On Sun, 3 Oct 2010, Ben Franksen wrote:
Christopher Done wrote:
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.
A simpler example is
main = print Nothing
This seems to be a different example, because "GHCi -Wall" says that the
type variable defaults to (). Thus 'Nothing' has monomorphic type at
runtime. The difference is certainly that 'print' requires a Show
instance, whereas Christopher's example does not require a type
constraint.
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