Haskell: Combine integers in a list of tuples [duplicate]

问题

This question already has answers here:

How to group similar items in a list using Haskell? (5 answers)

Closed 4 years ago.

I need to get from here:

[(2,"a"), (1,"a"), (1,"b"), (1,"c"), (2,"dd")]

to here:

[([1, 2], "a"), ([1], "b"), ([1], "c"), ([2], "dd")]

So far I have

combineInts listTuple = someFunc (map (\(num, str) -> ([num], str)) listTuple)

where "someFunc" is the bit I still need to figure out and implement. I believe it should utilize foldr, map, and/or intercalate to accomplish my goal. Any ideas?

回答1:

This should do:

import Data.Function (on)
import Data.List (groupBy, sort, sortBy)

out = map f . groupBy ((==) `on` snd) . sortBy (compare `on` snd) $ input
  where input = [(2,"a"),(1,"a"),(1,"b"),(1,"c"),(2,"dd")]
        f xs@(x:_) = (sort $ map fst xs, snd x)

main = print out

回答2:

You can do this using groupBy if you order the list first:

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

map (\l@((_,c):_) -> (map fst l, c))  $ groupBy (on (==) snd) $ sortBy (comparing snd) inputList

来源：https://stackoverflow.com/questions/32769930/haskell-combine-integers-in-a-list-of-tuples