New monads/MonadUnique

From NewMonads, copied from old wiki.

MonadUnique

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

"CaleGibbard/BSDLicense"

{-# 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)