#7495: Rebindable list syntax?
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Reporter: nwf | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 7.6.1 | Keywords:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
Failure: None/Unknown | Blockedby:
Blocking: | Related:
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First, sorry if I've missed an earlier request for this in trac; a few
searches did not turn up anything relevant.
I've recently taken to doing a lot of work with heterogenous lists (thanks
to the DataKinds work) and find the forced-cons-and-nil style of writing
lists (e.g. "a:+b:+c:+HN") to be sort of unpleasant.
Would it be possible to allow rebinding list-literal syntax? Off the top
of my head I think something like the following might be workable, if only
I could stop [] and (:) from being in scope, even with
-XNoImplicitPrelude. (Example requires -XDataKinds -XFlexibleInstances
-XGADTs -XMultiParamTypeClasses -XTypeOperators)
{{{
class HasNil a where
([]) :: a
isNil :: a -> Bool
class HasCons e l l' | e l -> l', l' -> e l where
(:) :: e -> l -> l'
uncons :: l' -> Maybe (e,l)
-- For homogeneous lists...
instance HasNil [a] where
([]) = ([])
isNil = null
instance (a ~ a1, a ~ a2) => HasCons a [a1] [a2] where
(:) = (:)
uncons [] = Nothing
uncons (x:xs) = Just (x,xs)
-- For HLists...
data HList as where
HN :: HList '[]
(:+) :: a -> HList as -> HList (a ': as)
instance HasNil (HList '[]) where
([]) = HN
isNil = const True
instance (a ~ a1, as ~ as1) => HasCons a (HList as) (HList (a1 ': as1))
where
(:) = (:+)
uncons (a :+ as) = Just (a,as)
}}}
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7495>
GHC <http://www.haskell.org/ghc/>
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