Question

Given a list of tuples like this:

dic = [(1,"aa"),(1,"cc"),(2,"aa"),(3,"ff"),(3,"gg"),(1,"bb")]

How to group items of dic resulting in a list grp where,

grp  = [(1,["aa","bb","cc"]), (2, ["aa"]), (3, ["ff","gg"])]

I’m actually a newcomer to Haskell…and seems to be falling in love with it..
Using group or groupBy in Data.List will only group similar adjacent items in a list.
I wrote an inefficient function for this, but it results in memory failures as I need to process a very large coded string list. Hope you would help me find a more efficient way.

Answer 1

Here’s my solution:

import Data.Function (on)
import Data.List (sortBy, groupBy)
import Data.Ord (comparing)

myGroup :: (Eq a, Ord a) => [(a, b)] -> [(a, [b])]
myGroup = map (\l -> (fst . head $ l, map snd l)) . groupBy ((==) `on` fst)
          . sortBy (comparing fst)

This works by first sorting the list with sortBy:

[(1,"aa"),(1,"cc"),(2,"aa"),(3,"ff"),(3,"gg"),(1,"bb")]     
=> [(1,"aa"),(1,"bb"),(1,"cc"),(2,"aa"),(3,"ff"),(3,"gg")]

then grouping the list elements by the associated key with groupBy:

[(1,"aa"),(1,"bb"),(1,"cc"),(2,"aa"),(3,"ff"),(3,"gg")] 
=> [[(1,"aa"),(1,"bb"),(1,"cc")],[(2,"aa")],[(3,"ff"),(3,"gg")]]

and then transforming the grouped items to tuples with map:

[[(1,"aa"),(1,"bb"),(1,"cc")],[(2,"aa")],[(3,"ff"),(3,"gg")]] 
=> [(1,["aa","bb","cc"]), (2, ["aa"]), (3, ["ff","gg"])]`)

Testing:

> myGroup dic
[(1,["aa","bb","cc"]),(2,["aa"]),(3,["ff","gg"])]