#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): We must treat data constructors uniformly with pattern synonyms. For example, it should definitely be OK to have {{{ module M( pattern P, foo, bar ) where -- The "pattern P" exports data constructor P data T = P | Q foo :: T foo = P bar :: T -> Int bar P = 1 bar Q = 2 module A where import M -- T is not in scope w1 = bar P -- Uses P as an expression w2 = case foo of P -> 1 -- Uses P as a pattern }}} 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: {{{ module M( T, pattern AbsP, pattern Q ) where data T = P Int | Q pattern AbsP = P }}} But I really wanted `AbsP` to be uni-directional and according to the current spec [wiki:PatternSynonyms] it is "implicitly bidirectional". I suggest we allow the user to control whether a pattern synonym is bidirectional or not. I'm not sure what syntax to use for that. We want to be able to say * Uni-directional pattern synonym * Bi-directional pattern synonym * Bi-directional pattern synonym with explicit user code for the reverse direction -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/5144#comment:23 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler