Question

when I try to compile this piece of code

prod [] = 1
prod (x:xs) = x * prod xs

ff :: (Num a) => a -> a -> a
ff x n = prod [(x - n + 1) .. x]

I get following error:

a.hs:5:15:
    Could not deduce (Enum a)
     arising from the arithmetic sequence `(x - n + 1) .. x'
    from the context (Num a)
      bound by the type signature for ff :: Num a => a -> a -> a
      at a.hs:5:1-32
    Possible fix:
      add (Enum a) to the context of
        the type signature for ff :: Num a => a -> a -> a
    In the first argument of `prod', namely `[(x - n + 1) .. x]'
    In the expression: prod [(x - n + 1) .. x]
    In an equation for `ff': ff x n = prod [(x - n + 1) .. x]

what is wrong with this code? When I substitute Double for a everything is all right.

Answer 1

[i .. j] is shorthand for enumFromTo i j. enumFromTo is part of the Enum typeclass, and not part of Num (you still need Num to use + and - though).

So you need to say that a implements Enum as well as implementing Num:

ff :: (Num a, Enum a) => a -> a -> a
ff x n = prod [(x - n + 1) .. x]

It works with Double because Double implements both these typeclasses.