[GHC] #13290: Data constructors should not have RULES

[GHC]

#13290: Data constructors should not have RULES
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           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:
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 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

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13290>
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