transform_with_group.hs

{-
The file is based on: 
https://ocharles.org.uk/blog/guest-posts/2014-12-07-list-comprehensions.html
-}

{-# LANGUAGE TransformListComp, RecordWildCards #-}
import GHC.Exts (groupWith, the)
import Data.Ord (comparing)
import Data.List (sortBy)

{-
The group syntax is

then group by <proj_func> [using <group_func>]

"group_func" is via which the grouping is made. It has a type signature of "Ord b => (a -> b) -> [a] -> [[a]]. "proj_func" is the first argument of "group_func" and this is the part of the input structure via which the grouping is made.
-}

data Character = Character {
    first_name :: String,
    last_name :: String,
    birth_year :: Int
    } deriving (Show, Eq)

friends :: [Character]
friends = [
    Character "Bubba"   "Johnes"    1975,
    Character "Daniel"  "Papp"      1983,
    Character "Uh"      "Oh"        1983,
    Character "Joe"     "Smith"     1975,
    Character "Osama"   "Bin Laden" 1955]

-- Sorts based on the total length of strings in the group
groupFunc f = sortBy (comparing (length . concatMap last_name)) . groupWith f

stuff :: [Character] -> [(Int, [String])]
stuff tbl = [(the birth_year, last_name)  -- Just notice the "the" function in front of birth_year
            | Character{..} <- tbl,
              then group by birth_year using groupFunc
            ]

main = print $ stuff friends