CODEWITHSUNDEEP
haskell
Hick
Hick

Reputation: 36404

Unable to take a sum of a list in Haskell

fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

sum' :: (Integral a) => [a] -> a
sum' [] = 0
sum' [a] = foldr (+) 0 ([a])

main = do putStrLn "Enter a number:"
          num <- readLn
          sum' (take num fibs)

That's my code to take the sum of a recursively produced list.

I could have done the foldr operation at take num fibs, but I wanted more control over the list and wanted a way in which I could take the sum of particular elements of a list rather than the whole list.

Where am I going wrong?

Upvotes: 1

Views: 176

Answers (1)

Deestan
Deestan

Reputation: 17136

Your sum' function is incorrect:

sum' [a] = foldr (+) 0 ([a])

This will only accept, and sum, a list of 1 element. :-)

Change it to:

sum' a = foldr (+) 0 a

And it will work.

You can also delete the redundant sum' [] = 0 line, and extend the type to sum' :: (Num a) => [a] -> a.

Upvotes: 5

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