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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)

Trac metadata

Trac field	Value
Version	7.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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