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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: calling function but ignoring results
Date: Tue, 29 Jun 2021 22:34:41 +0200
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On 2021-06-29 21:52, Jeffrey R. Carter wrote:
> On 6/29/21 9:25 PM, Matt Borchers wrote:
>> It is not very often that ignoring a function result is okay, but I 
>> have run across many instances of the following block structure in 
>> code over the years:
>>
>> declare
>>      dont_care : BOOLEAN;
>> begin
>>      dont_care := foo( x, y );
>> end;
> 
> This sort of thing usually indicates a design problem.

True. But there are exceptions from the rule:

1. Bindings to other languages. You normally have the result and you 
quite often ignore it because it is useless.

2. Side effects of the call. Normally you would overload a function with 
a procedure, like here:

    function Pop (Stack : not null access Stack_Type) return Element;
    procedure Pop (Stack : in out Stack_Type);

But you will have a problem (which BTW the proposal fail to address) 
when overloading is solely on the result:

    function Get (File : File_Type) return Integer;
    function Get (File : File_Type) return Float;
    function Get (File : File_Type) return String;

Now if you wanted to skip an integer in the file you could not do it in 
the proposed way:

    @ := Get (Standard_Input); -- Ambiguous

    @ := Integer'(Get (Standard_Input)); -- Ugly

And of course, as many times proposed before a better syntax would be 
the null statement:

    null <expression>; -- Does not address the problem either

>> Is there a Ada 202x feature to support calling functions and ignoring 
>> the result?
> If you want to use a language that allows this, then you probably 
> shouldn't be developing S/W.

I would not say that pieces like:

    declare
       Ignore : constant Bar := Foo (Baz);
    begin
       null;
    end;

or

   if Foo (X) then null; end if;

are good engineering. But excessive overloading may lead to unexpected 
collisions if you have many parameters with defaults.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de