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kiltek
kiltek

Reputation: 3211

Haskell list comprehension with "x is drawn from a function"

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?

Upvotes: 0

Views: 415

Answers (1)

Don Stewart
Don Stewart

Reputation: 137977

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].

Upvotes: 4

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