How to simplified this function in Haskell?

Question

I think writing code in this way is redundant. Regardless of what the type constructors are, the return values are all the same. Is there a way to write the return values once for all?

data End = Leftend (Int,Int) | Rightend (Int, Int)
            deriving (Eq, Ord, Show)


cmp:: End->End->Ordering
cmp (Leftend (l, h1))  (Rightend (r,h2))
        | l < r = LT
        | l == r = EQ
        | l > r = GT
cmp (Leftend (l, h1))  (Leftend (r,h2))
        | l < r = LT
        | l == r = EQ
        | l > r = GT
cmp (Rightend (l, h1))  (Rightend (r,h2))
        | l < r = LT
        | l == r = EQ
        | l > r = GT
cmp (Rightend (l, h1))  (Leftend (r,h2))
        | l < r = LT
        | l == r = EQ
        | l > r = GT

This isn't exactly a duplicate, but treads a lot of the same ground as stackoverflow.com/questions/32158110/…. — amalloy
– amalloy, Commented May 9, 2020 at 6:15
I propose refactoring your data declaration to something like data Side = Left | Right; data End = End { side :: Side, width :: Int, height :: Int } or similar. Then cmp = comparing width, and is probably not even worth giving a name to. — Daniel Wagner
– Daniel Wagner, Commented May 9, 2020 at 6:18
For starters, this pattern | l < r = LT | l == r = EQ | l > r = GT that keeps appearing is just compare l r — luqui
– luqui, Commented May 9, 2020 at 6:55

Daniel Wagner · Accepted Answer · 2020-05-09 06:14:41Z

11

I guess...

import Data.Ord

discard :: End -> (Int, Int)
discard (Leftend v) = v
discard (Rightend v) = v

cmp :: End -> End -> Ordering
cmp = comparing (fst . discard)

answered May 9, 2020 at 6:14

Daniel Wagner

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