michael rice wrote:
Still exploring monads. I don't understand why the type signature for double is OK,
It isn't. The |a| and |b| variables must unify: Prelude> :t \x -> Just (x+x) \x -> Just (x+x) :: (Num a) => a -> Maybe a Prelude> :t (\x -> Just (x+x)) :: Num a => a -> Maybe b <interactive>:1:13: Couldn't match expected type `b' against inferred type `a' `b' is a rigid type variable bound by the polymorphic type `forall a b. (Num a) => a -> Maybe b' at <interactive>:1:0 `a' is a rigid type variable bound by the polymorphic type `forall a b. (Num a) => a -> Maybe b' at <interactive>:1:0 In the first argument of `Just', namely `(x + x)' In the expression: Just (x + x)
but not the one for iota.
Again the |a| and |b| must unify. Also, Int isn't a type class (did you mean Num?) Also, the use of (..) requires the Enum class. Prelude> :t \n -> [1..n] \n -> [1..n] :: (Enum t, Num t) => t -> [t] Prelude> [3,4,5] >>= (\n -> [1..n]) [1,2,3,1,2,3,4,1,2,3,4,5] -- Live well, ~wren