Memorable random words from a dictionary

passwords.hs

module Main (main) where
import Control.Monad

import Data.Char

import System.IO
import System.Environment
import System.Random
import System.Exit

infixr 3 &?

-- |Composites two predicates together using AND
andPred :: (a -> Bool) -> (a -> Bool) -> a -> Bool
andPred p q n = (p n) && (q n)

-- |Infix version of andPred
(&?) = andPred


-- |Predicate which words must satisfy to be chosen
wordPred :: String -> Bool
wordPred = ((< 10) . length)  &? -- Short words
           (not . elem '\'')  &? -- Don't have apostrophes
           (all isAsciiLower) &? -- Contain only lower-case ASCII
           ((/= 's') . last)     -- Are not plural

main :: IO ()
main = do
    -- Validate args
    args <- getArgs
    when ((length args) < 2) $ do
      prog <- getProgName
      hPutStrLn stderr $ "Usage: " ++ prog ++ " <path-to-dict> <numwords>"
      exitWith =<< exitFailure

    -- First argument, specifying number of words to choose
    let (path:words:_) = args
    let numWords = read words :: Int
    gen <- getStdGen
    dict <- openFile path ReadMode

    -- Words in the dictionary
    words <- (liftM lines) . hGetContents $ dict
    
    -- Infinite list of random selections from dictionary
    let rs = filter wordPred . map (words !!) $
             randomRs (0, (length words) - 1) gen

    mapM_ putStrLn . take numWords $ rs