Arie Peterson wrote: > > However, if I type an apparently equivalent let expression into Hugs > > directly, then I get the value 4 as expected > > > > let k = 2 ; f :: Int -> Int -> Int ; f x y = x * y in f k k > > > > Why is there a difference in behaviour? > > Here, there is no defaulting, 'k' has the polymorphic type you expect, and > the use of 'k' as an argument to the monomorphically typed 'f' chooses the > right instance of 'Num'.
Well, there is no defaulting at the stage of type checking when k is given type Num a => a, the monomorphism restriction applies and this type is not generalised to forall a . (Num a) => a, then the use of k forces the type variable a to be Int, and then there is no longer any need for defaulting. So k gets a monotype which is determined by its usage, you cannot do e.g. let k = 2 ; f :: Int -> Int -> Int ; f x y = x * y in (f k k, 1/k) whereas let k :: Num a => a; k = 2; ... is possible. Defaulting in Haskell 98 happens so late that this file k = 2 f :: Int -> Int -> Int f x y = x * y r = f k k is okay. Alas, Hugs does not comply in this respect, see http://cvs.haskell.org/Hugs/pages/users_guide/haskell98.html at the end of 5.1.3. All the best, Christian Sievers _______________________________________________ Haskell mailing list Haskell@haskell.org http://www.haskell.org/mailman/listinfo/haskell
