# New monads/MonadSupply

### From HaskellWiki

Here is a simple monad/monad transformer for computations which consume values from a (finite or infinite) supply. Note that due to pattern matching, running out of supply in a non-MonadZero monad will cause an error.

{-# OPTIONS_GHC -fglasgow-exts #-} module MonadSupply (SupplyT, MonadSupply, supply, Supply, evalSupplyT, evalSupply, runSupplyT, runSupply) where import Control.Monad import Control.Monad.Identity import Control.Monad.State newtype SupplyT s m a = SupplyT (StateT [s] m a) deriving (Functor, Monad, MonadTrans, MonadIO) newtype Supply s a = Supply (SupplyT s Identity a) deriving (Functor, Monad, MonadSupply s) class Monad m => MonadSupply s m | m -> s where supply :: m s instance Monad m => MonadSupply s (SupplyT s m) where supply = SupplyT $ do (x:xs) <- get put xs return x evalSupplyT (SupplyT s) supp = evalStateT s supp evalSupply (Supply s) supp = evalSupplyT s supp runSupplyT (SupplyT s) supp = runStateT s supp runSupply (Supply s) supp = runSupplyT s supp

As an example, if we want to supply unique strings for use as variable names, the following specialisation of runSupply (or its obvious analogue with runSupplyT) might be handy:

runSupplyVars x = runSupply x vars where vars = [replicate k ['a'..'z'] | k <- [1..]] >>= sequence