The Concurrent ML Reference Manual

The `SyncVar` structure

Synopsis

signature SYNC_VAR structure SyncVar : SYNC_VAR

The SyncVar structure provides Id-style synchronous variables (or memory cells). These variables have two states: empty and full. An attempt to read a value from an empty variable blocks the calling thread until there is a value available. An attempt to put a value into a variable that is full results in the Put exception being raised. There are two kinds of synchronous variables: I-variables are write-once, while M-variables are mutable.

Interface

exception Put type 'a ivar val iVar : unit -> 'a ivar val iPut : 'a ivar * 'a -> unit val iGet : 'a ivar -> 'a val iGetEvt : 'a ivar -> 'a event val iGetPoll : 'a ivar -> 'a option val sameIVar : 'a ivar * 'a ivar -> bool type 'a mvar val mVar : unit -> 'a mvar val mVarInit : 'a -> 'a mvar val mPut : 'a mvar * 'a -> unit val mTake : 'a mvar -> 'a val mTakeEvt : 'a mvar -> 'a event val mGet : 'a mvar -> 'a val mGetEvt : 'a mvar -> 'a event val mTakePoll : 'a mvar -> 'a option val mGetPoll : 'a mvar -> 'a option val mSwap : 'a mvar * 'a -> 'a val mSwapEvt : 'a mvar * 'a -> 'a event val sameMVar : 'a mvar * 'a mvar -> bool

Description

exception Put

This exception is raised when an attempt is made to put a value into a value that is already full (see iPut and mPut).

type 'a ivar

This is the type constructor for I-structured variables. I-structured variables are write-once variables that provide synchronization on read operations. They are especially useful for one-shot communications, such as reply messages in client/server protocols, and can also be used to implement shared incremental data structures.

iVar ()

creates a new empty I-variable.

iPut (iv, x)

fills the I-variable iv with the value x. Any threads that are blocked on iv will be resumed. If iv already has a value in it, then the Put exception is raised.

iGet iv

returns the contents of the I-variable iv. If the variable is empty, then the calling thread blocks until the variable becomes full.

iGetEvt iv

returns an event-value that represents the iGet operation on iv.

val iGetPoll : 'a ivar -> 'a option

This is a non-blocking version of iGet. If the corresponding blocking form would block, then it returns NONE; otherwise it returns SOME of the variable's contents.

sameIVar (iv1, iv2)

returns true, if iv1 and iv2 are the same I-variable.

type 'a mvar

This is the type constructor for M-structured variables. Unlike ivar values, M-structured variables may be updated multiple times. Like I-variables, however, they may only be written if they are empty.

mVar ()

creates a new empty M-variable.

mVarInit x

creates a new M-variable initialized to x.

mPut (mv, x)

fills the M-variable mv with the value x. Any threads that are blocked on mv will be resumed. If mv already has a value in it, then the Put exception is raised.

mTake mv

removes and returns the contents of the M-variable mv making it empty. If the variable is already empty, then the calling thread is blocked until a value is available.

mTakeEvt mv

returns an event-value that represents the mTake operation on mv.

mGet mv

returns the contents of the M-variable mv without emptying the variable; if the variable is empty, then the thread blocks until a value is available. It is equivalent to the code:

let val x = mTake mv in mPut(mv, x); x end

mGetEvt mv

returns an event-value that represents the mGet operation on mv.

val mTakePoll : 'a mvar -> 'a option val mGetPoll : 'a mvar -> 'a option

These are non-blocking versions of mTake and mGet (respectively). If the corresponding blocking form would block, then they return NONE; otherwise they return SOME of the variable's contents.

mSwap (mv, newV)

puts the value newV into the M-variable mv and returns the previous contents. If the variable is empty, then the thread blocks until a value is available. It is equivalent to the code:

let val x = mTake mv in mPut(mv, newV); x end

except that mSwap is executed atomically.

mSwapEvt (mv, newV)

returns an event-value that represents the mSwap operation on mv and newV.

sameMVar (mv1, mv2)

returns true, if mv1 and mv2 are the same M-variable.

Discussion

I-variables provide a useful mechanism for implementing the reply communication in request/reply protocols (in cases where the server does not care if the reply is accepted). They may also be used to implement incremental data structures and streams; for example, the Multicast structure uses I-variables to implement its multicast channels.

A disciplined use of M-variables can provide an atomic read-modify-write operation.

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