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We have at various times discussed how to allow a module to export a data constructor that can be used as a pattern but not as a constructor (in expressions), because the data type is abstract and you should only use a smart constructor. With pattern synonyms you can (almost) do that

What if you could just export `pattern P` (where `P` is the name of the data constructor) without having to define `AbsP`? Would that be taking

#5144: Pattern synonyms -------------------------------------+------------------------------------ Reporter: simonpj | Owner: cactus Type: feature request | Status: new Priority: normal | Milestone: _|_ Component: Compiler | Version: Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Unknown/Multiple Type of failure: None/Unknown | Difficulty: Unknown Test Case: | Blocked By: Blocking: | Related Tickets: -------------------------------------+------------------------------------ Comment (by simonpj): Replying to [comment:25 cactus]: things too far? That would treat pattern synonyms and data constructors differently, which I am keen to avoid. Under what you propose: * If P is a data constructor then `pattern P` would export P uni- directionally (pattern only) * If P is a (bi-directonal) pattern synonym then `pattern P` would export P bi-directionally. I don't like that... a data constructor is precisely a (degenerate) bidirectional pattern. I suppose you could put the directionality in the export list: {{{ module M( pattern P, -- Uni-directional constructor Q -- Bidirectional ) where pattern P x = [x] pattern Q y = [y,y] }}} But I don't want to go this way. If you export P you get all of what P is. Otherwise we'll get into exporting it unidirectionally from one module, bidirectionally from another, then importing both of those, and combining the directinoality of those imports. Too complicated. Maybe `datacon P`, or `view P`, rather than `pattern P` would address Ian's worry? -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5144#comment:27 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler