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Re: Re: Perl6 and Extreme Programming

by Jenda (Abbot)
on May 26, 2002 at 22:42 UTC ( #169440=note: print w/ replies, xml ) Need Help??


in reply to Re: Perl6 and Extreme Programming
in thread Perl6 and Extreme Programming

I am afraid you have it wrong. "Currying" and "higher-order functions" are two unrelated features that only have one thing in common: they often appear in functional languages. A function is "higher-order" if it accepts a function as it's parameter or if it returns a function (it "created").

Higher order functions are long supported by Perl 5, even though we usualy do not use that name. (I guess so that we do not scare people away. Higher-order functions sounds too "functional".)

sub Map { my $sub = shift(); my @new_arr; # want foreach my $item (@_) { push @new_arr, $sub->($item); } @new_arr; } sub foo {$_[0] + 1} @increased = Map \&foo, (1,2,3); print "@increased\n";
On the other hand a function is curried if it returns a "partialy applied" function if called with only the first few parameters. Of course then it is also higer-order actualy, but that's not the point.
sub Map { my $sub = shift(); if (@_) { # we are calling it normaly my @new_arr; # want foreach my $item (@_) { push @new_arr, $sub->($item); } return @new_arr; } else { # only the function is passed return sub { my @new_arr; # want foreach my $item (@_) { push @new_arr, $sub->($item); } return @new_arr; } } } sub foo {$_[0] + 1}; $inc = Map \&foo; @increased = $inc->(1,2,3); print "@increased\n";

(Next day) Actually ... the second function is not a good example. It's both curried and uncurried. If I wanted a plain curried version I'd define it as

sub Map { my $sub = shift(); return sub { my @new_arr; # want foreach my $item (@_) { push @new_arr, $sub->($item); } return @new_arr; } }
and then would call it either as
@incremented = Map(\&foo)->(1,2,3)
or
$Inc = Map(\&foo); @incremented = $Inc->(1,2,3)
As you can see the syntax is not too nice in Perl. Currying is a bit alien to Perl, it looks much more natural in modern functional languages.

  Jenda


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