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Data.StateVar | Portability | portable | Stability | stable | Maintainer | sven.panne@aedion.de |
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Description |
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
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Synopsis |
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Readable State Variables
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The class of all readable state variables.
| | Methods | | Read the value of a state variable.
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data GettableStateVar a | Source |
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A concrete implementation of a read-only state variable, carrying an IO
action to read the value.
| Instances | |
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Construct a GettableStateVar from an IO action.
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Writable State Variables
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The class of all writable state variables.
| | Methods | | Write a new value into a state variable.
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data SettableStateVar a | Source |
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A concrete implementation of a write-only state variable, carrying an IO
action to write the new value.
| Instances | |
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Construct a SettableStateVar from an IO action.
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General State Variables
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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 | |
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Construct a StateVar from two IO actions, one for reading and one for
writing.
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Utility Functions
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A modificator convenience function, transforming the contents of a state
variable with a given funtion.
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A variant of $= which is strict in the value to be set.
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A variant of $~ which is strict in the transformed value.
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Produced by Haddock version 2.4.2 |