CODEWITHSUNDEEP
haskell
user3231622
user3231622

Reputation: 341

Convert list op tuples into two lists of one-elemnt tuple

In haskell I'm trying to acheive to convert a list like [(1,2),(3,4),(5,6)] into two lists: [1,3,5] and [2,4,6] using a recursive function only

I tried this so far, but it doesn't work

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

however, when I only try to make a list of the first value in the tuples, it does work:

unpair [] = []  
unpair ((x,y):xs) = x : unpair(xs)

Any idea what I'm doing wrong?

Upvotes: 0

Views: 317

Answers (1)

Sassa NF
Sassa NF

Reputation: 5406

The type of unpair xs is ([a], [b]), so you can't append an element to it like to a list. But you can use let statement to pattern-match the result from unpair xs, then construct the tuple with the new head element added to each.

I won't quote the actual answer because this looks like home work.

The reason the second implementation works is because in that one the type of unpair is [(a,b)]->[a].

Upvotes: 4

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