Tuple function in Haskell

2
votes

I have the function

getCode :: [(a, Int)] -> [a]
getCode = concatMap (uncurry replicate)`

I expect 

getCode [(‘a’,4),(‘b’,1),(‘a’,3),(‘b’,1)]

to output

“aaaabaaab”

Instead I get an error that it does not match with the expected data type [(a,Int)], and the actual data type is [(Int,a)]. How do i change it to make it work? Or any other way?

1
listhaskelltupleshigher-order-functions

1 Answers

2
votes

You can swap the parameters of replicate with the flip function:

flip :: (a -> b -> c) -> b -> a -> c

flip f takes its (first) two arguments in the reverse order of f.

getCode :: [(a, Int)] -> [a]
getCode = concatMap (uncurry (flip replicate))

Or use swap from Data.Tuple to do the same thing with a tuple instead of function parameters:

getCode :: [(a, Int)] -> [a]
getCode = concatMap (uncurry replicate . swap)