send + more = money

send-more-money-naiive.hs

module SendMoreMoney where

import Control.Monad
import Control.Monad.Trans.State

digits :: [Int]
digits = [0..9]

select :: StateT [a] [] a
select = StateT go
  where
    go :: [a] -> [(a, [a])]
    go [] = []
    go (x:xs) = (x,xs) : do
        (y,ys) <- go xs
        return (y,x:ys)

val :: [Int] -> Int
val = foldl (\acc n -> acc*10 + n) 0

--   S E N D
-- + M O R E
-- ---------
-- M O N E Y
sendMoreMoney :: StateT [Int] [] (Int, Int, Int)
sendMoreMoney = do
    s <- select
    e <- select
    n <- select
    d <- select
    m <- select
    o <- select
    r <- select
    y <- select

    let send = val [s,e,n,d]
        more = val [m,o,r,e]
        money = val [m,o,n,e,y]

    guard (s /= 0)
    guard (m /= 0)
    guard (send + more == money)

    return (send, more, money)