New monads/MonadUnique

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From New monads, copied from old wiki.

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


{-# OPTIONS_GHC -fglasgow-exts #-}

module MonadUnique
        ( UniqueT,
          evalUnique )

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)


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)