( Starting and controlling the interpreter

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 19.4 Starting and controlling the interpreter
 In almost every case, your first `gh_' call will be:
  -- Function: void gh_enter (int ARGC, char *ARGV[], void
      Starts up a Scheme interpreter with all the builtin Scheme
      primitives.  `gh_enter()' never exits, and the user's code should
      all be in the `MAIN_PROG()' function.  `argc' and `argv' will be
      passed to MAIN_PROG.
       -- Function: void main_prog (int ARGC, char *ARGV[])
           This is the user's main program.  It will be invoked by
           `gh_enter()' after Guile has been started up.
      Note that you can use `gh_repl' inside `gh_enter' (in other words,
      inside the code for `main-prog') if you want the program to be
      controlled by a Scheme read-eval-print loop.
    A convenience routine which enters the Guile interpreter with the
 standard Guile read-eval-print loop ("REPL") is:
  -- Function: void gh_repl (int ARGC, char *ARGV[])
      Enters the Scheme interpreter giving control to the Scheme REPL.
      Arguments are processed as if the Guile program `guile' were being
      Note that `gh_repl' should be used _inside_ `gh_enter', since any
      Guile interpreter calls are meaningless unless they happen in the
      context of the interpreter.
      Also note that when you use `gh_repl', your program will be
      controlled by Guile's REPL (which is written in Scheme and has many
      useful features).  Use straight C code inside `gh_enter' if you
      want to maintain execution control in your C program.
    You will typically use `gh_enter' and `gh_repl' when you want a
 Guile interpreter enhanced by your own libraries, but otherwise quite
 normal.  For example, to build a Guile-derived program that includes
 some random number routines "GSL" (GNU Scientific Library), you would
 write a C program that looks like this:
      #include <guile/gh.h>
      #include <gsl_ran.h>
      /* random number suite */
      SCM gw_ran_seed(SCM s)
        return SCM_UNSPECIFIED;
      SCM gw_ran_random()
        SCM x;
        x = gh_ulong2scm(gsl_ran_random());
        return x;
      SCM gw_ran_uniform()
        SCM x;
        x = gh_double2scm(gsl_ran_uniform());
        return x;
      SCM gw_ran_max()
        return gh_double2scm(gsl_ran_max());
        /* random number suite */
        gh_new_procedure("gsl-ran-seed", gw_ran_seed, 1, 0, 0);
        gh_new_procedure("gsl-ran-random", gw_ran_random, 0, 0, 0);
        gh_new_procedure("gsl-ran-uniform", gw_ran_uniform, 0, 0, 0);
        gh_new_procedure("gsl-ran-max", gw_ran_max, 0, 0, 0);
      main_prog (int argc, char *argv[])
        gh_repl(argc, argv);
      main (int argc, char *argv[])
        gh_enter (argc, argv, main_prog);
    Then, supposing the C program is in `guile-gsl.c', you could compile
 it with `gcc -o guile-gsl guile-gsl.c -lguile -lgsl'.
    The resulting program `guile-gsl' would have new primitive
 procedures `gsl-ran-random', `gsl-ran-gaussian' and so forth.
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