Haskell representation of F#-style Active Patterns

active.hs

-- https://www.reddit.com/r/haskell/comments/3huexy/what_are_haskellers_critiques_of_f_and_ocaml/cuisrmj

{-# LANGUAGE PatternSynonyms #-}
{-# LANGUAGE ViewPatterns #-}

module Main where

import Text.Read (readMaybe)

pattern Even <- ((`mod` 2) -> 0)
pattern Odd <- ((`mod` 2) -> 1)

f :: Int -> String
f n = case n of
    Even -> show n ++ " is even"
    Odd -> show n ++ " is odd"
    -- For totality: _ -> error "Impossible"

pattern IntLiteral :: Int -> String
pattern IntLiteral i <- (readMaybe -> Just i)

g :: String -> String
g n = case n of
    IntLiteral i -> show i ++ " is a valid int literal"
    _ -> n ++ " is an invalid int literal"

main :: IO ()
main = do
    mapM_ (putStrLn . f) [0 .. 3]
    putStrLn $ g "123"
    putStrLn $ g "abc"