Programming Praxis – Fortune

In today’s Programming Praxis exercise, our goal is to implement the fortune Unix command line tool. Let’s get started, shall we?

Some imports:

import Control.Monad
import System.Environment
import System.Random

Since we wrote some code in a previous exercise to select a random item from a list we can just use that here again:

chance :: Int -> Int -> a -> a -> IO a
chance x y a b = fmap (\r -> if r < x then a else b) $ randomRIO (0, y-1)

fortune :: [a] -> IO a
fortune = foldM (\a (n,x) -> chance 1 n x a) undefined . zip [1..]

All that’s left to do to implement the fortune program is to read the appropriate file and choose a random line.

main :: IO ()
main = putStrLn =<< fortune . lines =<<
       readFile . head . (++ ["fortunes.txt"]) =<< getArgs

Four lines versus the 23 of the C version. Not bad.

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