The type defaulting in GHCi with Typeable

The type defaulting in GHCi works in less cases with GHC 7.8.1-rc2 than with older versions, though there is no change in the related part of the documentation (i.e., "2.4.7 Type defaulting in GHCi").

skata@kata58:~$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.8.0.20140228
skata@kata58:~$ ghci
GHCi, version 7.8.0.20140228: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :m +Data.Typeable
Prelude Data.Typeable> let {f :: Read a => (a->Bool) -> Char; f = undefined}
Prelude Data.Typeable> f (\x -> (x == 3))
*** Exception: Prelude.undefined                                                 <== This is the expected result.
Prelude Data.Typeable> let {f :: Typeable a => (a->Bool) -> Char; f = undefined}
Prelude Data.Typeable> f (\x -> (x == 3))                                  <== Type defaulting does not work in this case.

<interactive>:6:1:
    No instance for (Typeable a0) arising from a use of ‘f’
    The type variable ‘a0’ is ambiguous
    Note: there are several potential instances:
      instance [overlap ok] Typeable ()
        -- Defined in ‘Data.Typeable.Internal’
      instance [overlap ok] Typeable Bool
        -- Defined in ‘Data.Typeable.Internal’
      instance [overlap ok] Typeable Char
        -- Defined in ‘Data.Typeable.Internal’
      ...plus 14 others
    In the expression: f (\ x -> (x == 3))
    In an equation for ‘it’: it = f (\ x -> (x == 3))

<interactive>:6:13:
    No instance for (Eq a0) arising from a use of ‘==’
    The type variable ‘a0’ is ambiguous
    Relevant bindings include x :: a0 (bound at <interactive>:6:5)
    Note: there are several potential instances:
      instance Eq a => Eq (GHC.Real.Ratio a) -- Defined in ‘GHC.Real’
      instance Eq Integer -- Defined in ‘integer-gmp:GHC.Integer.Type’
      instance Eq () -- Defined in ‘GHC.Classes’
      ...plus 33 others
    In the expression: (x == 3)
    In the first argument of ‘f’, namely ‘(\ x -> (x == 3))’
    In the expression: f (\ x -> (x == 3))

<interactive>:6:16:
    No instance for (Num a0) arising from the literal ‘3’
    The type variable ‘a0’ is ambiguous
    Relevant bindings include x :: a0 (bound at <interactive>:6:5)
    Note: there are several potential instances:
      instance Num Double -- Defined in ‘GHC.Float’
      instance Num Float -- Defined in ‘GHC.Float’
      instance Integral a => Num (GHC.Real.Ratio a)
        -- Defined in ‘GHC.Real’
      ...plus 7 others
    In the second argument of ‘(==)’, namely ‘3’
    In the expression: (x == 3)
    In the first argument of ‘f’, namely ‘(\ x -> (x == 3))’

On the other hand,

skata@kata58:~$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 7.4.1
skata@kata58:~$ ghci
GHCi, version 7.4.1: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> :m +Data.Typeable
Prelude Data.Typeable> let {f :: Read a => (a->Bool) -> Char; f = undefined}
Prelude Data.Typeable> f (\x -> (x == 3))
*** Exception: Prelude.undefined
Prelude Data.Typeable> let {f :: Typeable a => (a->Bool) -> Char; f = undefined}
Prelude Data.Typeable> f (\x -> (x == 3))
*** Exception: Prelude.undefined

I think the old behavior is compliant with the documentation.

Trac metadata

Trac field	Value
Version	7.8.1-rc2
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler (Type checker)
Test case
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