Re: [Haskell-beginners] Monads in javascript

16 Jul 2011

      Martin,
...
z' = [1,2,3] >>= (\y1->[y1+1] >>= (\y2->return (y1*y2)))
I'm not a Haskell expert, but I think the idea here is that the second
lambda is defined *inside* the first one. I think that's the only way
that it can access the first lambda's argument.

I don't know much Javascript, but here is a small Perl snippet that
(kinda) implements the list monad and your example:

use strict ;
use Data::Dumper ;

sub bindM {
        my $xs = shift ;
        my $fx2ys = shift ;

        # maps and concats in one shot
        my @ys = map {@{$fx2ys->($_)}} @{$xs} ;
        return \@ys ;
}

sub returnM {
        my $x = shift ;

        return [$x] ;
}

sub z {
        my $arg = [1,2,3] ;

        bindM($arg, sub {
                my $y1 = shift ;
                bindM([$y1 + 1], sub {
                        my $y2 = shift ;
                        returnM($y1 * $y2) ;
                }) ;
        }) ;
}

print Dumper(z()), "\n" ;

Since $y1 is local to te first lambda, you have to define the second one inside.

Patrick
...
works again, which misled me to believe that (>>=) associates to the right.
In javascript a lambda would look like this
f = function(y1) {return [y1+1]}
I cannot see how I could possible write a function (>>=) which chains such
lambdas such that I can still access the arguments outside the function
bodies. It is like there are always parentheses around the lambdas. Well there
actually are braces in javascript, but it can't be just the syntax.
So what is javascript missing? Is it because in haskell a lambda is just an
expression wheras in javascript it is something special?
--
Martin
_______________________________________________
Beginners mailing list
Beginners@haskell.org
http://www.haskell.org/mailman/listinfo/beginners
-- 
=====================
Patrick LeBoutillier
Rosemère, Québec, Canada