Re: Problems Interfacing Modula-2 with GNU Readline library

[Alice Osako]

    One minor error in the test
      code, in that I had somehow copied an older version of the code.
      The line 'Fin(input)' should be at the start of the loop, not the
      end.

Alice Osako:

>       
>       I am working on a
>         medium-sized project in Modula-2 (specifically, an
>         implementation of the MAL interpreter - see https://github.com/kanaka/mal
>         for details), and one of the soft requirements/recommended
>         implementation details is editable input and input history. For
>         this purpose, I intend to use the existing Readline library, and
>         to this end I created a simple definition module as a binding. 
>         
>         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>         DEFINITION MODULE FOR "C" readline;
>         
>         FROM SYSTEM IMPORT ADDRESS;
>         
>         EXPORT UNQUALIFIED readline, add_history;
>         
>         PROCEDURE readline (prompt: ARRAY OF CHAR) : ADDRESS;
>         
>         PROCEDURE add_history (line: ARRAY OF CHAR);
>         
>         END readline.
>         ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       
>         However, I am running into a problem when testing these
>         bindings. When I run the following test program:
>         
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>         MODULE rl_test;
>         
>         FROM SYSTEM IMPORT ADDRESS;
>         
>         FROM DynamicStrings IMPORT String, Fin, Length,
>                                                               
>         InitStringCharStar,
>                                                               CopyOut,
>         Equal;
>         
>         FROM StrIO IMPORT WriteString, WriteLn;
>         
>         FROM readline IMPORT readline, add_history;
>         
>         VAR
>            input, exitCondition: String;
>            temp: ARRAY [0..1024] OF CHAR;
>         
>         BEGIN
>            exitCondition := InitString("exit");
>            WHILE NOT Equal(input, exitCondition) DO
>               input := InitStringCharStar(readline("Write something:
>         "));
>         
>               IF Length(input) > 0 THEN
>                  WriteString("You Wrote: ");
>                  CopyOut(temp, input);
>         
>                  WriteString(temp);
>                  WriteLn;
>                  add_history(temp);
>               ELSE
>                  WriteString("No input");
>                  WriteLn;
>               END;
>               Fin(input);
>            END;
>         
>         END rl_test.
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       
>         I expected that this would result in something like the
>         following:
>         
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       Write something: foo
>       You Wrote: foo
>       Write something: bar
>       You Wrote: bar
>       Write something:
>         No input
>       Write something: bar
>       You Wrote: bar
>       Write something: quux
>       You Wrote: quux
>       Write something: exit
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       
>         However, the output I actually got was:
>         
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       Write something: foo
>         Write something: bar
>         Write something: 
>         Write something: bar
>         Write something: quux
>         Write something: exit
>         You Wrote: foo
>         You Wrote: bar
>         No input
>         You Wrote: bar
>         You Wrote: quux
>         You Wrote: exit
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>         
>         So my issue is, why is the response not getting flushed until
>         after the input is completed?
>         
>         Oddly enough, when I add 
>         
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>         FROM InOut IMPORT WriteCard;
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       
>         and insert this following the input but before the IF statement:
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>            
>         WriteCard(Length(input), 3);
>               WriteLn;
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>         
>         The result is different:
>         
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>       Write something: foo
>           3Write something: bar
>         
>         You Wrote: foo
>           3Write something: 
>         
>         You Wrote: bar
>           0Write something: bar
>         
>         No input
>           3Write something: quux
>         
>         You Wrote: bar
>           4Write something: exit
>         
>         You Wrote: quux
>           4
>         You Wrote: exit
>       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
>         
>         Why this particular delayed interleaving is happening, and why
>         the WriteLn is getting ignored,  is puzzling to me.
>         
>         Is there some known issue with interfacing the C libraries, or
>         the Readline library specifically, that I need to be aware of?
>         
>