has to be a procedure accepting one or two arguments. This function sets the `repl` display function. That is, to display the result of its evaluations, `repl` invokes `proc` giving it the evaluated expression as first argument and the current output port (or a file in case of transcript) as second argument. `Set-repl-printer!` returns the former `repl` display function.
For instance, one may write:

    1:=> (define x (cons 1 2))         -| X
    1:=> (define y (cons x x))         -| Y
    1:=> y                             -| (#0=(1 . 2) . #0#)
    1:=> (set-repl-printer! display)   -| #<procedure:83b8c70.-2>
    1:=> y                             -| ((1 . 2) 1 . 2)

native-repl-printer | bigloo procedure

Returns the native (default) `repl` display function.

expand exp | bigloo procedure

Returns the value of `exp` after all macro expansions have been performed.

expand-once exp | bigloo procedure

Returns the value of `exp` after one macro expansion has been performed.

It is possible to specify files which have to be loaded when the interpreter is invoked. For this, see section see Compiler Description.
If a Bigloo file starts with the line:

     #! bigloo-command-name

and if this file is executable (in the meaning of the system) and if the user tries to execute it, Bigloo will evaluate it. Note also that SRFI-22 support enables to run any Unix interpreter (see SRFIs).

load filename | bigloo procedure

loadq filename | bigloo procedure

`Filename` should be a string naming an existing file which contains Bigloo source code. This file is searched in the current directory and in all the directories mentioned in the variable `*load-path*`. The `load` procedure reads expressions and definitions from the file, evaluating them sequentially. If the file loaded is a module (i.e. if it begins with a regular module clause), load behaves as module initialization. Otherwise, this function returns the result of the last evaluation. The function `oadq` differs from the function `load` in the sense that `loadq` does not print any intermediate evaluations.
Both functions return the full path of the loaded file.

loada filename | bigloo procedure

Loads an ``access file'', which allows the interpreter to find the modules imported by a loaded module. It returns the full path of the loaded file.

*load-path* | bigloo variable

A list of search paths for the `load` functions.

dynamic-load filename #!optional (init init-point) | bigloo procedure

Loads a shared library named `filename`.
Important note: The function `dynamic-load` can only be used from interpreters linked against dynamic libraries. In particular, the `dynamic-load` function can be issued from the `bigloo` command if and only if the option `--sharedcompiler=yes` has been used when configuring Bigloo. If the `bigloo` command is not linked against dynamic libraries and if `dynamic-load` is required inside a read-eval-print loop (REPL) it exists a simple workaround. It consists in implementing a new REPL and linking it against dynamic libraries. This can be done as:

    $ cat > new-repl.scm <<EOF
    (module new-repl)
    (repl)
    EOF
    $ bigloo new-repl.scm -o new-repl
    $ new-repl
    1:=> (dynamic-load ...)

The function `dynamic-load` returns the name of the loaded library. If `init-point` is specified and if it is a string and if the library defines a function named `init-point`, this function is called when the library is loaded. `Init-point` is a C identifier, not a Scheme identifier. In order to set the C name a Scheme function, use the extern `export` clause (see Section see C Interface). If the `init-point` is provided and is not a string, no initialization function is called after the library is loaded. If the `init-point` value is not provided, once the library is loaded, `dynamic-load` uses the Bigloo default entry point. Normally you should not provide an `init-point` to `dynamic-load` unless you known what you are doing. When producing C code, to force the Bigloo compiler to emit such a default entry point, use the `-dload-sym` compilation option (see Section see Compiler Description). This option is useless when using the JVM code generator. Let's assume a Linux system and two Bigloo modules. The first:

    (module mod1
       (eval (export foo))
       (export (foo x)))

    (define (foo x)
       (print "foo: " x))

    (foo 4)

The second:

    (module mod2
       (import (mod1 "mod1.scm"))
       (eval (export bar))
       (export (bar x)))

    (define (bar x)
       (print "bar: " x))

    (bar 5)

If these modules are compiled as:

    $ bigloo mod1.scm -c -o mod1.o
    $ bigloo mod2.scm -c -o mod2.o -dload-sym

Then, if a shared library is built using these two modules (note that on non Linux systems, a different command line is required):

    $ ld -G -o lib.so mod1.o mod2.o

Then, `lib.so` cant be dynamically loaded and the variables it defines used such as :

    $ bigloo -i
    (dynamic-load "lib.so")
         -| foo: 4
           bar: 5
    1:=> (foo 6)
         -| foo: 7

As the example illustrates, when Bigloo modules are dynamically loaded, they are initialized. This initialization is ensure only if `dynamic-load` is called with exactly one parameter. If `dynamic-load` is called with two parameters, it is of the responsibility of the program to initialize the dynamically loaded module before using any Scheme reference.
Note: In order to let the loaded module accesses the variables defined by the loader application, special compilation flags must be used (e.g., `-rdynamic` under the Linux operating system). `Dynamic-load` is implemented on the top of the `dlopen` facility. For more information read the `dlopen` and `ld` manuals.

dynamic-unload filename | bigloo procedure

On the operating system that supports this facility, unloads a shared library. Returns `#t` on success. Returns `#f` otherwise.

*dynamic-load-path* | bigloo variable

A list of search paths for the `dynamic-load` functions.

transcript-on filename | procedure

transcript-off | procedure

### 21.3 Eval command line options

This section presents the Bigloo compiler options that impact the interaction between compiled and interpreted code. The whole list of the Bigloo compiler options can be found in The Bigloo command line.

- `-i` Don't compile a module, interpret it!
- `-export-all` Make all the bindings defined by the compiled module available from the interpreter.
- `-export-export` Make all the bindings exported by the compiled module available from the interpreter.
- `-export-mutable` Make all the bindings exported by the compiled module mutable from outside the module. This option is dangerous! Either all the modules composing the application must be compiled with or without `-export-mutable`. It is impossible to mix `-export-mutable` enabled and disabled compilations.

### 21.4 Eval and the foreign interface

To be able to get access to foreign functions within the Bigloo interpreter, some extra measurements have to be taken. The foreign functions have to be present in the interpreter binary, which means you have to compile a custom interpreter. This is described in Section Using C bindings within the interpreter.

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Last update Tue Jan 6 18:43:50 2009.