NAME

FP::TransparentLazy - lazy evaluation with transparent evaluation

SYNOPSIS

    use FP::TransparentLazy;

    # This is the same SYNOPSIS as in FP::Lazy but with most `force`
    # calls removed, and slightly differing behaviour in places
    # (e.g. `$a + 2` will evaluate the thunk here and thus give
    # division by zero):

    my $a = lazy { 1 / 0 };
    eval {
        # $a's evaluation is forced here
        print $a
    };
    like $@, qr/^Illegal division by zero/;

    eval {
        $a + 2
    };
    like $@, qr/^Illegal division by zero/;

    my $count = 0;
    my $b = lazy { $count++; 1 / 2 };
    is is_promise($b), 1;
    is $count, 0;
    is $b, 1/2; # increments $count
    is $count, 1;
    # $b is still a promise at this point (although an evaluated one):
    is is_promise($b), 1;
    is $b, 1/2; # does not increment $count anymore
    is $count, 1;

    # The following stores result of `force $b` back into $b
    FORCE $b;
    is is_promise($b), undef;
    is $b, 1/2;
    is $count, 1;

    # Note that lazy evaluation and mutation usually doesn't mix well -
    # lazy programs better be purely functional. Here $tot depends not
    # just on the inputs, but also on how many elements were evaluated:
    use FP::Stream qw(stream_map); # uses `lazy` internally
    use FP::List;
    my $tot = 0;
    my $l = stream_map sub {
        my ($x) = @_;
        $tot += $x;
        $x*$x
    }, list (5,7,8);
    is $tot, 0;
    is $l->first, 25;
    is $tot, 5;
    is $l->length, 3;
    is $tot, 20;

    # Also note that `local` does mutation (even if in a somewhat
    # controlled way):
    our $foo = "";
    sub moo {
        my ($bar) = @_;
        local $foo = "Hello";
        lazy { "$foo $bar" }
    }
    is moo("you")->force, " you";
    is moo("you"), " you";

    # runtime conditional lazyness:

    sub condprom {
        my ($cond) = @_;
        lazy_if { 1 / 0 } $cond
    }

    ok is_promise(condprom 1);

    eval {
        # immediate division by zero exception (still pays
        # the overhead of two subroutine calls, though)
        condprom 0
    };
    like $@, qr/^Illegal division by zero/;

DESCRIPTION

This implements a variant of FP::Lazy that forces promises automatically upon access (and writes their result back to the place they are forced from, like FP::Lazy's `FORCE` does). Otherwise the two are fully interchangeable.

NOTE: this is EXPERIMENTAL. Also, should this be merged with Data::Thunk ?

The drawback of transparency might be more confusion, as it's not directly visible anymore (neither in the debugger nor the source code) what's lazy. Also, transparent forcing will be a bit more expensive CPU wise. Please give feedback about your experiences!

SEE ALSO

FP::Lazy

NOTE

This is alpha software! Read the status section in the package README or on the website.