Foreign.Var

Variables

data Var a

This data type represents a piece of mutable, imperative state with possible side-effects. These tend to encapsulate all sorts tricky behavior in external libraries, and may well throw exceptions.

Inhabitants should satsify the following properties.

In the absence of concurrent mutation from other threads or a thrown exception:

do x <- getVar v; setVar v y; setVar v x

should restore the previous state.

Ideally, in the absence of thrown exceptions:

setVar v a >> getVar v

should return a, regardless of a. In practice some Vars only permit a very limited range of value assignments, and do not report failure.

The result of updateVar should also be compatible with the result of getting and setting separately, however, it may be more efficient or have better atomicity properties in a concurrent setting.

Constructors

Var

Fields

getVar :: IO a

Used by get

updateVar :: (a -> a) -> IO ()

Used by ($~)

updateVar' :: (a -> a) -> IO ()

Used by ($~!)

setVar :: a -> IO ()

Used by ($=)

Instances

HasGetter (Var a) a

HasSetter (Var a) a

HasUpdate (Var a) a a

Typeable (* -> *) Var

newVar

Arguments

:: IO a

getter

-> (a -> IO ())

setter

-> Var a

Build a Var form a getter and a setter.

mapVar :: (b -> a) -> (a -> b) -> Var a -> Var b

Change the type of a Var

newtype SettableVar a

Constructors

SettableVar (a -> IO ())

Instances

HasSetter (SettableVar a) a

Typeable (* -> *) SettableVar

type GettableVar = IO

Classes

class HasSetter t a | t -> a where

Methods

($=) :: MonadIO m => t -> a -> m () infixr 2

Instances

Storable a => HasSetter (Ptr a) a

HasSetter (TVar a) a

HasSetter (IORef a) a

HasSetter (SettableVar a) a

HasSetter (Var a) a

($=!) :: (HasSetter t a, MonadIO m) => t -> a -> m () infixr 2

class HasSetter t a => HasUpdate t a b | t -> a b where

Minimal complete definition

Nothing

Methods

($~) :: MonadIO m => t -> (a -> b) -> m () infixr 2

($~!) :: MonadIO m => t -> (a -> b) -> m () infixr 2

Instances

Storable a => HasUpdate (Ptr a) a a

HasUpdate (TVar a) a a

HasUpdate (IORef a) a a

HasUpdate (Var a) a a

class HasGetter t a | t -> a where

Methods

get :: MonadIO m => t -> m a

Instances

HasGetter (IO a) a

Storable a => HasGetter (Ptr a) a

HasGetter (STM a) a

HasGetter (TVar a) a

HasGetter (IORef a) a

HasGetter (Var a) a