Prints out every dictionary word that could be a FCC ham call sign by replacing the numbers 1,3,4,0 with the letter I,E,A,O

callwords.hs

import qualified Data.Char as C
import qualified Data.Set as S

main :: IO ()
main = do
    let alpha = ['A' .. 'Z']
        leetRegionNum = "IEAO"
        oneByOnes =
            [[a, n, b] | a <- "KNW"
                       , n <- leetRegionNum
                       , b <- alpha]
        oneByTwos =
            [a ++ [b] | a <- oneByOnes
                      , b <- alpha]
        oneByThrees =
            [a ++ [b] | a <- oneByTwos
                      , b <- alpha]
        twoByOnes =
            [[a, b, n, c] | a <- "AKNW"
                          , b <- alpha
                          , n <- leetRegionNum
                          , b `S.notMember`
                                if a == 'A'
                                    then (S.fromList "HLMNOPQRSTUVWXYZ")
                                    else (S.fromList "HLP")
                          , c <- alpha]
        twoByTwos =
            [a ++ [b] | a <- twoByOnes
                      , b <- alpha]
        twoByThrees =
            [a ++ [b] | a <- twoByTwos
                      , b <- alpha]
        calls =
            S.fromList
                (concat
                     [ oneByOnes
                     , oneByTwos
                     , oneByThrees
                     , twoByOnes
                     , twoByTwos
                     , twoByThrees])
    dict <-
        pure . S.fromList . lines . map C.toUpper . filter (/= '\'') =<<
        readFile "/usr/share/dict/american-english"
    mapM_ putStrLn (S.intersection calls dict)

Output looks like:

ABACI
ABACK
ABACUS
ABAFT
ABASE
ABASED
ABASES
ABASH
ABATE
ABATED