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Alternative instance for Data.Functor.Compose causes <<loop>>s with some types

    • Open Issue created by glaebhoerl @trac-glaebhoerl

    many and some are omitted and are filled in by their defaults from Alternative, instead of re-using the definitions provided for f; this manifests as infinite grammars being created in the case of the Earley parsing library, resulting in <<loop>> errors at runtime.

    The fix is just to add:

        many = Compose . fmap sequenceA . many . getCompose
    some = Compose . fmap sequenceA . some . getCompose

    Related twitter thread: https://twitter.com/ollfredo/status/1067498140100628480

    (It seems like this bug has been around for some time: https://github.com/feuerbach/regex-applicative/issues/19)

    Trac metadata
    Trac field Value
    Version 8.6.2
    Type Bug
    TypeOfFailure OtherFailure
    Priority normal
    Resolution Unresolved
    Component libraries/base
    Test case
    Differential revisions
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    Related
    Blocking
    CC
    Operating system
    Architecture
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