UnsafeMaybe.hs

{-# LANGUAGE NoImplicitPrelude #-}
{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}
module Unsafe.Maybe where

import GHC.Prim (Addr#,nullAddr#,addrToAny#,anyToAddr#,eqAddr#,unsafeCoerce#)
import GHC.Magic (runRW#)
import GHC.Types (Any)
import Unsafe.Coerce (unsafeCoerce)

newtype Maybe a = UnsafeMaybe Any

just :: a -> Maybe a
just a = UnsafeMaybe (unsafeCoerce a)

nothing :: Maybe a
nothing = case addrToAny# nullAddr# of
  (# m #) -> UnsafeMaybe m

maybe :: b -> (a -> b) -> Maybe a -> b
maybe n f (UnsafeMaybe pt) =
  case runRW# (anyToAddr# pt) of
    (# _, addr# #) -> case eqAddr# nullAddr# addr# of
      0# -> case addrToAny# addr# of
        (# a #) -> f a
      _ -> n