http://qs1969.pair.com?node_id=908767


in reply to Re: (almost) foldl
in thread (almost) foldl

I came up with a similar, and arguably neater implementation:

sub sum{ ( shift()//return 0 ) + &sum } print sum( 1,2,3);; 6 print sum( 1 .. 100 );; Deep recursion on subroutine "main::sum" at 5050

but rejected it because 'sum' is a (glob) variable.

In theory, it is possible to avoid the naming of the sub, thereby achieving "anonymous recursion", by using the Y-combinator. And the Y-combinator has been achieved in Perl by a former regular here.

Putting it together you get:

print Y( sub{my$rec=shift; sub{(shift()//return 0) + &$rec }})->(1 .. +100);; 5050

But whilst that achieves the Y-combinators goal of recursion without adding the sub to the permanent namespace, it still requires the naming of the pesky closure $rec.

And of course, requires you to add the Y-combinator to the permanent namespace first:

sub Y { my ( $curried_rec ) = @_; sub { my ( $f1 ) = @_; $curried_rec->( sub { $f1->( $f1 )->( @_ ) } ) }->( sub { my ( $f2 ) = @_; $curried_rec->( sub { $f2->( $f2 )->( @_ ) } ) } ) }

And that's already more obfuscation as I want to wrap my brain around, even in an obfuscation section post!

My final thought is that the simplest mechanism that fits the OPs breif is just:

C:\test>perl -Mstrict -wE"say eval join'+',()" C:\test>perl -Mstrict -wE"say eval join'+',0" 0 C:\test>perl -Mstrict -wE"say eval join'+',-1" -1 C:\test>perl -Mstrict -wE"say eval join'+',-1..+3" 5

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