Data.StateVar

State variables are references in the IO monad, like IORefs or parts of the OpenGL state. Note that state variables are not neccessarily writable or readable, they may come in read-only or write-only flavours, too. As a very simple example for a state variable, consider an explicitly allocated memory buffer. This buffer can easily be converted into a StateVar:

 makeStateVarFromPtr :: Storable a => Ptr a -> StateVar a
 makeStateVarFromPtr p = makeStateVar (peek p) (poke p)

The example below puts 11 into a state variable (i.e. into the buffer), increments the contents of the state variable by 22, and finally prints the resulting content:

   do p <- malloc :: IO (Ptr Int)
      let v = makeStateVarFromPtr p
      v $= 11
      v $~ (+ 22)
      x <- get v
      print x

IORefs are state variables, too, so an example with them looks extremely similiar:

   do v <- newIORef (0 :: Int)
      v $= 11
      v $~ (+ 22)
      x <- get v
      print x

Readable State Variables

class HasGetter g where

The class of all readable state variables.

Methods

get :: g a -> IO a

Read the value of a state variable.

Instances

HasGetter IORef

HasGetter StateVar

HasGetter GettableStateVar

data GettableStateVar a

A concrete implementation of a read-only state variable, carrying an IO action to read the value.

Instances

HasGetter GettableStateVar

makeGettableStateVar :: IO a -> GettableStateVar a

Construct a GettableStateVar from an IO action.

Writable State Variables

class HasSetter s where

The class of all writable state variables.

Methods

($=) :: s a -> a -> IO ()

Write a new value into a state variable.

Instances

HasSetter IORef

HasSetter StateVar

HasSetter SettableStateVar

data SettableStateVar a

A concrete implementation of a write-only state variable, carrying an IO action to write the new value.

Instances

HasSetter SettableStateVar

makeSettableStateVar :: (a -> IO ()) -> SettableStateVar a

Construct a SettableStateVar from an IO action.

General State Variables

data StateVar a

A concrete implementation of a readable and writable state variable, carrying one IO action to read the value and another IO action to write the new value.

Instances

HasSetter StateVar

HasGetter StateVar

makeStateVar :: IO a -> (a -> IO ()) -> StateVar a

Construct a StateVar from two IO actions, one for reading and one for writing.

Utility Functions

($~) :: (HasGetter v, HasSetter v) => v a -> (a -> a) -> IO ()

A modificator convenience function, transforming the contents of a state variable with a given funtion.

($=!) :: HasSetter s => s a -> a -> IO ()

A variant of $= which is strict in the value to be set.

($~!) :: (HasGetter v, HasSetter v) => v a -> (a -> a) -> IO ()

A variant of $~ which is strict in the transformed value.