Henning Thielemann wrote:
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.
Right. I always forget about defaulting. This is an obscure feature of the language. Are there any programs that rely on defaulting and could not be easily re-written so as not to? Cheers Ben