# New monads/MonadSupply

From HaskellWiki

From New monads, copied from old wiki

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.State
newtype SupplyT s m a = SupplyT (StateT [s] m a)
deriving (Functor, Monad, MonadTrans, MonadIO)
newtype Supply s a = Supply (SupplyT s Maybe 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
```