PatternSynonyms example

fib.hs

{-# LANGUAGE ViewPatterns #-}
{-# LANGUAGE PatternSynonyms #-}
import System.Environment
import Data.Ratio
import Control.Applicative ((<$>))

viewRatio x = (numerator x, denominator x)
pattern p :% q <- (viewRatio -> (p, q))

main = do
    [p, q] <- map read <$> getArgs
    let x = p % q
    putStr $ show x ++ ": "
    mapM_ (putStr . (++" ") . show) $ fib x
    putStrLn ""

fib   (1 :% q) = [q]
fib x@(p :% q) = let s = q `div` p + 1
                 in s : fib (x - (1 % s))