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

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<sect1 id="Overview" role="nobrk">
  <title>Overview</title>

  <para><emphasis role="bold">E</emphasis>nterprise <emphasis
  role="bold">C</emphasis>ontrol <emphasis role="bold">L</emphasis>anguage
  (ECL)<indexterm>
      <primary><emphasis role="bold">ECL</emphasis></primary>
    </indexterm> has been designed specifically for huge data projects using
  the LexisNexis High Performance Computer Cluster (HPCC)<indexterm>
      <primary><emphasis role="bold">HPCC</emphasis></primary>
    </indexterm>. ECL’s extreme scalability comes from a design that allows
  you to leverage every query you create for re-use in subsequent queries as
  needed. To do this, ECL takes a Dictionary approach to building queries
  wherein each ECL definition defines an expression. Each previous Definition
  can then be used in succeeding ECL definitions—<emphasis>the language
  extends itself as you use it</emphasis>.</para>

  <sect2 id="Attributes_versus_Actions">
    <title>Definitions versus Actions</title>

    <para>Functionally, there are two types of ECL code: Definitions (AKA
    Attribute definitions) and executable Actions. Actions are not valid for
    use in expressions because they do not return values. Most ECL code is
    composed of definitions.</para>

    <para>Definitions only define <emphasis>what</emphasis> is to be done,
    they do not actually execute. This means that the ECL programmer should
    think in terms of writing code that specifies <emphasis>what</emphasis> to
    do rather than <emphasis>how</emphasis> to do it. This is an important
    concept in that, the programmer is telling the supercomputer
    <emphasis>what</emphasis> needs to happen and not directing <emphasis>how
    </emphasis>it must be accomplished. This frees the super-computer to
    optimize the actual execution in any way it needs to produce the desired
    result.</para>

    <para>A second consideration is: the order that Definitions appear in
    source code does not define their execution order—ECL is a non-procedural
    language<indexterm>
        <primary>non-procedural language</primary>
      </indexterm>. When an Action (such as OUTPUT) executes, all the
    Definitions it needs to use (drilling down to the lowest level Definitions
    upon which others are built) are compiled and optimized—in other words,
    unlike other programming languages, there is no inherent execution order
    implicit in the order that definitions appear in source code (although
    there is a necessary order for compilation to occur without error—forward
    reference<indexterm>
        <primary>forward reference</primary>
      </indexterm>s are not allowed). This concept of “orderless execution”
    requires a different mindset from standard, order-dependent programming
    languages because it makes the code appear to execute “all at
    once.”</para>
  </sect2>

  <sect2 id="Syntax_Issues">
    <title>Syntax Issues</title>

    <para>ECL is not case-sensitive. White space is ignored, allowing
    formatting for readability as needed.</para>

    <para>Comments in ECL code are supported. Block comments must be delimited
    with /* and */.</para>

    <programlisting>/* this is a block comment - the terminator can be on the same line
or any succeeding line – everything in between is ignored */</programlisting>

    <para>Single-line comments must begin with //.</para>

    <programlisting>// this is a one-line comment</programlisting>

    <para>ECL uses the standard <emphasis>object.property</emphasis> syntax
    used by many other programming languages (however, ECL is not an
    object-oriented language) to qualify Definition scope and disambiguate
    field references within tables:</para>

    <programlisting>ModuleName.Definition //reference an definition from another module/folder</programlisting>

    <programlisting>Dataset.Field       //reference a field in a dataset or recordset</programlisting>
  </sect2>
</sect1>