Created
December 6, 2020 00:41
-
-
Save crdueck/5616b77a13be70b1530b0e243650b475 to your computer and use it in GitHub Desktop.
Type family inference
This file contains bidirectional Unicode text that may be interpreted or compiled differently than what appears below. To review, open the file in an editor that reveals hidden Unicode characters.
Learn more about bidirectional Unicode characters
{-# LANGUAGE GADTs, TypeFamilies, DataKinds, TypeOperators #-} | |
import Type.Reflection | |
type family Map f xs where | |
Map f '[] = '[] | |
Map f (x ': xs) = f x ': Map f xs | |
data HList xs where | |
HNil :: HList '[] | |
HCons :: a -> HList xs -> HList (a ': xs) | |
foo :: HList (Map TypeRep xs) -> HList xs | |
foo HNil = HNil | |
{- | |
main.hs:15:12: error: | |
* Could not deduce: xs ~ '[] | |
from the context: Map TypeRep xs ~ '[] | |
bound by a pattern with constructor: HNil :: HList '[], | |
in an equation for `foo' | |
at main.hs:15:5-8 | |
`xs' is a rigid type variable bound by | |
the type signature for: | |
foo :: forall (m :: * -> *) (xs :: [*]). | |
Monad m => | |
HList (Map TypeRep xs) -> m (HList xs) | |
at main.hs:14:1-56 | |
Expected type: m (HList xs) | |
Actual type: m (HList '[]) | |
* In the expression: return HNil | |
In an equation for `foo': foo HNil = return HNil | |
* Relevant bindings include | |
foo :: HList (Map TypeRep xs) -> m (HList xs) | |
(bound at main.hs:15:1) | |
| | |
15 | foo HNil = return HNil | |
| ^^^^^^^^^^^ | |
-} |
Sign up for free
to join this conversation on GitHub.
Already have an account?
Sign in to comment