Haskell syntax error in my function

Question

I have tried to write a function that takes a list pair, and swaps the pair elements

inverse :: [(a,b)] -> [(b,a)]
inverse [] = []
inverse (x,y):xs = (y:x): inverse xs

I have loaded this function via Prelude, it gives me following error:

mydefs.hs:11:1: Parse error in pattern: inverse

This is line 11, inverse (x,y):xs = (y:x): inverse xs

A better way of doing this would be making a function `swap (a,b) = (b,a)` and making `inverse = map swap`. — AJF, Jan 11 '15 at 10:32
Possible duplicate of [Haskell: Parse error in pattern](https://stackoverflow.com/questions/8561762/haskell-parse-error-in-pattern) — Chris Martin, Sep 06 '17 at 08:02

thefourtheye · Accepted Answer · 2015-01-11T09:18:00.250

6

You just have to surround the unpacked tuple and the rest of the list, like this

inverse ((x, y):xs) = (y, x) : inverse xs

Apart from this, you can use the Data.Tuple package's swap function, like this

Prelude> import Data.Tuple
Prelude Data.Tuple> map swap [(1, 2), (3, 4)]
[(2,1),(4,3)]

edited Jan 11 '15 at 09:18

answered Jan 11 '15 at 09:10

thefourtheye

1 Answers1