[metapost] Still experimenting with MPlib

[Nicola]

Hi,
I'm still experimenting with MPlib, and there is something that I do not 
understand. I hope you can help me. I have tried the following program:

void imp_init_mp_instance()
{
   MP_options *opt = mp_options();
   opt->ini_version = 0;
   opt->mem_name = "mpost";
   opt->command_line = NULL;
   opt->noninteractive = 1;
   imp = mp_initialize(opt);
   if (!imp) { exit(EXIT_FAILURE); }
   mp_execute(imp, "\\", 1); // Flush MetaPost introductory text
}


int main(void)
{
   imp_init_mp_instance();

   char *line[] = {
     "if 1=0:",
     "message \"tsk\";",
     "fi;"
   };

   for (int i = 0; i < 3; i++) {
     mp_execute(imp, line[i], strlen(line[i]));
     mp_run_data *cmd_result = mp_rundata(imp);
     printf("%s", cmd_result->term_out.data);
     printf("Status: %d\n", mp_status(imp));
   }
}

This results in the MP instance being in an error state, and the error 
message is the following:

! Missing `:' has been inserted.
<inserted text>
                 :
<to be read again>
                    fi
<*> fi
       ;
! A statement can't begin with `:'.
<to be read again>
                    :
<to be read again>
                    fi
<*> fi
       ;
! Extra tokens will be flushed.
<to be read again>
                    :
<to be read again>
                    fi
<*> fi
       ;

Of course, this doesn't work as in mpost's prompt, as I had naïvely 
expected. So, should mp_execute() be fed with complete statements only?

It is not clear to me what happens internally when mp_execute() is 
passed an incomplete statement. For example, if I call:

mp_execute(imp, "x=1", 3); // Note the missing semi-colon
mp_execute(imp, "show x;", 7);

this acts as if x=1 was ignored (`show x` prints `>> x`). But the 
example above seems to indicate that this is not the case (if incomplete 
statements were ignored, I'd get an `! Extra fi.` error after the third 
invocation).



Nicola