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From Timothy Farkas <...@datatorrent.com>
Subject Re: APEXCORE-1972: ExpressionEvaluator - quasi-Java Expression Language
Date Thu, 14 Jan 2016 08:13:27 GMT
Hey Chinmay,

It is possible the user may want to define a syntax radically different
from Java style syntax. I've proposed a syntax similar to yours to one of
our users for expression evaluation but they didn't like it because the
syntax needs to be something easily validated by a UI. It may be possible
that the user wants expressions to be sent to an operator as json, because
that is easy to parse and validate in javascript:

{"param":"myField1","operation":">","threshold":"1"}

If the syntax is pluggable then an implementation of this syntax could be
swapped with with an implementation of the syntax you've already provided
without changing operator code.

Thanks,
Tim

On Wed, Jan 13, 2016 at 11:34 PM, Chinmay Kolhatkar <chinmay@datatorrent.com
> wrote:

> Hi tim,
>
> I agree with you. The expression here we're talking about is really a
> quasi-Java expression. One can do anything which java does.
> Only change with java syntax here is how variables are accessed.
>
> They're by default accessed as ${...}.
>
> But this is also made configurable via a setter property on
> ExpressionEvaluator. The pull request has it.
>
> If this is what you're talking about then this is taken care of.
>
> Thanks,
> Chinmay.
>
>
> On Thu, Jan 14, 2016 at 1:00 PM, Timothy Farkas <tim@datatorrent.com>
> wrote:
>
> > 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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