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GHC 9.10.3 · lts/ghc-9.10.x · c74966e · 2026-09-27

ModuleStateVar-1.2.2Haskell2010

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 could 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

However, Ptr can be used directly through the same API:

  do p <- malloc :: IO (Ptr Int)
     p $= 11
     p $~ (+ 22)
     x <- get p
     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
  • 3 types
  • 3 classes
  • 5 values
  • PackageStateVar-1.2.2
  • Exports11
  • LanguageHaskell2010
  • LicenceBSD-3-Clause
  • SourceStateVar.hs

Readable State Variables

3 declarations
classclass HasGetter t a | t -> a where
#

This is the class of all readable state variables.

Methods

Instances7HasGetter, …
typetype GettableStateVar = IO
#

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

Writable State Variables

4 declarations
classclass HasSetter t a | t -> a where
#

This is the class of all writable state variables.

Methods

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

    Write a new value into a state variable.

Instances6HasSetter
value($=!) :: (HasSetter t a, MonadIO m) => t -> a -> m ()
#

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

Updatable State Variables

4 declarations
classclass HasSetter t b => HasUpdate t a b | t -> a b where
#

This is the class of all updatable state variables.

Methods

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

    Transform the contents of a state variable with a given funtion.

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

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

Instances5HasUpdate
datadata 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. 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 <- get v; v $= y; v $= x

should restore the previous state.

  • Ideally, in the absence of thrown exceptions:

v $= a >> get v

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

Constructors

Instances3HasGetter, HasSetter, HasUpdate