Meta-programming Tools for Ml

Compile-time reeective ML (CRML) is an extension to Standard ML providing a metalanguage for program manipulationn10]. The original motivation was the automatic generation of map and fold like combinators for arbitrary datatypess4, 9]. The mechanisms developed to achieve this are far more general than we at rst realized, providing the foundation for a rich class of language extensions expressed as source to source translations. Any programming task where the construction of a program can be described by an algorithm can be captured directly and naturally in CRML. In such a paradigm the object language (the language of the constructed programs) and the meta language (the language in which the algorithm is expressed) can be two distinct languages. In CRML both these languages are ML. The object language is encoded in data structures which are predeened by the system. CRML supplies a user level interface (syntactic sugar) so that the object language programs look like the programs they represent. It is the right language for expressing programs that manipulate programs. We have used CRML to implement \just plain macros", ML language processors, and experimental ML language extensionss2, 3]. An example, described later, is the extension of ML to support attribute grammar computations over ML datatypes. The class of ML extensions that can be rapidly prototyped with CRML is limited only by your imagination. The meta-programming paradigm supported by CRML supplies more than just macro-like transformations. CRML can be used to generate programs that require semantic as well as syntactic information, and to encode new abstractions not possible in ordinary functional languages. CRML is implemented as an extension to the Standard ML of New Jersey compilerr1]. Like the original ML and its French cousins, CRML uses an \object-language" facility to support reeection. Unlike these other tools, however, the object-language is just another instance of CRML (the metalanguage). There is a distinguished set of datatypes, including pat, exp and typ, that encode the abstract syntax of the object-language. CRML uses \object brackets" (written << >>) to construct object-language expressions with an embedded parser. Object-language expressions can escape to the metalanguage with an escape (written`), similar to the anti-quotation (,) in LISP. Most meta-programming can be accomplished using only the object bracket notation; detailed knowledge of the distinguished datatypes is often unnecessary. An example use of the object-language escape mechanism is: let val five = <<5>> in <> end. This could be …