Opened 16 months ago

Last modified 12 months ago

#7495 new feature request

Rebindable list syntax?

Reported by: nwf Owned by:
Priority: normal Milestone: 7.8.3
Component: Compiler Version: 7.6.1
Keywords: Cc: giorgidze@…, info@…, jeroen.weijers@…, eir@…
Operating System: Unknown/Multiple Architecture: Unknown/Multiple
Type of failure: None/Unknown Difficulty: Unknown
Test Case: Blocked By:
Blocking: Related Tickets:


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)

Change History (5)

comment:1 Changed 16 months ago by simonpj

  • Cc giorgidze@… info@… jeroen.weijers@… added
  • Difficulty set to Unknown

Yes, and an implementation is well advanced; see OverloadedLists. I've cc'd the George, Achim, and Wejers, who are doing it.


comment:2 Changed 14 months ago by monoidal

Should this be marked as fixed?

comment:3 Changed 14 months ago by nwf

I don't think the extant proposal at OverloadedLists addresses the heterogeneous types I was after?

comment:4 Changed 14 months ago by goldfire

  • Cc eir@… added

I agree with nwf. It seems the current implementation can't deal with heterogeneous lists. I, too, would enjoy being able to use nicer notation for my lists.

comment:5 Changed 12 months ago by igloo

  • Milestone set to 7.8.1
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