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Parsec Coffee-Script =================== <firstname.lastname@example.org> 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.