Perl 6 module to assist with creating awesome subsets
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Subset::Helper - create awesome subsets


    use Subset::Helper;

    subset Positive of Int
        where subset-is * > 0, 'Value must be above zero';

    my Positive $x = 42; # success
    my Positive $x = -2; # Fails with 'Value must be above zero';


This module solves two inconviniences with Perl 6's subsets:

1) Display of useful error messages when type check fails
2) Avoid evaluating subset's condition for `Any` values,
    which is what happens with optional parameters



    subset Positive of Int where subset-is * > 0;

    subset RoverCam of Str where subset-is
        { $_  set <MAST CHEMCAM FHAZ RHAZ> },
        'Valid cameras are MAST, CHEMCAM, FHAZ, and RHAZ';

Takes one mandatory positional argument, which is the code to execute to check the validity of value, and an optional descriptive error message to show when the value doesn't match the subset.

Note: type Any always matches the subset. This exists to make it possible to cleanly define subsets for optional parameters, for which the type check is still called, even when they aren't provided in the sub/method calls.


You can't declare our scoped subsets within roles. If you're using this module, however, that error will instead point to the end of the declaration, saying expecting any of: postfix. Simply prefix your subset with my


Rakudo may evaluate whether a value matches the subset TWICE: once to check the match and once to get sensible error information.

Thus, currently the error message you provide is printed twice.

Patches to fix this are welcome.


Fork this module on GitHub:


To report bugs or request features, please use


Zoffix Znet (


You can use and distribute this module under the terms of the The Artistic License 2.0. See the LICENSE file included in this distribution for complete details.