Re: [Gm2] Returning a string constant

[Andreas Fischlin]

Dear Scott,

In standard Modula-2 yes, this is illegal. Wirth (1983) writes on p. 52: "Only one value, however, can be returned as the result of a function. This value, moreover, cannot be of a structured type."

Please consider the following three arguments: (i) Functional procedures may be used in expressions, and using a string in an _expression_ doesn't make much sense. (ii) You have plenty of alternatives: Simply use procedure parameters and there is no need to return an object such as a string from a functional procedure (see alternative code provided below). (iii) Code does not look very good and it is a bad habit from C/Unix programmers to always use functional procedures and then to return the actual results as a side effect from the call to the procedure. In a easy to read programing style returning complex data from a procedure is best documented by using a procedure that is not a functional procedure and returns all the data through procedure parameters.

Considering this your example can be rewritten as:

MODULE test1;

FROM InOut IMPORT WriteString, WriteLn;
FROM Strings IMPORT Assign;

TYPE
  testresult = ARRAY [0..9] OF CHAR;

PROCEDURE test(VAR s: ARRAY OF CHAR);
BEGIN
  (* s := "test"; may be accepted by some compilers, but is not
   recommended, since not safe. It is better to use Strings.Assign *)
  Assign("test",s);
  (* there is no need to have an explicit RETURN statement here *)
END test;

BEGIN
  test(testresult);
  WriteString('Test results = "');
  WriteString(testresult);
  WriteString('"');
  WriteLn;
END test1.

Hope this helps and answers your question.

Regards,
Andreas

Cited references:
--------------------
Wirth, N., 1983 (2 (corr.) ed.). Programming in Modula-2. Springer-Verlag: Berlin a.o, 176 pp.


Scott Robinson wrote:

Hello all,

I was wondering if the Modula-2 code below is supposed to be illegal?

MODULE test1;

FROM InOut IMPORT WriteString, WriteLn;

TYPE
  testresult = ARRAY [0..9] OF CHAR;

PROCEDURE test() : testresult;
BEGIN
  (* Is this legal? *)
  RETURN "test";
END test;

BEGIN
  WriteString('Test results = "');
  WriteString(test());
  WriteString('"');
  WriteLn;
END test1.

The Modula-2 compiler is giving me this:

#gm2 -c -g test1.mod
test1.mod: In function ‘test’:
test1.mod:11: error: conversion to non-scalar type requested

Thanks,
Scott



_______________________________________________
gm2 mailing list
address@hidden
http://lists.nongnu.org/mailman/listinfo/gm2

--

________________________________________________________________________
Dr. Andreas Fischlin, Ph.D., Group Director

Terrestrial Systems Ecology
Institute of Integrative Biology: Ecology, Evolution, Infectious Disease
Department of Environmental Sciences, ETH Zurich

Address:
ETH Zurich, CHN E21.1
8092 Zurich, Switzerland

Phone: +41 44 633-6090 / Fax: +41 44 633-1136
http://www.sysecol.ethz.ch/Staff/af/
http://www.sysecol.ethz.ch/

     _/_/_/ _/_/_/ _/  _/
    _/       _/   _/  _/   Eidgenoessische Technische Hochschule Zuerich
   _/_/_/   _/   _/_/_/   Swiss Federal Institute of Technology Zurich
  _/       _/   _/  _/   Ecole polytechnique federale de Zurich
 _/_/_/   _/   _/  _/   Politecnico federale de Zurigo

             Make it as simple as possible, but distrust it!
________________________________________________________________________