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Date: Fri, 16 Nov 2012 11:12:49 +0100
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: Ada202X : Adding functors
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Date: 2012-11-16T11:12:47+01:00
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On 16.11.12 01:15, Peter C. Chapin wrote:
> val resultList = filter(myList, _ % 2 == 0)

Incidentally, I have just again had to turn a short expression
like this into a regular function, since keeping it as an
expression would have made the program unreadable.

I think that "growing up" to become a function is the fate of many
expressions playing the role of a function.

In the language I need to use, filtering looks like

   my @resultList = grep { _ % 2 == 0 } @$myList;

Actually, I also need to sort records in $myList in reverse order by
a field named "id". Language support is structurally similar to
filtering. Special sort variables $a and $b provided by the language:

   my @resultList = sort { $b->{"id"} <=> $a->{"id"} } @$myList;

Again, an unnamed expression, $b->{"id"} <=> $a->{"id"}.

It turned out at some point that the components of $myList needed a
more complex comparison, since "id" would not establish the desired
order. Therefore, the statement now is,

   my @resultList = sort $by_latest @$myList;

where $by_latest is a code reference, a "function pointer".  But! As
almost always, the function would not need to carry its environment
around.

So the situation made me end up with the Perl equivalent of a plain
old Ada generic or function pointer, roughly,

   generic
      type T(<>) is private;
      with Comp (a, b: T) return int;
      -- ...
   function sort (items : in out List) return List;

This is not the first time that an expression had to become a function.
To me, this development seems to be the more frequent case.

Maybe unnamed expressions work much better in the role of a gimmick,
for example, when introducing "generic functions" by demonstration.