What does Data.Semigroup ((<>)) do in this Haskell sort code?

Question

I know that the program can sort the list by Mfg or by Year(descending). That is defined on line 9 and 10. But what exactly is the code doing? What is (<>)?

import Data.Semigroup ((<>))
import Data.List (sort, sortOn)
import Data.Ord (comparing)

data Vehicle = Vehicle { mfg :: String, year  :: Int } deriving (Eq, Show)
mycars =[Vehicle "Saab" 2000, Vehicle "Volvo" 1995]

instance Ord Vehicle where
          compare = flip (comparing year) <> comparing mfg

main = do
          print $ sortOn mfg mycars
          print $ sort mycars

Unrelated to your question, but you're missing indentation before `compare =`. As a result, instead of defining that method in your `Ord` instance, you're creating a broken `Ord` instance and shadowing `compare` at top-level. — Joseph Sible-Reinstate Monica, Apr 06 '20 at 03:05
`comparing (Down . year)` might be more suggestive of the intended sorting order than `flip (comparing year)`. — chepner, Apr 06 '20 at 13:28
[cf.](https://stackoverflow.com/questions/11486436/composing-two-comparison-functions) — Will Ness, Apr 06 '20 at 15:53

score 4 · Accepted Answer · answered Apr 06 '20 at 03:00

import Data.Semigroup ((<>)) doesn't do anything useful in your program, and hasn't since GHC 7.10 was released in 2015. Before that, it brought the <> operator into scope so that compare = flip (comparing year) <> comparing mfg can use it. In GHC 7.10, that operator was added to the Prelude, so it's now always in scope even without importing it.

As for what <> does there, you're using it at the type Vehicle -> Vehicle -> Ordering. It comes from the Semigroup b => Semigroup (a -> b) instance (twice) and the Semigroup Ordering instance. The net effect is that after applying both Vehicles to it, it will use the result of the left-hand side comparison on them (flip (comparing year)), unless it's EQ, in which case it will use the right-hand side comparison (comparing mfg) on them instead. If you were to write out by hand exactly what it were doing, it would be this:

compare x y = case flip (comparing year) x y of
  LT -> LT
  EQ -> comparing mfg x y
  GT -> GT

What does Data.Semigroup ((<>)) do in this Haskell sort code?

1 Answers1