New monads/MonadSupply
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From New monads, copied from [http://haskell.org/hawiki/MonadSupply old wiki] 
From New monads, copied from [http://haskell.org/hawiki/MonadSupply old wiki] 

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−  = MonadSupply = 

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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 nonMonadZero monad will cause an error. 
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 nonMonadZero monad will cause an error. 
Revision as of 03:34, 5 November 2006
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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 nonMonadZero monad will cause an error.
{# OPTIONS_GHC fglasgowexts #} 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