here - Stefan-Marr.de

5. An Ownership-based MOP for Expressing Concurrency Abstractions
1 Agent = (
2 | state mailbox immutDomain |
3
4 read = unenforced ( ^ state )
5
6 " update blocks are of the form :
7 [: oldState | oldState produceNewState ]"
8 send : anUpdateBlock = unenforced (
9 mailbox nextPut : anUpdateBlock )
10
11 initialize = unenforced (
12 | domain |
13 mailbox := SharedQueue new .
14 immutDomain := ImmutableDomain new .
15 domain := AgentDomain new agent : self .
16 domain spawnHere : [
17 true whileTrue : [ self processUpdateBlock ]]. )
18
19 processUpdateBlock = (
20 | updateBlock newState |
21 updateBlock := mailbox waitForFirst .
22 newState := domain evaluateEnforced : [
23 updateBlock value : state ].
24 state := immutDomain adopt : newState .
25 mailbox removeFirst . ) )
Listing 5.2: Clojure agents implemented in SOM Smalltalk
For the purpose of this discussion, agents are represented by an object that
has the field state to represent the state of the agent and the field mailbox,
which holds incoming update requests. In addition to the object itself, an
agent has an associated Process, i. e., thread, which evaluates the incoming
update requests one by one. The mailbox is a SharedQueue object, i. e.,
a concurrent queue data structure. The queue implements #waitForFirst to
block the current thread until the queue has at least one element and then
return the element without removing it. This operation is complemented by
#removeFirst, which removes the first element of the queue.
The example uses these operations to implement the basic methods of the
agent. Thus, #read returns the current state and #send: enqueues an update
request for the agent’s state. The request is represented by anUpdateBlock,
which is a lambda, i. e., a block, with a single argument. The argument is the
old state, which is going to be replaced by the result the block returns.
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