Haskell - list operation: Parse error in pattern

Question

I am writing in Haskell a function that gets two lists of type Int and adds the values of one list to that one of the other.

for example: addElements [1,2,3] [4,5,6] will give the output: [5,7,9]

my function so far:

addElements :: [Int] -> [Int] -> [Int]
addElements [] [] = []
addElements x:xs [] = x:xs
addElements [] y:ys = y:ys
addElements x:xs y:ys = [x+y] ++ addElements xs ys

I keep getting the error:

Parse error in pattern: addElements Failed, modules loaded: none

I do not get any additional information - what have I done wrong?

I think you need parenthesis around the `x:xs` and `y:ys` pattern matches. — ryachza, Feb 27 '17 at 13:22
Possible duplicate of [Haskell: Parse error in pattern](https://stackoverflow.com/questions/8561762/haskell-parse-error-in-pattern) — Chris Martin, Sep 06 '17 at 07:43

score 8 · Accepted Answer · answered Feb 27 '17 at 13:22

You need parentheses around your patterns. It should be (x:xs), not x:xs on its own. That's what is causing the compiler confusion.

addElements :: [Int] -> [Int] -> [Int]
addElements [] [] = []
addElements (x:xs) [] = x:xs
addElements [] (y:ys) = y:ys
addElements (x:xs) (y:ys) = [x+y] ++ addElements xs ys

score 1 · Answer 2 · answered Feb 27 '17 at 15:12

1

Not an answer to the OP, but I just wanted to point out that the patterns can be simplified to:

addElements :: [Int] -> [Int] -> [Int]
addElements xs [] = xs
addElements [] ys = ys
addElements (x:xs) (y:ys) = (x+y) : addElements xs ys

answered Feb 27 '17 at 15:12

pat

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Haskell - list operation: Parse error in pattern

2 Answers2