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--- alvis:select [2013/12/15 07:54]
marcin [Select statement]
+++ — (current)
@@ Line 1: / Line 1: @@
-====== Select statement ======
-The **select** statement provides a selective wait for one or more alternatives. The statement may contain a series of **alt** clauses called **branches**. Each branch may be guarded. These guards divide branches into **open** and **closed** ones. A branch is called **open**, if it does not have a guard attached or its guard evaluates to **True**. Otherwise, a branch is called **closed**. To avoid indeterminism, if more than one branch is open the first of them is chosen to be executed. If all branches are closed, the corresponding agent is postponed until at least one branch is open.
-A **guard** is a Haskell logical expressions placed inside round brackets.
-The **select** statement is a multi-step statement. The first step consists of evaluating the guards and entering the corresponding branch (if possible). Then, a model executes steps inside the branch.
-<code>
-select {
-  alt (g1) {...}
-  alt (g2) {...}
-  ...
-}
-</code>
-**Listing 1. ** Select statement syntax: ''g1'', ''g2'' stand for guards
-Very often the **ready** function is used as a guard. The function takes a list of agent ports' names with in/out directions and returns **True** if at least one of them is ready for a communication immediately. For example, the guard (''a'' and ''b'' are ports of the corresponding agent):
-  ready [in(a), in(b)]
-evaluates to **True** if the agent can collect immediately a signal/value from at least one of these ports -- It means that another agent has already provided a signal/value.
-===== Example =====
-{{:alvis:example5.png|}}
-**Figure 1. ** Example communication diagram
-<code>
-agent A {
-  loop {
-    delay 10;
-    select {
-      alt (ready [out(a)] { out a; }
-      alt (ready [out(b)] { out b; }
-    }
-  }
-}
-agent B {
-...
-</code>
-**Listing 1. ** Example code layer
-In the above example, the ''A'' agent sends a signal through either the ''a'' or the ''b'' port depending on the requests of agents ''B'' and ''C''. If both port are ready, the ''a'' port is chosen.
-**See also:** [[:alvis:in|In statement]], [[:alvis:out|Out statement]]
-**[[:alvis:manual|Go back]]**