{-# LANGUAGE Trustworthy #-}

-----------------------------------------------------------------------------
-- |
-- Module      :  Data.Unique
-- Copyright   :  (c) The University of Glasgow 2001
-- License     :  BSD-style (see the file libraries/base/LICENSE)
--
-- Maintainer  :  libraries@haskell.org
-- Stability   :  experimental
-- Portability :  non-portable
--
-- An abstract interface to a unique symbol generator.
--
-----------------------------------------------------------------------------

module Data.Unique (
   -- * Unique objects
   Unique,
   newUnique,
   hashUnique
 ) where

import System.IO.Unsafe (unsafePerformIO)

import GHC.Num
import Data.IORef

-- $setup
-- >>> import Prelude

-- | An abstract unique object.  Objects of type 'Unique' may be
-- compared for equality and ordering and hashed into 'Int'.
--
-- >>> :{
-- do x <- newUnique
--    print (x == x)
--    y <- newUnique
--    print (x == y)
-- :}
-- True
-- False
newtype Unique = Unique Integer deriving (Unique -> Unique -> Bool
(Unique -> Unique -> Bool)
-> (Unique -> Unique -> Bool) -> Eq Unique
forall a. (a -> a -> Bool) -> (a -> a -> Bool) -> Eq a
/= :: Unique -> Unique -> Bool
$c/= :: Unique -> Unique -> Bool
== :: Unique -> Unique -> Bool
$c== :: Unique -> Unique -> Bool
Eq,Eq Unique
Eq Unique
-> (Unique -> Unique -> Ordering)
-> (Unique -> Unique -> Bool)
-> (Unique -> Unique -> Bool)
-> (Unique -> Unique -> Bool)
-> (Unique -> Unique -> Bool)
-> (Unique -> Unique -> Unique)
-> (Unique -> Unique -> Unique)
-> Ord Unique
Unique -> Unique -> Bool
Unique -> Unique -> Ordering
Unique -> Unique -> Unique
forall a.
Eq a
-> (a -> a -> Ordering)
-> (a -> a -> Bool)
-> (a -> a -> Bool)
-> (a -> a -> Bool)
-> (a -> a -> Bool)
-> (a -> a -> a)
-> (a -> a -> a)
-> Ord a
min :: Unique -> Unique -> Unique
$cmin :: Unique -> Unique -> Unique
max :: Unique -> Unique -> Unique
$cmax :: Unique -> Unique -> Unique
>= :: Unique -> Unique -> Bool
$c>= :: Unique -> Unique -> Bool
> :: Unique -> Unique -> Bool
$c> :: Unique -> Unique -> Bool
<= :: Unique -> Unique -> Bool
$c<= :: Unique -> Unique -> Bool
< :: Unique -> Unique -> Bool
$c< :: Unique -> Unique -> Bool
compare :: Unique -> Unique -> Ordering
$ccompare :: Unique -> Unique -> Ordering
Ord)

uniqSource :: IORef Integer
uniqSource :: IORef Integer
uniqSource = IO (IORef Integer) -> IORef Integer
forall a. IO a -> a
unsafePerformIO (Integer -> IO (IORef Integer)
forall a. a -> IO (IORef a)
newIORef Integer
0)
{-# NOINLINE uniqSource #-}

-- | Creates a new object of type 'Unique'.  The value returned will
-- not compare equal to any other value of type 'Unique' returned by
-- previous calls to 'newUnique'.  There is no limit on the number of
-- times 'newUnique' may be called.
newUnique :: IO Unique
newUnique :: IO Unique
newUnique = do
  Integer
r <- IORef Integer -> (Integer -> (Integer, Integer)) -> IO Integer
forall a b. IORef a -> (a -> (a, b)) -> IO b
atomicModifyIORef' IORef Integer
uniqSource ((Integer -> (Integer, Integer)) -> IO Integer)
-> (Integer -> (Integer, Integer)) -> IO Integer
forall a b. (a -> b) -> a -> b
$ \Integer
x -> let z :: Integer
z = Integer
xInteger -> Integer -> Integer
forall a. Num a => a -> a -> a
+Integer
1 in (Integer
z,Integer
z)
  Unique -> IO Unique
forall (m :: * -> *) a. Monad m => a -> m a
return (Integer -> Unique
Unique Integer
r)

-- SDM (18/3/2010): changed from MVar to STM.  This fixes
--  1. there was no async exception protection
--  2. there was a space leak (now new value is strict)
--  3. using atomicModifyIORef would be slightly quicker, but can
--     suffer from adverse scheduling issues (see #3838)
--  4. also, the STM version is faster.

-- SDM (30/4/2012): changed to IORef using atomicModifyIORef.  Reasons:
--  1. STM version could not be used inside unsafePerformIO, if it
--     happened to be poked inside an STM transaction.
--  2. IORef version can be used with unsafeIOToSTM inside STM,
--     because if the transaction retries then we just get a new
--     Unique.
--  3. IORef version is very slightly faster.

-- IGL (08/06/2013): changed to using atomicModifyIORef' instead.
--  This feels a little safer, from the point of view of not leaking
--  memory, but the resulting core is identical.

-- | Hashes a 'Unique' into an 'Int'.  Two 'Unique's may hash to the
-- same value, although in practice this is unlikely.  The 'Int'
-- returned makes a good hash key.
hashUnique :: Unique -> Int
hashUnique :: Unique -> Int
hashUnique (Unique Integer
i) = Integer -> Int
integerToInt Integer
i