Question

I’m trying to write a Haskell function that checks if a list of integers is in order without using any of the already existing functions to order or check the order of the list. I have written the following code but I do not understand why it does not work. I get the error:

No instance for (Ord integer)
      arising from a use of `<='
    In the expression: x <= (head xs)

I don’t understand what this means. Is there a different way that I should be writing this function? Here is my code so far.

isordered :: [integer] -> Bool
isordered [] = True
isordered (x:[]) = True
isordered (x:xs)|x <= (head xs) = isordered xs
                |otherwise = False

Thanks in advance!!!

Answer 1

In Haskell type names start with capital letters and type variables start with lower case letters. So if you write integer, that’s a type variable. So your type is the same as [a] -> Bool, i.e. you take a list of anything and return a Bool. So since there’s no restriction on what type of item might be in the list, you’re not allowed to use <= on it.

To fix this you can either just change it to Integer, which is what you wanted, or add an Ord constraint like this: Ord a => [a] -> Bool. The latter will make your function work with any type that implements the Ord typeclass (which provides the comparison operators such as <=).