[Haskell-cafe] overlapping/Incoherent closed type families

[Carter Schonwald]

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 at 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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