Chainable.hs

{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances #-}
module Chainable (
  (==?), (/=?),
  (<?), (<=?), (>?), (>=?),
  isJust
) where

import Data.Maybe (isJust)
import Control.Monad (liftM, liftM2)

infixl 4 ==?
infixl 4 /=?
(==?),(/=?) :: (Eq b) => Maybe b -> b -> Maybe b
Just x ==? y    = if x == y then Just y else Nothing
Nothing ==? _   = Nothing
Just x /=? y    = if x /= y then Just y else Nothing
Nothing /=? _   = Nothing

infixl 4 <?
infixl 4 >?
infixl 4 <=?
infixl 4 >=?
(<?),(>?),(<=?),(>=?) :: (Ord b) => Maybe b -> b -> Maybe b
Just x <? y     = case compare x y of LT -> Just y ; _ -> Nothing
Nothing <? _    = Nothing
Just x >? y     = case compare x y of GT -> Just y ; _ -> Nothing
Nothing >? _    = Nothing
Just x <=? y    = case compare x y of GT -> Nothing; _ -> Just y
Nothing <=? _   = Nothing
Just x >=? y    = case compare x y of LT -> Nothing; _ -> Just y
Nothing >=? _   = Nothing

-- convenience instance so that we don't need to do 
--  Just 0 <=? x <? 100
instance Num a => Num (Maybe a) where
  (+) = liftM2 (+)
  (*) = liftM2 (*)
  (-) = liftM2 (-)
  negate = liftM negate
  abs = liftM abs
  signum = liftM signum
  fromInteger = Just . fromInteger