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Asked: 2026-05-21T09:01:00+00:00

The code below starting with digits returns a List of integers. The code starting

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The code below starting with digits returns a List of integers. The code starting with altSumOfDigits should return just the same list, but instead Prelude is complaining about a type error, which i don’t understand at this point.

Couldn't match expected type `a' against inferred type `Integer'


digits' :: Integer -> [Integer]
digits' 0 = []
digits' x = (x `mod` 10) : digits' (x `div` 10)

digits :: Integer -> [Integer]
digits 0 = [0]
digits x = reverse $ digits' x

altSumOfDigits :: Integer -> [a]
altSumOfDigits num = [ x | x <- (digits num)]

(I know that altSumOfDigits num = [ x | x <- (digits num)] is rather useless in this form. I’m going to extend its functionality with an if-expression and operations on the single Integers later.)
Any explanations why this doesn’t work?

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1 Answer

  1. Editorial Team

    Change (or remove) the type of altSumOfDigits to return a [Integer].

    Including the error in the question is good form, and here it is:

    Couldn't match type `a' with `Integer'

    The type of altSumOfDigits takes a list of Integer, but returns a list of any type a, according to the type you have declared. So it is polymorphic in its return type.

    However, you call digits from within altSumOfDigits, which must return a list of Integer only. There is thus no way your altSumOfDigits could return anything other than a list of [Integer].

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