In Haskell, how can I use the built in sortBy function to sort a list of pairs(tuple)?

July 30, 2023 by Tarik

You need to construct your function sortGT, so that it compares pairs the way you want it:

sortGT (a1, b1) (a2, b2)
  | a1 < a2 = GT
  | a1 > a2 = LT
  | a1 == a2 = compare b1 b2

Using this you get the following results (I used ghci):

*Main Data.List> sortBy sortGT [(1, "b"), (1, "a"), (2, "b"), (2, "a")]
[(2,"a"),(2,"b"),(1,"a"),(1,"b")]