yeah... I dont think close type families can match on the first one, thought its interesting to ask if they should be able to.... On Sat, Mar 15, 2014 at 10:21 AM, Silvio Frischknecht < silvio.frischi@gmail.com> wrote:
Hi
I have been playing around a bit with closed type families. However, I somehow always bump my head at the fact that things usually doesn't work for Num without specifying the type.
Here is an example.
{-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE DataKinds #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-} {-# LANGUAGE IncoherentInstances #-} module Main where
import Data.Typeable
type family UnMaybed a where UnMaybed (Maybe a) = a UnMaybed a = a
class UnMaybe x where unMaybe :: x -> UnMaybed x
instance UnMaybe (Maybe a) where unMaybe (Just a) = a
instance (UnMaybed a ~ a) => UnMaybe a where unMaybe a = a
main = do print $ unMaybe 'c' print $ unMaybe (1::Int) print $ unMaybe (Just 1) print $ unMaybe 1 -- this line does not compile
everything except the last line will compile.
../Example.hs:23:17: Occurs check: cannot construct the infinite type: s0 ~ UnMaybed s0 The type variable ‘s0’ is ambiguous In the second argument of ‘($)’, namely ‘unMaybe 1’ In a stmt of a 'do' block: print $ unMaybe 1
Now I know this is because numbers are polymorphic and (Maybe a) could be an instance of Num. I think for normal overlapping typeclasses this dilemma can be solved by using the IncoherentInstances PRAGMA. Anyway, I wanted to ask if there is a way to make this work in type families?
I also thought about specifying Num explicitly in UnMaybed
type family UnMaybed a where unMaybed (Num a => a) = a UnMaybed (Maybe a) = a UnMaybed a = a
This compiles but i think the first case will never be matched this is probably a bug.
Silvio
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