Control.Monad.Supply

Computation type:

Computations that require a supply of values.

Binding strategy:

Applicative values are functions that consume an input from a supply to produce a value.

Useful for:

Providing a supply of unique names or other values to computations needing them.

Zero and plus:

Identical to the underlying implementations (if any) of empty, <|>, mzero and mplus.

The Supply s a monad represents a computation that consumes a supply of s's to produce a value of type a. One example use is to simplify computations that require the generation of unique names. The Supply monad can be used to provide a stream of unique names to such a computation.

The MonadSupply Class

class Monad m => MonadSupply s f m | m -> s, m -> f where

The MonadSupply class provides access to the functions needed to construct supply-consuming computations in a monad transformer stack.

Methods

supply :: (s -> f a) -> m a

Supply a construction function with an s value from the supply, the f in the function's type refers to the monad wrapped by SupplyT.

provide :: (s -> a) -> m a

Provide a non-monadic construction function with an s value from the supply and automatically lift its result into the f monad that SupplyT wraps.

Instances

MonadSupply s f m => MonadSupply s f (MaybeT m)

MonadSupply s f m => MonadSupply s f (ListT m)

MonadSupply s f m => MonadSupply s f (IdentityT m)

(Monoid w, MonadSupply s f m) => MonadSupply s f (WriterT w m)

(Monoid w, MonadSupply s f m) => MonadSupply s f (WriterT w m)

MonadSupply s f m => MonadSupply s f (StateT s m)

MonadSupply s f m => MonadSupply s f (StateT s m)

MonadSupply s f m => MonadSupply s f (ReaderT r m)

(Error e, MonadSupply s f m) => MonadSupply s f (ErrorT e m)

MonadSupply s f m => MonadSupply s f (ContT r m)

(Functor m, Monad m) => MonadSupply s m (SupplyT s m)

(Monoid w, MonadSupply s f m) => MonadSupply s f (RWST r w s m)

(Monoid w, MonadSupply s f m) => MonadSupply s f (RWST r w s m)

demand :: MonadSupply s f m => m s

Demand an s value from the supply.

The Supply Monad

type Supply s a = SupplyT s Identity a

The Supply monad.

Computations consume values of type s from a supply of values.

return ignores the supply of values, while >>= passes the supply to the second argument after the first argument is done consuming values.

withSupply :: (s' -> s) -> Supply s a -> Supply s' a

Change the type of values consumed by a Supply computation.

runSupply :: Supply s a -> (s -> s) -> s -> a

Run a supply consuming computation, using a generation function and initial value to compute the values consumed by the Supply computation.

runListSupply :: Supply s a -> [s] -> Either (Supply s a) a

Feed a supply consuming computation from a list until the computation finishes or the list runs out. If the list does not contain sufficient elements, runListSupply returns uncompleted computation.

runMonadSupply :: Monad m => Supply s a -> m s -> m a

Feed a supply consuming computation from a monadic action until the computation finishes.

The Supply Monad Transformer

data SupplyT s m a

The Supply transformer.

Composes Supply with an underlying monad, allowing it to be used monad in transformer stacks.

The resulting SupplyT value has Alternative and MonadPlus instances if the underlying monad has such these instances.

Instances

(Functor m, Monad m) => MonadSupply s m (SupplyT s m)

MonadTrans (SupplyT s)

(Functor m, Monad m) => Monad (SupplyT s m)

Functor f => Functor (SupplyT s f)

(Functor m, MonadPlus m) => MonadPlus (SupplyT s m)

(Functor m, Monad m) => Applicative (SupplyT s m)

(Alternative m, Monad m) => Alternative (SupplyT s m)

(Functor m, MonadIO m) => MonadIO (SupplyT s m)

withSupplyT :: Functor f => (s' -> s) -> SupplyT s f a -> SupplyT s' f a

Change the type of values consumed by a SupplyT computation.

runSupplyT :: Monad m => SupplyT s m a -> (s -> s) -> s -> m a

Run a supply consuming computation, using a generation function and initial value to compute the values consumed by the SupplyT computation.

runListSupplyT :: Monad m => SupplyT s m a -> [s] -> m (Either (SupplyT s m a) a)

Feed a supply consuming computation from a list until the computation finishes or the list runs out. If the list does not contain sufficient elements, runListSupplyT returns uncompleted computation.

runMonadSupplyT :: Monad m => SupplyT s m a -> m s -> m a

Feed a supply consuming computation from a monadic action until the computation finishes.

module Control.Monad

module Control.Monad.Trans