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

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE sect1 PUBLIC "-//OASIS//DTD DocBook XML V4.5//EN"
"http://www.oasis-open.org/docbook/xml/4.5/docbookx.dtd">
<sect1 id="Attribute_Definition">
  <title>Definitions</title>

  <para>Each ECL definition<indexterm>
      <primary>ECL definition</primary>
    </indexterm> is the basic building block of ECL. A definition specifies
  <emphasis>what </emphasis>is done but not <emphasis>how</emphasis> it is to
  be done. Definitions can be thought of as a highly developed form of
  macro-substitution, making each succeeding definition more and more highly
  leveraged upon the work that has gone before. This results in extremely
  efficient query construction.</para>

  <para>All definitions take the form:</para>

  <para><emphasis role="bold">[</emphasis><emphasis>Scope</emphasis><emphasis
  role="bold">] [</emphasis><emphasis>ValueType</emphasis><emphasis
  role="bold">] Name<indexterm>
      <primary>Name</primary>
    </indexterm> [ </emphasis>(<emphasis>parms</emphasis>) <emphasis
  role="bold">] := Expression<indexterm>
      <primary>Expression</primary>
    </indexterm> [ </emphasis><emphasis>:WorkflowService</emphasis><emphasis
  role="bold">] ;</emphasis></para>

  <para>The Definition Operator<indexterm>
      <primary>Definition Operator</primary>
    </indexterm> (<emphasis role="bold"> :=</emphasis> read as “is defined
  as”) defines an expression. On the left side of the operator is an optional
  <emphasis>Scope<indexterm>
      <primary>Scope</primary>
    </indexterm></emphasis> (see <emphasis role="bold">Attribute
  Visibility</emphasis>), <emphasis>ValueType<indexterm>
      <primary>Value Type</primary>
    </indexterm></emphasis> (see <emphasis role="bold">Value
  Types</emphasis>), and any parameters<indexterm>
      <primary>parameters</primary>
    </indexterm> (<emphasis>parms</emphasis>) it may take (see <emphasis
  role="bold">Functions (Parameter Passing)</emphasis>). On the right side is
  the expression that produces the result and optionally a colon (:) and a
  comma-delimited list of <emphasis>WorkflowServices</emphasis> (see <emphasis
  role="bold">Workflow Services</emphasis>). A definition must be explicitly
  terminated with a semi-colon (;). The Definition name can be used in
  subsequent definitions:</para>

  <programlisting>MyFirstDefinition := 5; //defined as 5
MySecondDefinition := MyFirstDefinition + 5; //this is 10</programlisting>

  <sect2 id="Attribute_Name_Rules">
    <title>Definition Name Rules</title>

    <para>Definition name<indexterm>
        <primary>Definition Name</primary>
      </indexterm>s begin with a letter and may contain only letters, numbers,
    or underscores (_).</para>

    <programlisting>My_First_Definition1 := 5; // valid name
My First Definition := 5;  // INVALID name, spaces not allowed</programlisting>

    <para>You may name a Definition with the name of a previously created
    module in the ECL Repository, if the attribute is defined with an explicit
    <emphasis>ValueType</emphasis>.</para>
  </sect2>

  <sect2 id="Reserved_Words">
    <title>Reserved Words</title>

    <para>ECL keywords<indexterm>
        <primary>ECL keywords</primary>
      </indexterm>, built-in functions and their options are reserved
    words<indexterm>
        <primary>Reserved Words</primary>
      </indexterm>, but they are generally reserved only in the context within
    which they are valid for use. Even in that context, you may use reserved
    words as field or attribute names, provided you explicitly disambiguate
    them, as in this example:</para>

    <programlisting>ds2 := DEDUP(ds, ds.all, ALL); //ds.all is the 'all' field in the
                               //ds dataset - not DEDUP’s ALL option</programlisting>

    <para>However, it is still a good idea to avoid using ECL keywords as
    attribute or field names.</para>
  </sect2>

  <sect2 id="Attribute_Naming">
    <title>Definition Naming</title>

    <para>Use descriptive names for all EXPORTed and SHARED Definitions. This
    will make your code more readable. The naming convention adopted
    throughout the ECL documentation and training courses is as
    follows:</para>

    <programlisting><emphasis role="bold">Definition Type</emphasis>     <emphasis
        role="bold">Are Named
</emphasis>Boolean              Is...
Set Definition       Set...
Record Set           ...DatasetName</programlisting>

    <para>For example:</para>

    <programlisting>IsTrue := TRUE;                         // a BOOLEAN Definition
SetNumbers := [1,2,3,4,5];              // a Set Definition
R_People := People(firstname[1] = 'R'); // a Record Set Definition</programlisting>
  </sect2>
</sect1>