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Author image Christian Jaeger
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FP::Lazy - lazy evaluation (delayed evaluation, promises)


    use FP::Lazy;

    my $a = lazy { 1 / 0 };
    eval {
        print force $a
    like $@, qr/^Illegal division by zero/;

    eval {
        $a + 2
    like $@, qr/^non-auto-forcing promise accessed via 0\+ operation/;

    my $count = 0;
    my $b = lazy { $count++; 1 / 2 };
    is is_promise($b), 1;
    is $count, 0;
    is force($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 force($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;
    }, 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";

    # runtime conditional lazyness:

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

    is is_promise(condprom 1), 1;

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


This implements promises, a data type that represents an unevaluated or evaluated computation. The computation represented by a promise is only ever evaluated once, after which point its result is saved in the promise, and subsequent requests for evaluation are simply returning the saved value.

    my $p = lazy { "......" }; # returns a promise that represents the computation
                               # given in the block of code

    force $p;  # runs the block of code and stores the result within the
               # promise and also returns it

    FORCE $p; # or FORCE $p,$q,$r;
              # in addition to running force, stores back the resulting
              # value into the variable given as argument ($p, $q, and $r
              # respectively (the commented example forces 3 (possibly)
              # separate values))

    is is_promise($p), undef; # returns true iff $x holds a promise


The thunk (code body) of a promise is always evaluated in scalar context, even if it is being forced in list or void context.


The name `lazy` for the delaying form was chosen because it seems what most frameworks for functional programming on non-functional programming languages are using, as well as Ocaml. We don't want to stand in the way of what people expect, after all.

Scheme calls the lazy evaluation form `delay`. This seems to make sense, as that's a verb, unlike `lazy`. There's a conceptually different way to introduce lazyness, which is to change the language to be lazy by default, and `lazy` could be misunderstood to be a form that changes the language in its scope to be that. Both for this current (slight?) risk for misinterpretation, and to reserve it for possible future implementation of this latter feature, it seems to be wise to use `delay` and not `lazy` for what this module offers.

What should we do?

(To experiment with the style, or in case you're stubborn, you can explicitely import `delay` or import the `:all` export tag to get it.)


If the thunk of a promise throws an exception, the promise will remain unevaluated. This is the easiest (and most efficient) thing to do, but there remains a question about the safety: if the data source is read-once (like reading lines from files), and the exception happens after the read, then forcing the promise again will fetch and store the next line, hence a line will be lost. Since exceptions can happen because of out of memory conditions or from signal handlers, this will be of real concern in some situations.

Provide safe promises for these situations? (But that would mean that they need to be implemented in C as Perl does not offer the features to implement them safely, correct?)

FP_Show_show: instead of "DUMMY", show file/line of the thunk's definition?


Lazy code can be difficult to debug because the context in which the code that evaluates a promise runs is not the same context in which the promise was captured. There are two approaches to make this easier:

$ENV{DEBUG_FP_LAZY} = "1" or local $FP::Lazy::debug=1 -- captures a backtrace in every promise (slow, of course!). Use the optionally exported lazy_backtrace function to get the backtrace (or look at it via the repl's :d (Data::Dumper) mode).

$ENV{DEBUG_FP_LAZY} = "eager" or local $FP::Lazy::eager=1 -- completely turns of any lazyness (except for lazyLight, currently); easy stack traces and flow logic but of course the program behaves differently; beware of infinite lists!



Alternative Data::Thunk, but see note in TODO file about problems.

Alternative Scalar::Defer?


FP::Mixin::Utils -- Lazy implements this as a fallback (lower priority than forcing the promise and finding the method on the result)


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