Hi all,
I'm working on a family of languages in Racket, and I'm running into a
conceptual issue with the information flow of the project. I have a
standard core language whose syntax is s-expressions, with some macros in
the expander. Then, there is a surface language (compiled into
s-expressions with brag) which directly translates into the core language.
There will also be other surface languages in the future, forming a
tower/tree of languages that compile into simpler ones, so I want to make
sure I get this pattern right.
Conceptually, I would want the surface language to fully "compile" into the
core language before the core language tries to do anything. As I develop
the surface language, I don't think I should have to concern myself with
the inner workings of the core language; I should be able to just produce
core language code, and then have the core language handle it from there. I
find myself running into an issue, however, where the core language starts
to do computations before the translation is fully complete. This is an
issue because the macros of the core language are taking in the syntax of
the surface language which hasn't been translated yet, and so the core
language doesn't recognize this syntax and breaks.
As a small example, consider the following:
core-language-expander.rkt
(define-syntax (A-core stx)
(syntax-parse stx #:datum-literals (b-core)
[(A-core (b-core x)) #'(printf "A-core got value ~a.~n" x)]))
surface-language-expander.rkt
(define-syntax-rule (b-surface x) (b-core x))
(define-syntax (A-surface stx)
(syntax-parse stx
[(A-surface expr) #'(A-core expr)]))
Here, if we call `(A-surface (b-surface "hello"))`, we would want it to
take the `A-surface`, and convert it into `A-core`, and then expand `(b-surface
"hello")`. Since `A-core` is implemented as a macro, though, it takes
`(b-surface
"hello")` unexpanded, and so tries to evaluate `(A-core (b-surface "hello"))`,
which fails.
Since `b-core` isn't defined as a function/macro in the core language (it
is just a piece of syntax used in `A-core`), we can't just evaluate `(b-surface
"hello")` as a value before passing it in. If we try using `local-expand`
or something similar to handle the expansion as syntax, there isn't an
obvious way to make it only expand macros defined in the surface language
expander (and not those defined in the core language expander), which turns
out to be an issue in my case.
Do any of you recommend a resource for understanding the strategy of taking
a stack of languages and compiling down one layer at a time? It seems like
the notion of a tower of languages would be fairly natural, yet the only
success I have had is with some hacks with recursive local-expand that feel
extremely unnatural, and also require further hacks to get around things
like syntax-tainting.
Thanks so much!
Thomas Del Vecchio
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