#13290: Data constructors should not have RULES -------------------------------------+------------------------------------- Reporter: simonpj | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.0.1 Keywords: | Operating System: Unknown/Multiple Architecture: | Type of failure: None/Unknown Unknown/Multiple | Test Case: | Blocked By: Blocking: | Related Tickets: Differential Rev(s): | Wiki Page: -------------------------------------+------------------------------------- GHC has never (knowingly) supported rules for data constructors, like {{{ {-# RULES "BadBadBad" Just [x,y] = Just [] #-} Notice that the rule is for the ''constructor itself''. Presumably the intent is that any occurrence of `Just` appplied to a two-element list will rewrite to `Just []`. But currently they are accidentally allowed through, and then behave in mysterious ways because of constructor wrappers. Duncan Coutts says {{{
Well I've certainly tried to use that in the past. A previous version of the cbor lib which used a different representation did a lot of matching on constructors to re-arrange input to an interpreter, until I discovered that GHC actually uses constructor wrappers and that matching on constructors was thus not reliable }}} I think we should simply make it illegal for now. If you really want it, use a smart constructor thus {{{ mkJust x = Just x {-# INLINE [0] mkJust #-} {-# RULES “Good” mkJust [x,y] = mkJust [] #-} }}} So let us
* Check in that you don't try to write a rule for a data constructor. * Document in the user manual -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13290 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler