# New monads/MonadUnique

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## Latest revision as of 07:33, 10 December 2006

From New monads, copied from old wiki.

This is a simple (trivial) monad transformer for supplying unique integer values to an algorithm.

## [edit] 1 MonadUnique

{-# OPTIONS_GHC -fglasgow-exts #-} module MonadUnique ( UniqueT, Unique, MonadUnique, fresh, evalUniqueT, evalUnique ) where import Control.Monad import Control.Monad.State import Control.Monad.Identity newtype UniqueT m a = UniqueT (StateT Integer m a) deriving (Functor, Monad, MonadTrans, MonadIO) newtype Unique a = Unique (UniqueT Identity a) deriving (Functor, Monad, MonadUnique) class Monad m => MonadUnique m where fresh :: m Integer instance (Monad m) => MonadUnique (UniqueT m) where fresh = UniqueT $ do n <- get put (succ n) return n evalUniqueT (UniqueT s) = evalStateT s 0 evalUnique (Unique s) = runIdentity (evalUniqueT s)

## [edit] 2 STSupply

There is also a simple way to get the same functionality in the ST Monad. Here's a quick module to construct infinite supplies of unique values in the ST monad:

module STSupply (Unique, createSupply) where import Control.Monad.ST import Data.STRef newtype Unique = Unique Integer deriving (Eq, Ord) createSupply :: ST s (ST s Unique) createSupply = do v <- newSTRef $ Unique 0 return $ do Unique x <- readSTRef v writeSTRef v $ Unique (x+1) return $ Unique x

A test example:

import Control.Monad.ST import STSupply main = print test1 test1 = runST supplyTest where supplyTest = do fresh <- createSupply x <- fresh y <- fresh return (x == x, x == y, x < y)