[Haskell-cafe] Type classes in Typing Haskell in Haskell

[Alexander Bau]

Hi,

recently I tried the Typing Haskell in Haskell library. But I was  
wondering why this program type checks:

> -- plusMfun is standard '+': Num a => a -> a -> a
> test =  let Just classEnv = ( addCoreClasses <:> addNumClasses )  
> initialEnv
>       e = Ap ( Ap (Var "+") (Lit $ LitStr "3")) (Lit $ LitStr "5")
>       impl = ("foo",[([],e)])
>   in
>     putStrLn $ pretty $ runTI $ tiImpls classEnv [plusMfun] [impl]

I was expecting some kind of typechecking error, because [Char] is no
instance of Num. But I get this:

> ([isIn1 cNum (TAp tList tChar)],
>  ["foo" :>:
>     Forall []
>       ([] :=>
>          (TAp tList tChar))])

The predicate says that if [Char] would be in Num then the type of the  
expression would be [Char]. But actually [Char] isn't in Num. So, how do I  
provoke a type check error in this case?

Thanks in advance.