> For the same reasons I would use a `let` around a code block -- it's an easy > way to isolate computation and create reusable elements. What would a more > normal way be when using syntax-parse?
The #:with syntax-parse "pattern directive" does the same thing, but with less parens. https://docs.racket-lang.org/syntax/stxparse-specifying.html?#%28tech._pattern._directive%29 > The syntax->datum part is because I want it to default to #t whereas > (attribute missing-optional-value) would be #f and offer no way to > distinguish between a missing argument (which should be interpreted as #t) > and an explicit #f. I see. IMO it's awkward because it's trying to handle three cases for only two possible outputs. I think the more Racket-standard way to do this is to only have 2 possibilities: explicit argument or no argument. See the struct options for examples, eg #:mutable or #:omit-define-syntaxes >> #lang racket >> (require (for-syntax racket racket/syntax syntax/parse)) >> (define-syntax (foo stx) >> (syntax-parse stx >> [(foo name (~optional flag)) >> #:with propthing (if (attribute flag) >> #'(#:property prop:foo (delay "stuff")) >> #'()) >> #`(begin >> (define-values (prop:foo foo? foo-ref) >> (make-struct-type-property 'foo 'can-impersonate)) >> (struct name (id) (~@ . propthing) #:transparent) >> (name 'bob))])) >> >> (foo person) >> (foo thing #f) >> >> answer 3) A more "rackety" way would be if you included a part of the >> output as the optional input. (this would invert your default case >> though). This is the more natural use case for the ~? and ~@ patterns >> because you no longer need the extra "if": >> >> #lang racket >> (require (for-syntax racket racket/syntax syntax/parse)) >> (define-syntax (foo stx) >> (syntax-parse stx >> [(foo name (~optional prop-val)) >> #`(begin >> (define-values (prop:foo foo? foo-ref) >> (make-struct-type-property 'foo 'can-impersonate)) >> (struct name (id) (~? (~@ #:property prop:foo (delay >> prop-val))) #:transparent) >> (name 'bob))])) >> >> (foo person) >> (foo thing "stuff") > > > Yep. That's how I would have done it if the prop-val were something simple, > but this example was highly simplified for clarity In point of fact the > contents of the `delay` are going to be a significant pile of reflectance > data built off the struct transformer. Not something that the user should > care about aside from saying "yes, do / don't include this." > >> >> >> On Wed, May 15, 2019 at 10:56 AM David Storrs <david.sto...@gmail.com> wrote: >> > >> > I'd like to find a general mechanism, when writing macro code, to say "If >> > this optional argument was supplied, generate this code. If not, generate >> > this other code", where "this other code" might be nothing at all. I feel >> > like this should be simple, but my brain is failing. >> > >> > As an example, consider a macro that generates a struct; the macro has an >> > optional argument that controls whether a property is added, and it >> > defaults to 'add it'. I thought I could do the following, but it doesn't >> > compile because the bits aren't spliced properly and I'm not sure what I'm >> > missing. Note that the syntax->datum part is because I want it to default >> > to #t whereas (attribute missing-optional-value) would be #f and offer no >> > way to distinguish between a missing argument and an explicit #f. Again, >> > I feel like there should be a simpler way. >> > >> > #lang racket >> > (require (for-syntax racket racket/syntax syntax/parse)) >> > (define-syntax (foo stx) >> > (syntax-parse stx >> > [(foo name (~optional arg:boolean)) >> > (with-syntax ([propthing >> > (if (syntax->datum #'(~? arg #t)) >> > #'(#:property prop:foo (delay "stuff")) >> > #'())]) >> > #`(begin >> > (define-values (prop:foo foo? foo-ref) >> > (make-struct-type-property 'foo 'can-impersonate)) >> > (struct name (id) (~@ propthing) #:transparent) >> > (name 'bob)))])) >> > >> > (foo person) >> > (foo thing #f) >> > >> > >> > >> > >> > >> > -- >> > 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/CAE8gKofM%3Dep0B%2BY0YXOq5uqooh-jDTBr7mZ9yDK%2BFrVVVbEHjA%40mail.gmail.com. >> > For more options, visit https://groups.google.com/d/optout. > > -- > 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/CAE8gKocZmY6W1iuH77GHEM7_G6uePtAkCUZsBWnsijw60tW8YA%40mail.gmail.com. > For more options, visit https://groups.google.com/d/optout. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. 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