Data.Primitive.MutVar

Primitive boxed mutable variables. This is a generalization of Data.IORef, Data.STRef and Data.STRef.Lazy to work in any PrimMonad.

Documentation

data MutVar s a

A MutVar behaves like a single-element mutable array associated with a primitive state token.

Constructors

MutVar (MutVar# s a)

Instances

Eq (MutVar s a)

Defined in Data.Primitive.MutVar

Methods

(==) :: MutVar s a -> MutVar s a -> Bool

(/=) :: MutVar s a -> MutVar s a -> Bool

newMutVar :: PrimMonad m => a -> m (MutVar (PrimState m) a)

Create a new MutVar with the specified initial value.

readMutVar :: PrimMonad m => MutVar (PrimState m) a -> m a

Read the value of a MutVar.

writeMutVar :: PrimMonad m => MutVar (PrimState m) a -> a -> m ()

Write a new value into a MutVar.

atomicModifyMutVar :: PrimMonad m => MutVar (PrimState m) a -> (a -> (a, b)) -> m b

Atomically mutate the contents of a MutVar.

This function is useful for using MutVar in a safe way in a multithreaded program. If you only have one MutVar, then using atomicModifyMutVar to access and modify it will prevent race conditions.

Extending the atomicity to multiple MutVars is problematic, so if you need to do anything more complicated, using MVar instead is a good idea.

atomicModifyMutVar does not apply the function strictly. This means if a program calls atomicModifyMutVar many times, but seldom uses the value, thunks will pile up in memory resulting in a space leak. To avoid this problem, use atomicModifyMutVar' instead.

atomicModifyMutVar' :: PrimMonad m => MutVar (PrimState m) a -> (a -> (a, b)) -> m b

Strict version of atomicModifyMutVar. This forces both the value stored in the MutVar as well as the value returned.

modifyMutVar :: PrimMonad m => MutVar (PrimState m) a -> (a -> a) -> m ()

Mutate the contents of a MutVar.

modifyMutVar does not apply the function strictly. This means if a program calls modifyMutVar many times, but seldom uses the value, thunks will pile up in memory resulting in a space leak. To avoid this problem, use modifyMutVar' instead.

modifyMutVar' :: PrimMonad m => MutVar (PrimState m) a -> (a -> a) -> m ()

Strict version of modifyMutVar.