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From: Paul Rubin <no.email@nospam.invalid>
Newsgroups: comp.lang.ada
Subject: Re: How to get Ada to ?cross the chasm??
Date: Wed, 02 May 2018 07:23:51 -0700
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Xref: reader02.eternal-september.org comp.lang.ada:51915
Date: 2018-05-02T07:23:51-07:00
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"Randy Brukardt" <randy@rrsoftware.com> writes:
> For instance:
>    Num_Outputs : constant Natural :=
>           (for P of Formal_Params => 
> Boolean'Pos(P.Is_Operation_Output))'Reduce("+",0);
>    -- Count "in out" and "out" parameters in a parameter list.
>
> Cool, but is this really making the code more readable?

Is that some kind of syntactic extension in the compiler?  I'm used to
reduce being an ordinary function.  But "+" in double quotes looks like
a character string, so the compiler must be figuring out what it does.

It also looks like you can add up booleans, where true=1 and false=0?
That is also surprising, if booleans are a separate type from integers.

> Put_Line("[" & (for I in Inputs'Range => (if I = Inputs'First then "" else 
> ", ") & "VN" & Inputs(I)) & "]");
>    -- Display a properly formatted list of inputs.
>
> Again, wouldn't this have been better written as a function rather than a 
> one-liner? And using "reduce" for string *construction* seems confusing at 
> best (nothing is being reduced here).

On the surface that looks like what other languages call a list
comprehension.  But then it seems to be concatenating all those strings?
So it's reducing a concatenation operation over the list of strings.

I agree that it looks confusing in Ada.  If the goal was only to support
parallel evaluation, there were probably easier ways.

> This should let Paul write the kind of code that he wants to write;

It's too syntactically clumsy to be convenient, but besides the
parallelism benefit, it might also be easier to formally verify than an
explicit loop would be.