Question

I got a custom Data type named Student which has marks for 2 subjects. I have created a function named average that calculates the average of two. All work fine.

My question is how can I sort list of students by their average?

data Student = Student
    {studentName :: String,
     subject1 :: Double,
     subject2 :: Double} deriving (Show)

average :: Student -> Double
average (Student _ sub1 sub2) = (sub1 + sub2) / 2

students :: [Student]
students = [Student "Dave"  50.0  40.0,
            Student "Joe"   65.0  90.0,
            Student "Ann"   75.0  82.0]

P.S. I am a beginner in Haskell and don’t know whether its got a
inbuilt average function but I prefer if I can sort my list in similar
way without using inbuilt average function (if it is there) as this
small test solution to be use with a different type of function
instead average.

Answer 1

import Data.Function (on)
import Data.List (sortBy)

studentsSortedByAverage = sortBy (compare `on` average) students

Note that those are backticks around on, not single quotes.

Here are links to docs for sortBy and on.

---

If you are using an old compiler that doesn’t come with Data.Function, here is the definition of on:

on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
(.*.) `on` f = \x y -> f x .*. f y