Which Compiler to Extend for a Small Project?

Andreas(R) asks: "While planning the design of my small programming language, and would appreciate some lessons learned from experienced programmers which have already tried this. I was investigating whether to start from an existing compiler and extend it. The compiler will be based on yacc, or bison. The programming language will be interpreted, object oriented and have higher order programming. Perl 1 seems like a decent starting point, as it's yacc based, and 5000 lines of code. Later versions of Perl are too large to get a good understanding of the whole program in a short period of time. Perl also has the right license (GPL). Is Python out of the question for such a project, since it's not GPL? What other small languages can be used instead? How do I go about designing a small programming language in practice, using what I already know about compiler theory?"

Comments

Holy cow!

[sidecut]

Can you give us a few more specifics on what the language will be used for? Will it be embedded? Database connected? Real-time? Interactive?

While this seems like beaucoup fun, I'd question the need to extend an existing language by altering the compiler. Towards that end, you might want to use LISP or Scheme, as language extension is built into the language. ( See what Paul Graham has to say about the subject)

A few ideas

[RevAaron]

First, there are two kinds of small languages:
1. small languages like lua, io, and scheme that are small in the built-in libraries and in the total distro. These three are great places to start- both are small, OOPish, allow higher-order programming by passing classes, objects, functions and methods as objects.

2. Then there are languages that are big in some ways, but small in syntax. Some of these are easier to extend than so-called "little languages." The reason is usually that their syntax is small, in an isolated place, easy to get at, and meant to be modified. The two best examples for this are Smalltalk and Lisp. Both of these languages satisfy your other requirements and really kick ass for extention. Unlike the above languages, the so-called little-languages, most Smalltalk and Lisp dialects have big, useful libraries. Unlike a big fat language like perl or C++, having a useful library doesn't mean that the language is a huge pain in the ass to extend.

Both Lisp and Smalltalk have a number of implementations. I am a big fan of Squeak Smalltalk, though systems like Little Smalltalk or even GNU Smalltalk maybe worth checking out.

A lot of people here have bad feelings about Lisp-like languages. It's a shame, since Scheme, ISLISP (OpenLisp is a great implementation) and Common Lisp are all *very* powerful languages. You can be quite productive with them once you get over the part about whining about parens. But Lisp may very well be the best option here, there is a long history of people writing custom-syntaxes and language extensions. Look up Common Lisp macros- power almost beyond comprehension, a lot of fun to play with, and with an elegance all its own.

There are examples of people writing a C-like syntaxes for various Scheme implementations. IIRC, Gambit-C (a Scheme to C compiler) comes with one. On Cliki, there are a bunch of other alternative Scheme syntaxes listed.

To, one of the big advantages to using a language in the second category is that syntax extension/modification is done in the language itself, rather than in C. With that comes the familiarity of the language you're creating and the other benefits you gain by using a high-level language like Smalltalk or Common Lisp.

Just some thoughts...