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
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
do v <- newIORef (0 :: Int) v $= 11 v $~ (+ 22) x <- get v print x
- class HasGetter g where
- get :: g a -> IO a
- data GettableStateVar a
- makeGettableStateVar :: IO a -> GettableStateVar a
- class HasSetter s where
- ($=) :: s a -> a -> IO ()
- data SettableStateVar a
- makeSettableStateVar :: (a -> IO ()) -> SettableStateVar a
- data StateVar a
- makeStateVar :: IO a -> (a -> IO ()) -> StateVar a
- ($~) :: (HasGetter v, HasSetter v) => v a -> (a -> a) -> IO ()
- ($=!) :: HasSetter s => s a -> a -> IO ()
- ($~!) :: (HasGetter v, HasSetter v) => v a -> (a -> a) -> IO ()
Readable State Variables
The class of all readable state variables.
A concrete implementation of a read-only state variable, carrying an IO action to read the value.
GettableStateVar from an IO action.
Writable State Variables
The class of all writable state variables.
A concrete implementation of a write-only state variable, carrying an IO action to write the new value.
SettableStateVar from an IO action.
General State Variables
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.
StateVar from two IO actions, one for reading and one for
A modificator convenience function, transforming the contents of a state variable with a given funtion.
A variant of
$= which is strict in the value to be set.