Destructure a List of Pairs

Question

Consider

.say for (1,2,2).rotor(2=>-1).map( -> ($a, $b) { $a - $b })

which works as expected. However,

.say for (1,2,2).pairs.rotor(2=>-1).map( -> ($a, $b) { $a.value - $b.value })

throws

Too few positionals passed to '<anon>'; expected 2 arguments but got 0 in sub-signature

Is this a bug or am I missing something?

This is Rakudo Star version 2019.03.1 built on MoarVM version 2019.03 implementing Perl 6.d.

Elizabeth Mattijsen · Accepted Answer · 2020-04-19T22:22:15.013

8

It is taking the Pair as a Capture, thus turning the Pair into a named argument:

$ raku -e '(a => 42, b => 666).map: -> |c { dd c }'
\(:a(42))
\(:b(666))

In your example, it then doesn't pass any positional arguments, thus causing the observed execution error.

jnthn++ for pointing this out.

edited Apr 19 '20 at 22:22

answered Apr 19 '20 at 20:46

Elizabeth Mattijsen

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Coercion to a `Capture` is precisely how destructuring works. – Jonathan Worthington Apr 19 '20 at 21:16

score 0 · Answer 2 · answered Apr 23 '20 at 20:00

0

btw - I was wondering why |c and not \c ... from the doc...

Inside a Signature, a Capture may be created by prefixing a sigilless parameter with a vertical bar |. This packs the remainder of the argument list into that parameter.

answered Apr 23 '20 at 20:00

librasteve

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Destructure a List of Pairs

2 Answers2