generalizing overloaded list syntax to Sized Lists, HLists, HRecords, etc
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 |
Test case | |
Differential revisions | |
BlockedBy | |
Related | |
Blocking | |
CC | |
Operating system | |
Architecture |