PWC171 - First-class Function


On with TASK #2 from The Weekly Challenge #171. Enjoy!

The challenge

Create sub compose($f, $g) which takes in two parameters $f and $g as subroutine refs and returns subroutine ref i.e. compose($f, $g)->($x) = $f->($g->($x))


$f = (one or more parameters function)
$g = (one or more parameters function)

$h = compose($f, $g)
$f->($g->($x,$y, ..)) == $h->($x, $y, ..) for any $x, $y, ...

The questions

I know I should pepper this specific task with tests… but can we please pretend I did just for asking here?

Also, this is somehow weird in the light of how broad the PWC audience is… it seems very skewed towards Perl. It this by intention?

More interestingly, can we assume that the $g function should be called in list context, and that the user knows that compose should be called in the same context as $f?

The solution

In an effort at being minimalistic, this is my solution in Perl:

sub compose ($f, $g) { sub { $f->($g->(@_)) } }

I assume this is right, until proven wrong. It’s so short that any Perl master will probably debunk it with just a glance.

I’m not so sure about the Raku alternative though, I feel like there’s so much more to it and so many corner cases. Which, I have to admit, do not really add to the whipuptitude.

sub compose (&f, &g) { sub {f(g(@_))} }

Again, I hope some Raku master will be so benevolent as to take a look and point out at least the most grandiose errors with this.

All in all, I guess manwar tried to start something with this challenge.

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