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non-ratchet right-hand branch of || causes backtracking into the alternation #6570

p6rt opened this issue Oct 4, 2017 · 1 comment


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@p6rt p6rt commented Oct 4, 2017

Migrated from (status was 'new')

Searchable as RT132219$

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@p6rt p6rt commented Oct 4, 2017

From @smls

When backtracking is disabled for an alternation (via `​:` modifier or
`​:ratchet` mode), this obviously means that once the alternation has
matched once it may no longer reconsider which branch it wants to select​:

  say "ab" ~~ / [ab || a ]​: b /; # Nil

But does it also mean that once the alternation has matched, the
selected branch *itself* may no longer do any backtracking?

I would have thought so, but Rakudo's current behavior is inconsistent​:

  say "ab" ~~ / [ .+ | x ]​: b /; # Nil
  say "ab" ~~ / [ .+ || x ]​: b /; # Nil

  say "ab" ~~ / [ x | .+ ]​: b /; # Nil
  say "ab" ~~ / [ x || .+ ]​: b /; # 「ab」

I.e. in most cases, it commits to what the alternation matched the first
time. But when

1) it is an `||` alternation,
2) and its *last* branch matched,
3) and said branch is a backtrackable atom which (unlike the alternation
itself) is not affected by `​:ratchet`,

then it looks like that backtrackable atom can reconsider what it wants
to match even after the regex execution has moved past the alternation.

I suppose another way to phrase the @​LARRY design question is this​: Does
an alternation select a particular match value (like a "function"), or
does it select a subregex (like a "macro")?

(PS​: I should note that the backtracking behaviors of || and | were even
less consistent before my NQP commit 263257a9c¹ [see RT #​130117²], which
was supposed to bring `||` in line. But I didn't discover this edge case
until now, and indeed am not sure how to fix it no matter which way it
*should* be fixed.)

[1] Raku/nqp@263257a

This is Rakudo version 2017.09-203-g98fae3d84 built on MoarVM version
implementing Perl 6.c.

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