Big-Data-HPC-Platform/docs/ECLLanguageReference/ECLR_mods/Basics-ActionsandAttrib.xml

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<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<sect1 id="Actions_and_Attributes">
  <title>Actions and Definitions</title>

  <para>While Definitions define expressions that may be evaluated, Actions
  trigger execution of a workunit that produces results that may be viewed. An
  Action may evaluate Definitions to produce its result. There are a number of
  built-in Actions in ECL (such as OUTPUT), and any expression (without a
  Definition name) is implicitly treated as an Action to produce the result of
  the expression.</para>

  <sect2 id="Functions_as_ActionsFunctions_as_Actions">
    <title>Expressions as Actions<indexterm>
        <primary>Expressions as Actions</primary>
      </indexterm></title>

    <para>Fundamentally, any expression in can be treated as an Action. For
    example,</para>

    <programlisting>Attr1 := COUNT(Trades);     
Attr2 := MAX(Trades,trd_bal);
Attr3 := IF (1 = 0, 'A', 'B');</programlisting>

    <para>are all definitions, but without a definition name, they are simply
    expressions</para>

    <programlisting>COUNT(Trades);       //execute these expressions as Actions
MAX(Trades,trd_bal);
IF (1 = 0, 'A', 'B');</programlisting>

    <para>that are treated as actions, and as such, can directly generate
    result values by simply submitting them as queries to the supercomputer.
    Basically, any ECL expression can be used as an Action to instigate a
    workunit.</para>
  </sect2>

  <sect2 id="Attributes_as_ActionsAttributes_as_Actions">
    <title>Definitions as Actions<indexterm>
        <primary>Definitions as Actions</primary>
      </indexterm></title>

    <para>These same expression definitions can be executed by submitting the
    names of the Definitions as queries, like this:</para>

    <programlisting>Attr1; //These all generate the same result values
Attr2; // as the previous examples
Attr3;</programlisting>
  </sect2>

  <sect2 id="Actions_as_Attributes">
    <title>Actions as Definitions<indexterm>
        <primary>Actions as Definitions</primary>
      </indexterm></title>

    <para>Conversely, by simply giving any Action a Definition name it becomes
    a definition, therefore no longer a directly executable action. For
    example,</para>

    <programlisting>OUTPUT(Person);</programlisting>

    <para>is an action, but</para>

    <programlisting>Attr4 := OUTPUT(Person);</programlisting>

    <para>is a definition and does not immediately execute when submitted as
    part of a query. To execute the action inherent in the definition, you
    must execute the Definition name you've given to the Action, like
    this:</para>

    <programlisting>Attr4;    // run the previously defined OUTPUT(Person) action </programlisting>
  </sect2>

  <sect2 id="Debugging_Uses">
    <title>Debugging Uses</title>

    <para>This technique of directly executing a Definition as an Action is
    useful when debugging complex ECL code. You can send the Definition as a
    query to determine if intermediate values are correctly calculated before
    continuing on with more complex code.</para>
  </sect2>
</sect1>