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

Question

I am a beginner in Haskell so please bear with me. (Just started learning yesterday!) How can I sort a list of tuples primarily by their first components (highest to smallest) and secondarily by their second components (smallest to highest)? A sample input/output would be:

[(1, "b"), (1, "a"), (2, "b"), (2, "a")] (input)

[(1, "a"), (2, "a"), (1, "b"), (2, "b")] (middle step)

[(2, "a"), (2, "b"), (1, "a"), (1, "b")] (output)

I tried using the following but it gave wrong output:

sortGT a b = GT

sortBy sortGT lst

I am sure that I can do this by using sortBy only, but I can't figure it out myself. Any help would be highly appreciated!

question from:https://stackoverflow.com/questions/2349798/in-haskell-how-can-i-use-the-built-in-sortby-function-to-sort-a-list-of-pairst

Answer 1

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")]