CODEWITHSUNDEEP
macrosschemeracket
day
day

Reputation: 2352

Can the substitute-and-expand model of Scheme macros be improved?

As far as I know, the pattern&template-based Scheme macro system works by first pattern matching a macro invocation, obtaining a substitution in case of success, applying the resulted substitution to the corresponding template to build up a (maybe) partially-expanded expression, and then continuously expanding the resulted expression. If what I describe is true (please correct me otherwise), then it seems to me this building-up and expanding-again model is not efficient. Why does the expansion need to be done like this? Is it possible to finish the expansion by a run down the template once and for all?

Upvotes: 0

Views: 99

Answers (2)

Sylwester
Sylwester

Reputation: 48745

Keep in mind that macros can be handled define time for procedures and compile time for a whole program.

Also, a macro expansion might turn into another (or similar) macro form that needs expanding. Eg. you can make a macro that ends up as a cond expression which of course is a macro for nested if expressions in most Schemes.

Have you seen Alexpander? It evaluates a program (in one expression) and returns an equal program without macros.

Upvotes: 1

John Clements
John Clements

Reputation: 17203

The semantics of the macro system are specified in the way you describe. However, implementations are free to implement that specification any way they want; in particular, they could "inline" macro expansions ahead of time to speed the process of macro expansion.

I'm not aware of any Scheme implementations that do what you describe, and I would guess it's because macro expansion is not usually a big bottleneck in compilation.

Upvotes: 1

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