[plt-scheme] If expressions in Embedded PLT Scheme

From: Samuel Rebelsky (REBELSKY at Grinnell.EDU)
Date: Thu Oct 9 11:12:46 EDT 2008

Clearly, I wasn't clear.  So, let's try again:

(1) I've embedded PLT Scheme (previously MzScheme) in an application.

(2) The application is being used with a number of "legacy" Scheme  
files, some of which I don't have permission to modify.  (Others also  
use the application, so I don't even know about all of the files.)

(3) Those "legacy" Scheme files often use the two-parameter (no- 
alternative) if.

I'm trying to deal with #3 in the context of #2 and #1.

* Because of #2, I can't replace "if" by "when".

* Because of #2, I can't just put "#lang mzscheme" at the top of the  
Scheme programs.

* In the embedded PLT Scheme application, there are also calls to  
"inside PLT" C functions like scheme_apply and scheme_eval_string; I'm  
not sure how to put "#lang mzscheme" in those.

I'm attaching a sample application (which is much simpler than the  
real application; I can't just rewrite everything in Scheme).  Are  
there suggestions of what I could add/change to make it happily load  
the "legacy" Scheme files, many of which use the "no-alternative" form  
of "if"?

Note that I've tried making copies of the Scheme files and adding  
"#lang mzscheme" to the top.  However, then the embedded intepreter  
does't seem to have access to the procedures defined in the file.


-- Sam Rebelsky

      Given a Scheme "program" with a main procedure, loads that program
      and applies the procedure named 'main' to the values given on the
      command line (including the name of the scheme program).

      Requires -lmzscheme -lmzgc -ldl -lm

#include <scheme.h>
#include <stdio.h>
#include <stdlib.h>

main (int argc, char **argv)
   Scheme_Env *e;
   mz_jmp_buf * volatile save, fresh;

   scheme_set_stack_base (NULL, 1);

   /* Set up the Scheme environment. */
   e = scheme_basic_env ();
   scheme_set_collects_path (scheme_make_path ("/usr/local/plt/lib/plt/ 
   scheme_init_collection_paths (e, scheme_null);
   scheme_namespace_require (scheme_intern_symbol ("scheme/base"));

   /* Set up application-specific stuff. */
   /* Elided. */

   /* Prepare to execute code. */
   save = scheme_current_thread->error_buf;
   scheme_current_thread->error_buf = &fresh;
   if (scheme_setjmp (scheme_error_buf))
       /* It failed.  Give up. */
       exit (1);
       int i;
       Scheme_Object **args;
       Scheme_Object *result;

       /* Load the Scheme file. */
       scheme_load (argv[1]);

       /* Build the list of actual parameters. */
       args = malloc ((argc-1) * sizeof (Scheme_Object *));
       for (i = 1; i < argc; i++)
           args[i-1] = scheme_make_locale_string  (argv[i]);
         } /* for */

       /* Apply the main procedure, and print the result. */
       result = scheme_apply (scheme_eval_string ("main", e),  
(argc-1), args);
       printf ("%s\n", scheme_display_to_string (result, NULL));

   exit (0);
} /* main */

On Oct 8, 2008, at 12:14 PM, Carl Eastlund wrote:

> If you use "when" in place of "if", you'll get the one-clause if
> behavior, or you can use the legacy mzscheme behavior by putting
> "#lang mzscheme" at the top of your program (instead of "#lang
> scheme").
> --Carl

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