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--- alvis:in [2015/11/24 15:33]
marcin
+++ — (current)
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-====== In statement ======
-An agent can communicate with its outside world using **ports**. Each port can be used both as an input or an output one. The current role of a port is determined by two factors:
-  - Connections to the port in the corresponding communication diagram -- e.g. if a port ''p'' is used only as an input port for an one-way connection, it cannot be used as an output port.
-  - Statements used in the code layer -- e.g. if a port ''p'' is used only as an argument of the **in** statement, it is an input port even if all its connections are two-way ones.
-Any communication through a port can be a **pure communication** or a **value passing combination**.
-In case of a **pure communication**, the **in** statement collects a signal (without specified value) through the statement port.
-In case of a **value passing communication**, the **in** statement collects a value through the statement port (probably of a composed type) and assigns it to the statement parameter.
-The **in** statement as a **blocking communication** is a single-step statement.
-<code>
-in p;
-in p x;
-</code>
-**Listing 1. ** In statement syntax (blocking version): ''p'' stands for a port name and ''x'' stands for a parameter name.
-===== Example 1 =====
-{{:alvis:example3.png|}}
-**Figure 1. ** Example 1 communication diagram
-<code>
-agent A {
-  out a;
-}
-agent B {
-  in b;
-}
-</code>
-**Listing 2. ** Example 1 code layer
-The only task of agent ''A'' is sending a signal through its port ''a'' and the only task of agent ''B'' is collecting a signal through its port ''b''.
-A communication between two active agents can be initialised by any of them. The agent that initialises it, performs the **out** statement to provide some information and waits for the second agent to take it, or performs the **in** statement to express its readiness to collect some information and waits until the second agent provides it.
-===== Example 2 =====
-{{:alvis:example4.png|}}
-**Figure 2. ** Example 2 communication diagram
-<code>
-agent A {
-  out a;
-}
-agent B {
-  in b;
-}
-agent C {
-  in c;
-}
-</code>
-**Listing 2. ** Example 2 code layer
-In the above example the port ''A.a'' is connected with two other ports, but only one of them will collect the signal. If both agents ''B'' and ''C'' are waiting for the signal, the choice between them is indeterministic. To avoid indeterminism, different priorities for these agents should be assigned.
-A **blocking communication** means that if an agent initialises a communication with the **in** statement then it waits until another agent finishes it. The communication cannot be aborted.
-The **in** statement as an **non-blocking communication** may be a single-step or multi-step statement.
-<code>
-in (0) p x {
-  success { ... }
-  fail    { ... }
-}
-in (0) p x {
-  fail    { ... }
-}
-in (0) p x {
-  success { ... }
-}
-in (0) p x;
-in (0) p {
-  success { ... }
-  fail    { ... }
-}
-in (0) p {
-  fail    { ... }
-}
-in (0) p {
-  success { ... }
-}
-in (0) p;
-</code>
-**Listing 4. ** In statement syntax (non-blocking version): ''p'' stands for a port name and ''x'' stands for a parameter name.
-A communication with a passive agent denotes a procedure call.
-**See also:** [[:alvis:out|Out statement]], [[:alvis:proc|Proc statement]].
-**[[:alvis:manual|Go back]]**