CODEWITHSUNDEEP
haskell
user3389445
user3389445

Reputation:

Getting the sum of all even numbers in Haskell

I'm trying to get the sum of all even numbers with a function in Haskell, but it doesn't seem to work.

This is how my code looks like:

sumEvenNumbers :: [Int] -> Int
sumEvenNumbers n = sum . even

Anyone has a solution to this? Thanks!

Upvotes: 0

Views: 2123

Answers (2)

Emil
Emil

Reputation: 2167

The type of even is even :: (Integral a) => a -> Bool, meaning you give it a number and it tells you whether it is even or not. You are trying to apply this function to a list of numbers which doesn't make sense.

What you want instead is a function that gives you back a list of the numbers that are even. To accomplish that you want filter which has the type filter :: (a -> Bool) -> [a] -> [a], and it will apply the function to the original list and give back only those things that evaluated to True.

Additionally, when you are going to define a function point-free style, you don't need to (actually, language does not allow you to) give arguments to the function, so the n is unnecessary.

Upvotes: 4

user1804599
user1804599

Reputation:

You want sumEvenNumbers = sum . filter even (note the absence of the argument on the left-hand side of the equation). even doesn’t work on lists, but on single integers.

A more explicit equivalent is sumEvenNumbers xs = sum (filter even xs).

Upvotes: 6

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