Simple program that calculates the number of possible phone numbers of length 4 that have 2 or more equal digits, and displays them.

at_least_two_equal_digits.hs

import Data.Map (Map)
import qualified Data.Map as Map

countDistinct :: Ord a => [a] -> Int
countDistinct = Map.size . foldr fn Map.empty
    where fn e = Map.insertWith (+) e 1

validStrings :: [String]
validStrings = 
    filter ((`elem` [1..3]) . countDistinct) $ do
    a <- ['0'..'9']
    b <- ['0'..'9']
    c <- ['0'..'9']
    d <- ['0'..'9']
    return $ a:b:c:d:[]

main :: IO ()
main = do
    putStrLn $ "Total: " ++ (show . length) validStrings
    putStrLn "List:"
    mapM_ (putStrLn . ("    "++)) validStrings