Johan Lodin


Sub::Curry - Create curried subroutines


    use Sub::Curry;
    use Sub::Curry qw/ :CONST curry /; # Import spice constants
                                       # and the &curry function.

    #my $f1 = Sub::Curry::->new(\&foo, 1, 2); # Same as below.
    my $f1 = curry(\&foo, 1, 2);
    my $f2 = $cb1->new(3, 4);

    my $f3 = curry(\&foo, 1, HOLE, 3);
    my $f4 = $f3->new(2, 4);

    $f1->('a'); # foo(1, 2, 'a');
    $f2->('a'); # foo(1, 2, 3, 4, 'a');

    $f3->('a'); # foo(1, 'a', 3);
    $f4->('a'); # foo(1, 2, 3, 4, 'a');

    $f4->call('a'); # Same as $cb4->('a');


Sub::Curry is a module that provides the currying technique known from functional languages. This module, unlike many other modules that borrow techniques from functional languages, doesn't try to make Perl functional. Instead it tries to make currying Perlish.

This module aims to be a base for other modules that use/provide currying techniques.

This module supports a unique set of special spices (argument features). It doesn't just support holes, but also introduces antiholes, blackholes, whiteholes, and antispices. All these extra special spices effect how the spice is applied to the subroutine. They make functions such as &rcurry superfluous. See "Currying" and Sub::Curry::Cookbook.

An oft-missed feature is argument aliasing. This module preserves the aliasing.

Sub::Curry does explicit currying. For more automatic ways to use currying, look in the Sub::Curry::* namespace.

When version hits 1.00 the interface will be stable.

As of now, this is a beta release. It is and will continue to be compatible with Sub::Curry version 0.08.


Currying is when you attach arguments to subroutines. This is sometimes called "partial application". Currying is already done manually every here and there in existing Perl code. It typically looks like

    my $curried_foo = sub { foo($arg, @_) };
    # foo($arg, @more_args);

That's all there is to primitive currying: you store arguments. The stored arguments are called the spice. This module however, extends the concept further by introducing several special spices.

See the &call method for how special spices are treated if left when the original function will be called.

There's no need for a &rcurry subroutine--that's done with a blackhole, see "Right-currying" in Sub::Curry::Cookbook.

Holes - Sub::Curry::HOLE

A hole is what it sounds like: a gap in the argument list. Later, when the subroutine is called the holes are filled in. So if the spice is 1, <HOLE>, 3 and then 2, 4 is applied to the curried subroutine, the resulting argument list is 1, 2, 3, 4.

This can be handy if you want to curry a method. Just leave a hole as the first spice for the object.

Holes can be called "scalar inserters" that defaults to undef.

Antiholes - Sub::Curry::ANTIHOLE

An antihole put in a hole makes the hole disappear. If the spice is 1, <HOLE>, 3, <HOLE>, 4 and 2, <ANTIHOLE>, 5 is applied then the result will become 1, 2, 3, 4, 5.

Blackholes - Sub::Curry::BLACKHOLE

A blackhole is like a hole for lists that never gets full. There's an imaginary untouchable blackhole at the end of the spice. The blackhole thusly inserts the new spice before itself. The blackhole never gets full because nothing is ever stored in a blackhole as it isn't a hole really...

Blackholes are used to move the point of insertion from the end to somewhere else, so you can curry the end of the argument list.

Blackholes can be called "list inserters" that defaults to the empty list.

Whiteholes - Sub::Curry::WHITEHOLE

A whitehole removes the blackhole, but the spice that has been put into the blackhole remains since blackholes themselves don't store anything.

Antispices - Sub::Curry::ANTISPICE

An antispice is like a hole except that when it's filled it disappears. It's like a combination of a hole and an antihole. If the spice is 1, <ANTISPICE>, 3 and 2, 4 is applied, then the result will become 1, 3, 4.

This can be handy if you want to provide a function as a method. Just put an antispice to remove the object when called.


my $curried = Sub::Curry::->new($subref, @spice)

new is different depending on the invocant.

If the invocant is a class name then the first argument is the subroutine reference that should be curried, and following arguments are the spice. The special spices that can be used here are holes, blackholes, and antispices.

The returned value is a spiced up closure that also is a Sub::Curry object.

No special treatment is given if the subroutine reference is a Sub::Curry object, see the other form of new instead.

my $other = $curried->new(@spice)

If the invocant is an object then all arguments are the spice, and a new object will be returned.

This spice will not just be added to the previous spice. The arguments will be interpreted as arguments to the already curried subroutine and processed accordingly. This means that holes will be filled in, but unfilled holes remain holes. The same applies to the other special spices. If the spice doesn't hold spices that act on spices, i.e. antiholes and whiteholes, then the call $curried->new(@spice)->() is equivalent to curry($curried, @spice)->().

The new object won't be wrapped around the old -- that would be a performance hit. Instead the processed spice is put on the same subroutine that the old object spiced up. This is important to realize as the uncurried method will return the same subroutine reference for $curried and $other.

Here all special spices can be used. This is the only place where antiholes and whiteholes can be used.


Just an OO alias for dereferencing. I.e. $curried->call(...) is the same as $curried->(...).

Holes that are not filled in will become undef. Antispices and blackholes will be removed. Antiholes and whiteholes cannot be used here, due to optimization and implementation. If this is needed do $curried->new(@spice_with_antiholes_or_whiteholes)->() since the second form of new is the only place that handle antiholes.


Experimental! This may be removed in future versions.

In scalar context spice returns the length of the spice. In list context it returns the spice. This is the unprocessed spice. Special spices will be present.


Experimental! This may be removed in future versions.

Returns the original subroutine reference passed to the first form of new, that is the class invocation Sub::Curry::->new(...).


Returns a copy of the subroutine/object that isn't blessed, i.e. lost all its properties and possibility to invoke method calls. There's no speed gain in using the copy returned by cursed.

    my $f1 = curry(sub { ... }, @spice);
    my $f2 = $f1->cursed;

    $f1->();   # IDENTICAL
    $f2->();   # CALLS


No symbols are exported by default. :ALL exports all functions. :CONST exports all constants.



Perhaps you think it's tiresome to write Sub::Curry::->new and want a &curry function instead. Well, make one yourself!

    *curry = Sub::Curry::->new(Sub::Curry::->can('new'), Sub::Curry::);

OK, you don't have to do it yourself. You can do

    use Sub::Curry 'curry';

instead and let the module do that currying for you. Note that $c1 and $c2 in

    my $curried = curry(\&foo, @foo);

    my $c1      = curry($curried, @bar);
    my $c2      = $curried->new(@bar);

isn't equivalent. See the second form of new for an explanation.


See "Currying".


For backwardscompability the subroutine &Sub::Curry::Hole is provided. It takes one optional integer argument. If no argument is given one hole is returned. If an argument is given it returns that many holes in list context. The new way of doing that is

    (HOLE) x $n

where $n is the number of holes you want.


  • Doesn't handle prototypes

    If you feel the need for this module to handle prototypes in any way, please e-mail me with an idea of how you want it or an interface suggestion.


Don't do &$curried;, because that will break your program! See "What's the difference between calling a function as &foo and foo()?" in perlfaq7.

$curried->() is the recommended syntax.


This module has been partly inspired by the CPAN modules listed in "SEE ALSO" and credits go to David Helgason (CPAN ID: DAVIDH) who introduced holes to me by writing Sub::Curry versions 0.0x and passed me the namespace.


Johan Lodin <>


Copyright 2004 Johan Lodin. All rights reserved.

This library is free software; you can redistribute it and/or modify it under the same terms as Perl itself.





2 POD Errors

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=cut found outside a pod block. Skipping to next block.

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