[Haskell-cafe] Type Families: deleting from HList.
sergueyz at gmail.com
Sat Jul 31 16:22:49 EDT 2010
Is it possible to delete an element from heterogenous list using type
I can do it using multiparameter type classes:
class Del a list result
instance Del a (a,list) list
instance Del a list list' => Del a (a',list) list'
instance Del a () ()
I tried to express the same using type families:
type family Del a list
type instance Del a () = ()
type instance Del a (a,list) = list
type instance Del a (a',list) = (a',Del a list)
to no avail. I got "conflicting family instance declarations" error.
I tried to express that with associated type synonyms that contains
both multiparameter type classes and type families:
class HDel a list where type Del a list
instance HDel a () where type Del a () = ()
instance HDel a (a,list) where type Del a (a,list) = list
instance HDel a list => HDel a (a',list) where type Del a (a',list) =
(a',Del a list)
And I once again got "conflicting family instance declaration".
It looks like right now there is no way to express type equality or
inequality with type families, even when they are combined with
multiparameter type classes.
What is the right way to express something like that? Only MPTC?
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