Are you looking for `let/ec`?
(let/ec return
(define x (random 10))
(unless (even? x)
(log-info "x wasn't even, x = ~a" x)
(return -1))
(define y (random 10))
(unless (even? y)
(log-info "y wasn't even, y = ~a" y)
(return -1))
(+ x y))
Alex.
On Wednesday, October 28, 2020 at 6:54:44 PM UTC+8 jackh...@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.
>
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