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Parsec Coffee-Script
===================
<metalua@gmail.com>
About.
------
Parsec Coffee Script (PCS) is a parser / compiler for the
coffee-script language, based on parser combinators. The project's aim
is to add static metaprogramming (i.e. macros + syntax extensibility)
to Coffee Script (CS), similar to how Metalua adds such features to
Lua. The resulting compiler, once merged with the official compiler,
should be usable as a drop-in replacement for it.
Roadmap.
--------
The project is decomposed into several steps:
1. Development of the Lexer. Compared to the official CS lexer, it
must be single pass: no rewriting stage, direct handling of string
and regex interpolation. [IN PROGRESS]
2. Development of the generic parsec library, GrammarGenerator
(GG). It doesn't need to have a compact/fluid API yet: the final
API will be based on a static DSL, once PCS is bootstrapped and
able to define it. [DONE]
3. Implementation of the coffee-script parser (PCS) with GG
[IN PROGRESS].
4. Defining an Abstract Syntax Tree (AST) format [DONE].
5. Merge with Coffee-Script's compiler backend Node.js
At this stage, PCS will be a fully functional Coffee-Script compiler.
6. Implementation of splice (the fundamental operator of static
metaprogramming).
7. Development of a static DSL, making the definition of GG grammars
terser and more readable. At this stage, GG grammars are expected
to be more readable than Bison-inspired grammar definitions.
8. Lift (inverse operator of splice) + quasi-quotes
9. Focus on GG usability: good error messages on syntactically
incorrect inputs, error recovery for multiple errors detection,
automated bookkeeping of source/Abstract Syntax Tree (AST)
correspondence.
10. To be determined: macro hygiene, experimental language extensions,
modularisation of Coffee-Script (implement non-core features as
syntax extensions, in order to simplify the AST->js compiler)...
A word about static MP and parsec.
----------------------------------
Static metaprogramming allows to execute arbitrary code during
compilation, and to splice user-program-generated AST fragments into
the compiler-generated AST.
Parser combinators are a way to define grammars, especially suitable
to functional languages. Parsers are regular functions, which take
token streams and return AST. Parsers can be defined from scratch, as
regular functions; but the specificity of the parsec approach is the
use of combinators: functions which take simple parsers as arguments,
and return a more complex parser as a result.
Benefits of the parsec approach:
- In addition to standard combinators (sequence, choice,
repetition...), some more specific parsers can be defined,
e.g. expression generators properly handling prefix, infix and
suffix operators, precedence, associativity, etc. Ad-hoc support for
common combinators can be backed into Bison-like generators, but
can't be tailored unless one considers patching the compiler.
- Since parsers are first-class objects (functions), unusual
operations can be supported through specific functions. In a Bison +
host language 2-staged approach, there's a big impedance mismatch
when one wants to mess with the parsing stages in ways not intended
by Bison; if the parser merely combines plain functions, special
actions can be easily taken at any stage.
- Most importantly, parsecs can be modified dynamically. Therefore,
some code in a splice can extend the grammar being recognized in the
middle of a file being parsed. This allows a file to define and/or
load grammar extensions seamlessly, reclaiming most of Lisp's
flexibility, without the constraints Lisp-style macros put on
superficial syntax.