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Subset::Helper zef:raku-community-modules last updated on 2022-05-21
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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 inconveniences with Raku's subsets:

  * Display of useful error messages when type check fails

  * Avoid evaluating subset's condition for `Any` values, which is what happens with optional parameters.



**NOTE**: This functionality broke sometime since 2017 as the returned `Failure` for non-matching is somehow ignored, and the message specified will never be displayed. Sadly, this was not picked up by the tests, as these only checked for throwing **an** exception, not whether the exception was correctly worded.

Since the 2022.04 release of Rakudo, one can use the `will complain` trait instead:

use experimental :will-complain;  # this may become unnecessary

subset Positive of Int:D
  will complain { "need a positive integer, got: $_" }
  where * > 0;

my Positive $a = -2;
# Type check failed in assignment to $a; Need a positive integer, got: -2

my str @cameras = <MAST CHEMCAM FHAZ RHAZ>;
subset RoverCam of Str:D
  will complain { "valid cameras are: @cameras[]" }
  where * ∈ @cameras;

The original documentation:

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: undefined values are accepted by 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.


Zoffix Znet


Copyright 2016 - 2017 Zoffix Znet

Copyright 2018 - 2022 Raku Community

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.