Re: [Pyparsing] DSL using pyparsing

[Paul M. <pt...@au...>]

Asim -

I would work on this like they dig a tunnel - from both directions.  First I
would write out what my DSL syntax would be.  Then I would work backwards
and write out how this would be implemented in Python.  The purpose of the
pyparsing DSL converter is to find your DSL syntax, and then create the
related Python code.

Let's say your application was something about RGB colors, and A and B are
going to be colors which, when added together, result in a third color whose
RGB values are the respective sums of the R, G, and B values of A and B.
Let's say this is your color syntax:

A = < 100 10 50 >
B = < 0 0 0 >
C = A+B

The purpose of the DSL converter is to convert the "<>" syntax into a
Python-compatible API call (probably a constructor call to a Color class).
The "C=A+B" line probably doesn't need DSL handling, your Python Color class
can define the __add__() function to do the proper R, G, and B summing.

So all your DSL converter in this case would need to do would be to change:

A = < 100 10 50 >

To:

A = Color(100, 10, 50)

In pyparsing this is a very simple thing:

LT,GT = map(Suppress,"<>")
Integer = Word(nums)
colorDef = LT + integer*3 + RT
colorDef.setParseAction(lambda tokens : "Color(%s, %s, %s)" % tokens)

Then use colorDef.transformString() to convert the source read from the
imported file into valid Python, and then use that to compile into the
executable module.

Remember, you are generating a Python script from your DSL script, so you
don't have to worry about "carrying values forward from one line to the
next" - when your generated code gets compiled to Python bytecode, it will
do this for you.

Don't try to re-invent Python with your DSL.  You are *much* better off
using your DSL to *augment* the Python syntax.

Thanks for reading the article!

-- Paul


> -----Original Message-----
> From: Asim Malik [mailto:as...@ho...]
> Sent: Wednesday, June 10, 2009 1:58 PM
> To: pyp...@li...
> Subject: [Pyparsing] DSL using pyparsing
> 
> 
> Ok i am looking at building a DSL using pyparsing and am relatively new to
> python . Intially I am looking at something like this:
> 
> A = {build some list, this calls some underlying python api and creates
> say a class A}
> B = {build another list, this creates another python class B }
> C= A+B
> 
> Should i parse this as a single script or parse each statement? If i parse
> each statement how can i pass variables that were defined in the previous
> statement? As the complexity of the doamin increase, it might be the case
> that the construction of A depends on other statements. What i am looking
> for is the best approach.
> 
> Thanks
> 
> 
> 
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