1 Oct
2010
1 Oct
'10
9:36 a.m.
On 1 October 2010 15:27, Henning Thielemann
Given the following code, that is accepted by GHC:
data Exist = forall a. Exist a
exist :: Exist exist = Exist undefined
What type has the 'undefined' ?
I think its type is `a'.
So far I assumed that at runtime all objects have a concrete type. This seems not to be true.
Consider the following program: main = putStrLn $ show $ length [undefined :: a,undefined :: b] A concrete type of the element in list doesn't need to be determined at runtime, or any time. a unifies with b, and that unifies with x in length :: [x] -> Int.