TypeApplications and DefaultSignatures - problems deducing type variables.

The following example code throws an error.

The example code:

{-#LANGUAGE TypeApplications#-}
{-#LANGUAGE MultiParamTypeClasses #-}
{-#LANGUAGE AllowAmbiguousTypes #-}
{-#LANGUAGE FlexibleInstances #-}
{-#LANGUAGE ScopedTypeVariables #-}
{-#LANGUAGE DefaultSignatures #-}
module Test1 where

-- | Type a is only used for
-- type application.
class ToUse a where
    toUse :: Int -> Int

-- | The type used for
-- type application
data Default


-- | The instance using Default as type application.
-- To call use:
-- > toUse @Default 
instance ToUse Default where
    toUse a = 3*a

-- | Typeclass whose methods work
-- only with type application.
class (ToUse a) => Uses a b where
    uses :: b -> [b]
    -- | Default Signature, which generates the problem.
    -- It is the same as the normal one
    -- Comment it to 'fix' the bug.
    default uses :: b -> [b] 
    uses v = [v] 

-- | Normal instances, nothing special
instance Uses Default Int where
    uses v = take (toUse @Default 3) $ repeat v
-- | Another normal one
instance Uses Default String where
    uses v = take (toUse @Default 2) $ repeat v

-- | This one works nicely
instance (ToUse t, Uses t a, Uses t b) => Uses t (a,b) where
    uses (vl,vr) = zip ls rs
        where ls = uses @t vl
              rs = uses @t vr

-- | But this one doesn't.
-- Unless you comment the default signature.
instance (ToUse t, Uses t a, Uses t b, Uses t c) => Uses t (a,b,c)

The error:

    • Could not deduce (Uses a0 a)
        arising from a use of ‘Test1.$dmuses’
      from the context: (ToUse t, Uses t a, Uses t b, Uses t c)
        bound by the instance declaration at Test1.hs:47:10-66
      The type variable ‘a0’ is ambiguous
      Relevant bindings include
        uses :: (a, b, c) -> [(a, b, c)] (bound at Test1.hs:47:10)
    • In the expression: Test1.$dmuses
      In an equation for ‘uses’: uses = Test1.$dmuses
      In the instance declaration for ‘Uses t (a, b, c)’

Commenting out the default signature fixes the problem.

Trac metadata

Trac field	Value
Version	8.0.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC
Operating system
Architecture