Re: [GHC] #8583: Associated pattern synonyms

17 Jul 2015


      #8583: Associated pattern synonyms
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        Reporter:  cactus            |                   Owner:
            Type:  feature request   |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |  PatternSynonyms
 Type of failure:  None/Unknown      |            Architecture:
      Blocked By:  5144              |  Unknown/Multiple
 Related Tickets:                    |               Test Case:
                                     |                Blocking:
                                     |  Differential Revisions:
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Description changed by bgamari:

Old description:
...
The PatternSynonyms wiki page has a section on (typeclass-)associated
pattern synonyms:
{{{
class ListLike l where
      pattern Nil :: l a
      pattern Cons :: a -> l a -> a
      isNil :: l a -> Bool
      isNil Nil = True
      isNil (Cons _ _) = False
      append :: l a -> l a -> l a
instance ListLike [] where
      pattern Nil = []
      pattern Cons x xs = x:xs
      append = (++)
headOf :: (ListLike l) => l a -> Maybe a
   headOf Nil = Nothing
   headOf (Cons x _) = Just x
}}}
New description:

 The PatternSynonyms wiki page has a section on (typeclass-)associated
 pattern synonyms:

 {{{#!hs
 class ListLike l where
       pattern Nil :: l a
       pattern Cons :: a -> l a -> a
       isNil :: l a -> Bool
       isNil Nil = True
       isNil (Cons _ _) = False
       append :: l a -> l a -> l a

    instance ListLike [] where
       pattern Nil = []
       pattern Cons x xs = x:xs
       append = (++)

    headOf :: (ListLike l) => l a -> Maybe a
    headOf Nil = Nothing
    headOf (Cons x _) = Just x
 }}}

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/8583#comment:4
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