Add macro expansion to Pig Latin
--------------------------------
Key: PIG-1793
URL: https://issues.apache.org/jira/browse/PIG-1793
Project: Pig
Issue Type: New Feature
Reporter: Richard Ding
Assignee: Richard Ding
Fix For: 0.9.0
As production Pig scripts grow longer and longer, Pig Latin has a need to
integrate standard programming techniques of separation and code sharing
offered by functions and modules. A proposal of adding macro expansion to Pig
Latin is posted here: http://wiki.apache.org/pig/TuringCompletePig
Below is a brief summary of the proposed syntax (and examples):
* Macro Definition
The existing DEFINE keyword will be expanded to allow definitions of Pig
macros.
*Syntax*
{code}
define <name> (<params>) returns <aliases> {
<Pig Latin fragment>
};
{code}
*Example*
{code}
define my_macro(A, sortkey) returns C {
B = filter $A by my_filter(*);
$C = order B by $sortkey;
}
{code}
* Macro Expansion
*Syntax*
{code}
<aliases> = <macro name> (<params>);
{code}
*Example:* Use above macro in a Pig script:
{code}
X = load 'foo' as (user, address, phone);
Y = my_macro(X, user);
store Y into 'bar';
{code}
This script is expanded into the following Pig Latin statements:
{code}
X = load 'foo' as (user, address, phone);
macro_my_macro_B_1 = filter X by my_filter(*);
Y = order macro_my_macro_B_1 by user;
store Y into 'bar';
{code}
*Notes*
1. Any alias in the macro which isn't visible from outside will be prefixed
with macro name and suffixed with instance id to avoid namespace collision.
2. Macro expansion is not a complete replacement for function calls. Recursive
expansions are not supported.
* Macro Import
The new IMPORT keyword can be used to add macros defined in another Pig Latin
file.
*Syntax*
{code}
import <Pig Latin file name>;
{code}
*Example*
{code}
import my_macro.pig;
{code}
*Note:* All macro names are in the global namespace.
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