Convert List of Tuples to List of Lists Haskell

Question

I have [("m","n"),("p","q"),("r","s")]. How can I convert it to [["m","n"],["p","q"],["r","s"]]?

Can anyone please help me? Thanks.

Does this answer your question? Haskell List of tuples to list? — Bergi
– Bergi, Commented Jan 14, 2021 at 16:41

Zeta · Accepted Answer · 2017-12-22 11:46:35Z

19

Write a single function to convert a pair to a list:

pairToList :: (a, a) -> [a]
pairToList (x,y) = [x,y]

Then you only have to map pairToList:

tuplesToList :: [(a,a)] -> [[a]]
tuplesToList = map pairToList

Or in a single line:

map (\(x,y) -> [x,y])

answered Dec 8, 2013 at 2:43

Zeta

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2 Comments

Newbie question - what's the \ for?

@AbhijitSarkar Thats just syntax for defining a lambda.

András Kovács · Accepted Answer · 2013-12-08 10:18:46Z

14

Using lens you can do this succinctly for arbitrary length homogenous tuples:

import Control.Lens

map (^..each) [("m","n"),("p","q"),("r","s")] -- [["m","n"],["p","q"],["r","s"]]
map (^..each) [(1, 2, 3)] -- [[1, 2, 3]]

Note though that the lens library is complex and rather beginner-unfriendly.

answered Dec 8, 2013 at 10:18

András Kovács

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Ankur · Accepted Answer · 2013-12-08 06:49:09Z

10

List comprehension version:

[[x,y] | (x,y) <- [("m","n"),("p","q"),("r","s")]]

answered Dec 8, 2013 at 6:49

Ankur

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