Writing a DSL with Python

http://blog.fmeyer.org/entry/writing_a_domain_specific_language_dsl_with_python

by jgalvez • 17 years ago

43 21 17 years ago

blog.fmeyer.org

hedgehog2• 17 years ago

Wow. If you ever wanted curly braces or multi-line lambdas in Python this looks like the way to do it.

pavelludiq• 17 years ago

What exactly would people do with multi-line lambdas? I would definitely refactor any lambda with statements or more than one expression in it, give it a name, or use a list comprehension, so I've never seen a problem with them. Can anybody give me an example of a piece of code they think would look better if python had multi-line lambdas?

rw3• 17 years ago

If you give something a name, yet still only use it once, what's the point? It is just semantic clutter to do that (unless, of course, you can't use multiline lambdas...).

hedgehog2• 17 years ago

Those are just two of the complaints I hear most. A friend of mine actually went so far as to patch Python to use curly braces at the start of a project in Python. "Perfect Python" he called it. Personally I think deviating from PEP8 & standard idioms is likely to cause more pain in maintainability than you win in elegant syntax.

gaius• 17 years ago

Because functional languages discourage loops. This is just a really trivial example, but:

    # value x =[|1;2;3;4|];
    value x : array int = [|1; 2; 3; 4|]
    # for i = 0 to (Array.length x) - 1 do {
      Printf.printf "%d" x.(i);
      };
    1234- : unit = ()
    # Array.iter (fun y -> Printf.printf "%d" y) x;
    1234- : unit = ()

Basically the entire body of any for loop I would want to make an anonymous function. Then if I cared about the result I could do:

    # Array.map (fun y -> Printf.printf "%d" y) x;
    1234- : array unit = [|(); (); (); ()|]

Assuming the underlying platform supported it, and the function is pure, that's "free" parallelism.

dlat• 17 years ago

from __future__ import braces

srwh• 17 years ago

antlr, antlr, antlr

n8agrin• 17 years ago

Maybe provide a link next time?

http://www.antlr.org/

ramanujan• 17 years ago

I would recommend pyparsing for this. There was an excellent article on it in Python Magazine a few months ago.

Some links:

1. O'Reilly mini-book on pyparsing (probably best single ref) http://oreilly.com/catalog/9780596514235/

2. Quick example of pyparsing http://eikke.com/pyparsing-introduction-bnf-to-code/

3. Main pyparsing page (i know it's not very aesthetically pleasing. The code is much better, I promise) http://pyparsing.wikispaces.com/

4. Links to more on pyparsing here http://pyparsing.wikispaces.com/Publications

Python Magazine is excellent, by the way. Definitely worth the purchase price as they include a zip file with code. (Mandatory disclaimer: I have no stake in it at all. Just found it very useful, that's all).

spahl• 17 years ago

A very interesting DSL for network packet manipulation is Scapy(http://www.secdev.org/projects/scapy/).

It uses python itself as a DSL to make things like packet forging easy:

  >>> Ether()/IP()/TCP()/"GET / HTTP/1.0\r\n\r\n"
  <Ether type=0x800 |IP frag=0 proto=TCP |<TCP |<Raw load='GET / HTTP/1.0\r\n\r\n' |>>>>

Here the / operator was overloaded to stack layers.

extension• 17 years ago

Note that there is a world of difference between an in-language DSL and one with its own syntax that must be parsed, so much difference that they really should have different names. The former can effortlessly exchange data with and enjoy all the features of the host language. I'm convinced that the latter will one day be an artifact of less enlightened times.

tomlu709• 17 years ago

By convention the in-language form is called an internal DSL, the other an external ditto.

eru3• 17 years ago

Or embedded.

Writing a DSL with Python

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