This is a nice example of a macro design pattern that I think of as "partial expansion with trampolining". You don't need to deal with the internal definition context API, because you can return definitions to the macro expander, let it handle their interpretation, and then resume your work. Here's an implementation sketch:
1. First, make sure you're in an internal definition context: (guarded-block form ...) => (let () (guarded-block* form ...)). The guarded-block* helper macro has the invariant that it is always used in internal definition context. 2. If guarded-block* has at least one form, it partially expands it, using a stop list containing guard and Racket's primitive syntactic forms. Then it analyzes the partially-expanded form: - If it is a begin, it recurs with the begin's contents appended to the rest of its argument forms. - If it is a define-values or define-syntaxes form, it expands into (begin defn (guarded-block* form-rest ...)). The macro expander interprets the definition, adds it to the environment, etc. Then guarded-block* resumes with the rest of the forms (in the same definition context). - If it is a guard form, then you transform its contents and the rest of the forms into a cond expression, with a recursive call in the right place. - Anything else, assume it's an expression, and trampoline the same as for a definition. Also, because you're calling local-expand, you should disarm the result of local-expand and then call syntax-protect on the syntax you produce. If you don't disarm, then you might get "cannot use identifier tainted by macro transformer" errors. If you don't call syntax-protect, your macro can be misused to circumvent other macros' protection. I've attached an implementation. Ryan On Wed, Oct 28, 2020 at 11:54 AM Jack Firth <jackhfi...@gmail.com> wrote: > So I'm a little tired of writing code like this: > > (define x ...) > (cond > [(take-shortcut? x) (shortcut x)] > [else > (define y (compute-y x)) > (cond > [(take-other-shortcut? x y) (other-shortcut x y)] > [else > (define z ...) > (cond ...)])]) > > That is, I have some logic and that logic occasionally checks for > conditions that make the rest of the logic irrelevant, such as an empty or > false input or something else that should trigger an early exit. Each check > like this requires me to write a cond whose else clause wraps the > remainder of the body, leading to an awkward nesting of cond forms. I > don't have this issue when the early exits involve raising exceptions: in > those cases I can just use when and unless like so: > > (define x ...) > (unless (passes-check? x) (raise ...)) > (define y ...) > (unless (passes-other-check? x y) (raise ...)) > (define z ...) > ... > > I'm aware of a few macros in the racket ecosystem that try to solve this > problem. For example, Jay wrote a blog post > <http://jeapostrophe.github.io/2013-11-12-condd-post.html> that creates a > condd form that's like cond but allows embedded definitions using a #:do > keyword. I've also seen various approaches that use escape continuations to > implement the early exit. There's drawbacks I'm not happy about however: > > - > > For cond-like macros that allow embedded definitions, it looks too > different from regular straight-line Racket code. I like my function bodies > to be a sequence of definitions and expressions, with minimal nesting, just > like the when and unless version above. I don't have to use a keyword > or extra parentheses to signal whether a form is a definition or a when > / unless check in error-raising code, why should I have to do that in > code that uses early returns? > - > > Continuation-based solutions impose a nontrivial performance penalty > and have complex semantics. I don't like that the generated code behaves > differently from the cond tree I would normally write. What happens if > I stick an early exit inside a lambda? Or a thread? What if I set up a > continuation barrier? Does that matter? I don't know and I don't want to > think about that just to write what would be a simple if (condition) { > return ... } block in other languages. > > So I wrote a basic macro for this and I have some questions about how to > make it more robust. The macro is called guarded-block and it looks like > this: > > (guarded-block > (define x (random 10)) > (guard (even? x) else > (log-info "x wasn't even, x = ~a" x) > -1) > (define y (random 10)) > (guard (even? y) else > (log-info "y wasn't even, y = ~a" y) > -1) > (+ x y)) > > Each guard clause contains a condition that must be true for evaluation > to proceed, and if it isn't true the block takes the else branch and > finishes. So the above would expand into this: > > (block > (define x (random 10)) > (cond > [(not (even? x)) > (log-info "x wasn't even, x = ~a" x) > -1] > [else > (define y (random 10)) > (cond > [(not (even? y)) > (log-info "y wasn't even, y = ~a" y) > -1] > [else (+ x y)])])) > > This part I got working pretty easily. Where I hit problems, and where I'd > like some help, is trying to extend this to support two important features: > > - > > I should be able to define macros that *expand* into guard clauses. > This is important because I want to implement a (guard-match <pattern> > <expression> else <failure-body> ...) form that's like match-define > but with an early exit if the pattern match fails. I'd also really like to > add a simple (guard-define <id> <option-expression> else > <failure-body> ...) form that expects option-expression to produce an > option > > <https://gist.github.com/jackfirth/docs.racket-lang.org/rebellion/Option_Values.html> > (a value that is either (present v) or absent) and tries to unwrap it, > like the guard let construct in Swift > <https://www.hackingwithswift.com/sixty/10/3/unwrapping-with-guard>. > - > > Begin splicing. The begin form should splice guard statements into the > surrounding body. This is really an offshoot of the first requirement, > since implementing macros that expand to guard can involve expanding > into code like (begin (define some-temp-value ...) (guard ...) (define > some-result ...)). > > Having been around the Racket macro block before, I know I need to do some > kind of partial expansion here. But honestly I can't figure out how to use > local-expand, syntax-local-context, syntax-local-make-definition-context, > and the zoo of related tools. Can someone point me to some existing macros > that implement similar behavior? Or does anyone have general advice about > what to do here? I'm happy to share more examples of use cases I have for > guarded-block if that helps. > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/CAAXAoJVXCRo1CTk8rNDHmPZR_%2BhkMg2f%3DCb8T%3D8KFONUiM%2B_Hw%40mail.gmail.com > <https://groups.google.com/d/msgid/racket-users/CAAXAoJVXCRo1CTk8rNDHmPZR_%2BhkMg2f%3DCb8T%3D8KFONUiM%2B_Hw%40mail.gmail.com?utm_medium=email&utm_source=footer> > . > -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CANy33qnzo2SEpPJjAcuLwhDkHPq_F94J2a-Sb2SUJgwLbEjG_w%40mail.gmail.com.
guarded-block.rkt
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