On Sunday 03 October 2010 10:43:24, 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.
Yup. Prelude> print $ length [undefined :: a, undefined :: b] 2 Prelude> print $ length [undefined :: a, undefined :: Num b => b] <interactive>:1:32: Warning: Defaulting the following constraint(s) to type `Integer' `Num a' arising from an expression type signature at <interactive>:1:32-54 In the expression: undefined :: (Num b) => b In the first argument of `length', namely `[undefined :: a, undefined :: (Num b) => b]' In the second argument of `($)', namely `length [undefined :: a, undefined :: (Num b) => b]' 2 Prelude> print $ length [undefined :: a, undefined :: Num b => c -> b] <interactive>:1:32: Warning: Defaulting the following constraint(s) to type `Integer' `Num b' arising from an expression type signature at <interactive>:1:32-59 In the expression: undefined :: (Num b) => c -> b In the first argument of `length', namely `[undefined :: a, undefined :: (Num b) => c -> b]' In the second argument of `($)', namely `length [undefined :: a, undefined :: (Num b) => c -> b]' 2 At runtime, a type variable has to be either unconstrained or instantiated with a concrete type?