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Simpler `where` constraints for items of lists
Perl 6
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WhereList - Simpler where constraints for items of lists


use WhereList;

subset StrArray of Array where all-items Str, *.chars3, *.contains: any <o a e>;

say [<foo bar meow>] ~~ StrArray; # OUTPUT: «True␤»
say ['dddrrrrrrrrr'] ~~ StrArray; # OUTPUT: «False␤»
say [<ha ha ha ha >] ~~ StrArray; # OUTPUT: «False␤»
say [<ooh come onn>] ~~ StrArray; # OUTPUT: «True␤»

class Foo {
    has where all-items any Str|Nil|Int:D, * === Any

sub foo (
    @meows where all-items(/meow/), # <-- use parens to avoid gobbling of params that follow
    @barks where all-items(/woof/) = ['woof'], # <-- or to add defaults
) {


Type-constraining elements of list parameters, attributes, and subsets can be done with where clauses, however, they can quickly get overly complex when you want to constraint using multiple requirements. This module addresses that problem!



Defined as:

    sub all-items (+@matchers)

See SYNOPSIS for sample use.

Takes a list of matchers (anything that can be fed to .grep). Returns a Callable that a where clause can use to check whether all items in a list match all of these matchers. Matchers will be checked in the order provided, short-circuiting as soon as a matcher fails. If an exception occurs during matching, it will be turned into a Failure, gracefully failing the type check.

Notes and tips:

  • An empty list always matches the type constraint.
  • There's no thunking involved. where all-items .so is an error, as .so will be called on the list itself, not each of the elements and its return value will be used as a matcher. Use WhateverCode instead: where all-items *.so
  • All Seqs will be cached.
  • Don't forget to add parens around the args when more params follow this routine (see sub example in SYNOPSIS).
  • Don't drink bleach.
  • Donate to TPF.


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.

The META6.json file of this distribution may be distributed and modified without restrictions or attribution.

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