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.
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