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.