Wolfram Language Paclet Repository
Community-contributed installable additions to the Wolfram Language
Wolfram`Parser`
ParseChoiceLongest  |
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Where is PEG-ordered — it stops at the first alternative that matches — is POSIX-style: it runs every alternative and commits to the one that consumed the most input.
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Ties between equal-length successes resolve to the first listed alternative, mirroring 's left-bias.
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It cannot early-exit at the first hit, so it always runs all branches; it is slower than by that constant factor.
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The case it exists for: alternatives that share a leaf-level prefix and differ only in what follows, so the shorter alternative is a prefix of the correct one. Ordering longest-first cannot fix this when the alternatives are factored across separate rules — TPTP's , where both branches consume the same leading term but only the second reaches a trailing .
<fof_atomic_formula>::=<fof_plain_atomic_formula>|<fof_defined_atomic_formula>
=rhs
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There is no operator overload: lowers to (via ), never to . Call explicitly.
p1|p2
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On total failure the reported error is the furthest-advanced failure across all branches, and its set is the union of the per-branch expected sets at that position — the same diagnostic produces.
Expected
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Result type: whatever the winning branch returned.
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It is the combinator folds a rule's alternatives with under and, for length-ambiguous rules, .
"ChoiceMode""Longest"
"ChoiceMode""Auto"
Examples
(13)
Basic Examples
(3)
Two alternatives share the prefix ; longest-match takes the branch that consumes the whole input:
ab
In[1]:=
 | Parse |
 | ParseChoiceLongest |
 | ParseLiteral |
| ParseLiteral |
Out[1]=
abc
The same two alternatives under pick the shorter branch first, leaving unconsumed and failing the whole-input match:
c
In[1]:=
| Parse |
| ParseChoice |
| ParseLiteral |
| ParseLiteral |
Out[1]=
Failure
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Longest-match is independent of the order the alternatives are listed — the longer branch wins even when it comes first:
In[1]:=
| Parse |
| ParseChoiceLongest |
| ParseLiteral |
| ParseLiteral |
Out[1]=
abc
Scope
(3)
Properties & Relations
(5)
Possible Issues
(2)
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""