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As GHC documentation (section 7.4.4, Type operators) says: > "There is now some potential ambiguity in import and export lists; for example if you write import M( (+) ) do you mean the function (+) or the type constructor (+)? The default is the former, but with -XExplicitNamespaces (which is implied by -XExplicitTypeOperators) GHC allows you to specify the latter by preceding it with the keyword type" Turns out this ambiguity causes 4 of 6 reduce/reduce conflicts in GHC parser. All 4 conflicts arise from a single production: qcname : qvar | oqtycon Recursive inlining of 'qvar' and 'oqtycon' helps reveal the faulty productions: qcname : ... | '(' QVARSYM ')' | '(' VARSYM ')' | '(' '*' ')' | '(' '-' ')' These productions can either be parsed as variable or type constructor, but variable constuctor is always preferred. My patch removes ambiguity while preserving the existing behaviour: - all unambigous productions are left as-is - ambigous productions for variable constuctors are left - ambigous productions for type constructors are removed (there's no way they could be triggered) Updated comment. Test Plan: Tested with 'make fasttest' Reviewers: austin, simonpj, trofi, bgamari, simonmar Reviewed By: trofi, bgamari, simonmar Subscribers: thomie, mpickering Projects: #ghc Differential Revision: https://phabricator.haskell.org/D1111
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