> module Test where
>
> class Show a => Foo a where
> bar :: Read b => a -> b
> bar x = read (show x) :: b
> \end{code}
>
> yields:
> ----
> Test.hs:5:
> Could not deduce `Read b' (arising from use of `read' at
> test.hs:5)
> from the context: ()
> Probable cause: missing `Read b' in tcPolyExpr
> In an expression with expected type: forall b1. b1
> In an expression with a type signature:
> read (show x) :: forall b. b
Yes that is a bit misleading. It now says:
Probable cause: missing `Read b' in the type signature of an
expression
> BTW: What I intended to do boils down to:
> \begin{code}
> class Show a => Foo a where
> bar :: Read b => a -> b
> bar x = let r = stringBar (show x)
> in read r
>
> instance Foo String where
> bar x = stringBar x
>
> stringBar :: String -> String
> stringBar = id
> \end{code}
>
> which gives an error, because b cannot be unified with String
> (correct).
Right. The type you declared for bar says
bar :: FORALL b. Read b => a -> b
and plainly stringBar is less general than that.
The instance at String (i.e. [Char]) is illegal in H98.
Simon
