Re: [Gm2] Returning a string constant

[Iztok Kobal]

MODULE test1;

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

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


(* physical allocation of return value container is needed !!! *)

VAR
   s : testresult;

(* Now it is correct ;-) *)

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

Regards, Iztok


Andreas Fischlin wrote:
> 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
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> http://www.sysecol.ethz.ch/
>
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