Control.Monad.StateVar

Overloaded get and put for state variables (IORef, TVar, etc.) to make reading and writing more concise.

Overloaded get and put

class HasGet m v where

Methods

get :: v a -> m a

Read the value from the variable.

Instances

HasGet IO IORef

readIORef

HasGet STM TVar

readTVar

(HasGet m v, MonadTrans t, Monad m) => HasGet (t m) v

lift $ get v

HasGet (ST s) (STRef s)

readSTRef

HasGet (ST s) (STRef s)

readSTRef

class HasPut m v where

Methods

put :: v a -> a -> m ()

Write a new value to the variable.

Instances

HasPut IO IORef

writeIORef

HasPut STM TVar

writeTVar

(HasPut m v, MonadTrans t, Monad m) => HasPut (t m) v

lift $ put v a

HasPut (ST s) (STRef s)

writeSTRef

HasPut (ST s) (STRef s)

writeSTRef

put' :: HasPut m v => v a -> a -> m ()

Variant of put that forces the value before writing it.

modify :: (HasGet m v, HasPut m v, Monad m) => v a -> (a -> a) -> m ()

Modify the value inside the variable with the given function.

list <- newIORef [1,2,3]
modify list (0:)  -- prepend 0 to the list

modify' :: (HasGet m v, HasPut m v, Monad m) => v a -> (a -> a) -> m ()

Variant of modify that forces the result of the function.

tally <- newIORef (0 :: Int)
modify tally (+ 10)  -- add 10 to the tally

swap :: (HasGet m v, HasPut m v, Monad m) => v a -> a -> m a

Write a new value and return the old value.

Infix aliases

($=) :: HasPut m v => v a -> a -> m ()

Infix alias for put

($~) :: (HasGet m v, HasPut m v, Monad m) => v a -> (a -> a) -> m ()

Infix alias for modify

($=!) :: HasPut m v => v a -> a -> m ()

Infix alias for put'

($~!) :: (HasGet m v, HasPut m v, Monad m) => v a -> (a -> a) -> m ()

Infix alias for modify'