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From Timothy Farkas <...@datatorrent.com>
Subject Re: APEXCORE-1972: ExpressionEvaluator - quasi-Java Expression Language
Date Thu, 14 Jan 2016 07:30:45 GMT
Hi Chinmay,

I think this is a very good first implementation. My only concern is that
people will get caught up in the details of the syntax for this expression
language. I think a way to avoid that is to make the syntax pluggable. For
example is there an interface for expression evaluation which can be
implemented by different syntaxes? This would allow operators which use
expression evaluation to have the syntax configured via a property. That
way if someone discovers a limitation in the syntax down the line, a new
syntax can be introduced without breaking backwards compatibility and
without requiring operator code to be changed.

Thanks,
Tim

On Wed, Jan 13, 2016 at 10:52 PM, Chinmay Kolhatkar <chinmay@datatorrent.com
> wrote:

> Hi All,
>
> I'm working on APEXCORE-1972 which adds support for a quasi-Java Expression
> Language and its expression evaluator.
>
> All the detailed functionality and design details along with examples are
> present in Jira.
>
> I've summarized the ExpressionEvaluator feature & Expression language
> below.
>
> The pull request created for this at here:
> https://github.com/apache/incubator-apex-malhar/pull/170
>
> Please share your thought on this.
>
> Thanks,
> Chinmay.
>
>
> *Summary of functionality of ExpressionEvaluator:*
>
>    1. Support quasi-Java Expression which defines a single line executable
>    expression
>    2. Support for quasi-Java expression based function code, which will be
>    compiled on the fly and made available for execution.
>    3. Should support accessing multiple fields from multiples input POJOs
>    while addressing the conversion of private variables to public getter
>    method for all levels.
>    4. Should support nested field support
>    5. quasi-Java expressions should support operands to be mentioned in
>    following ways:
>       - ${input.fieldName}
>          - Access fieldName via a object name.
>       - ${fieldName}
>          - Accessing fieldName directly when a single object is registered
>          for operation.
>       - ${input}
>          - Accessing object variable directly
>       - ${input.fieldName.internalField}
>          - Accessing nested fields
>       6. There should be some predefined function provided to expression
>    writer which one can directly use in expression for invoking certain
>    functionality.
>    7. These are simple String based, Date time based etc functions.
>    8. On-need-basic one should be able to easily update Expression
>    Evaluator to easily add new predefined functions to be made available
> for
>    expression writer.
>    9. User of ExpressionEvaluator should be able to add a custom method
>    externally to be made available to in expression.
>    10. Though operands are defined, placeholder for the operand in
>    expression should be allowed to be overridden. By default, expression
>    language should support bash type syntax for operand - {…}
>    11. The library should not introduce any serialization related issues.
>    12. All the java operators should be supported.
>
> ​
>
> *The examples of quasi-Java Expression:*
>
>    - ${inp.field1}
>       - Will return value of field1 from registered input POJO.
>    - ${inp.field1} + ${inp.field2}
>       - Will return sum of field1 & field2 from given POJO
>    - ${field1} + ${field2}
>       - Equivalent to above
>    - ${inpA.field1} > ${inpA.field2} ? ${inpA.field3} : ${inpB.field3}
>       - Executes ternary expression and returns value accordingly. Works on
>       2 POJOs. inpA & inpB are two placeholder registered for given POJO
> with
>       ExpressionEvaluator library.
>    - pow(${inpA.field1}, ${inpB.field2})
>       - Executes pow function coming from java.lang.Math library. This and
>       other with lot other basic functions is available to expression
> writer out
>       of the box to use in expression.
>    - ${inpA.field1} > 0 ? ${inpB.innerPOJO.field3} :
>    ${inpA.innerPOJO.field3}
>       - Shows how nested POJOs can be accessed in expression. The variables
>       will evaluate to correct public getter method is required.
>    - ${inp.firstname} + “ “ + ${inp.lastname}
>       - Generate the full name as per given expression from firstname and
>       lastname field.
>    - long retVal=1; for (int i=0; ${inpF.value1}; i++) {retVal = retVal *
>    ${inpF.value1}; } return retVal;
>       - This tells a complete method content to ExpressionEvaluator. The
>       library create an executable and compiled method using this
> expression.
>
> ​
>

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