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
– AJF, Commented Jan 11, 2015 at 10:32

thefourtheye · Accepted Answer · 2015-01-11 09:18:00Z

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)]

answered Jan 11, 2015 at 9:10

thefourtheye

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