@sorawee It’s worth noting that define-syntaxes can only show up in fully-expanded code at the module top level—define-syntax used as an internal definition is guaranteed to go away after expansion. And that makes sense, when considered together with the things others have already pointed out.

@mbutterick does this work? https://docs.racket-lang.org/reference/generic-numbers.html?q=denominator#%28def._%28%28quote._~23~25kernel%29._numerator%29%29

Hey, I got a somewhat complicated question for syntax experts around here. I’m trying to attach meta-information to a syntax identifier at compile-time, by overriding the identifier defined by struct with my own (and keeping the old identifier somewhere for future reference). But once an identifier has been defined with define-syntax, it seems impossible to override it (I get an “already defined” error). Is there a better way to embed more info into a struct identifier at compile-time?

I’m doing all this because I need struct’s field names (and parent’s field names recursively), and they are not provided by extract-struct-info (only the final getters and setters)

I’d like to do something similar to what struct does: creating a struct:id and keeping it inside a syntax variable so that other structs can use it when inheriting. But as the identifier for my struct is already taken, I can’t use the exact same name.

Does all that make sense?

@jerome.martin.dev usually doing this requires that there’s a struct-type-property you can use like prop:match-expander. I think there’s a similar property for structs

I think prop:struct-info is what you want

but aren’t struct properties used at run-time?

maybe I could just… use make-struct-type and keep my own wrapper around that, or something… my head hurts

I have some questions. Do you want to attach the meta-information to existing structs defined by other libraries, or would you be fine with a macro that defines new structs with meta-information attached?

the later :slightly_smiling_face:

Okay that makes it slightly easier I think.

You shouldn’t need to go all the way down to make-struct-type.

What I’ve done in the past is define the struct with different id passed in for the #:name and #:constructor-name fields, and then use define-syntax with an instance of a compile-time normal+struct-info+extra-meta-information struct, which has prop:procedure for the normal behavior, prop:struct-info for the struct-info behavior, and prop:whatever-you-need-for-meta-information for your meta information.

I see

by "normal+struct-info+extra-meta-information struct" you mean a struct instance that exists only at compile-time?

or a syntax?

Like this: (begin (struct foo [field ...] option ... #:name internal-foo #:constructor-name internal-foo) (define-syntax foo (normal+struct-info+extra-meta-information (make-var-like-transformer #'internal-foo) (extract-struct-info (syntax-local-value #'internal-foo)) ...extra-meta-information...))) Where somewhere you have a struct definition for normal+struct-info+extra-meta-information in a begin-for-syntax: (begin-for-syntax (struct normal+struct-info+extra-meta-information [normal struct-info extra-meta-information] #:property prop:procedure (struct-field-index normal) #:property prop:struct-info (lambda (self) (normal+struct-info+extra-meta-information-struct-info self)) #:property prop:extra-meta-information ...stuff...))

I think I’m starting to understand, I just don’t get why you would need a prop:procedure though

Oh, that’s so that you can still use the struct name as a constructor, like you can use (foo 1 2 3) to make an instance of (struct foo [a b c])

ok I see! In my case, I made the macro that generates the struct to also generate a syntax used as the constructor, so I guess I won’t need this

I’ll try your method, thanks!

@jerome.martin.dev in answer to your earlier question, prop:struct-info is meant to be used at compile time, not runtime. So you’d use (define-syntax foo (make-my-meta-info)) where my-meta-info is a struct type with prop:struct-info.

And then your macros can use syntax-local-value to get the my-meta-info instance and use it to decide what code to generate

Ok, that’s what seemed the most logical, but I wasn’t sure I understood it correctly!

(oops I didn’t notice that that’s exactly what alex’s code does - long codeblocks are very hard to read on mobile)

it seems like it’s nearly working! Thanks a lot for your help!

so, brainfart here, I’m trying to call (system* "/usr/bin/ls" "/tmp/") from inside a serve/servlet, everything works outside the servlet, but always fails inside … I’m guessing this is because of sandbox but I can’t find the docs for it

is there a way to get a call to system* to work inside a servlet?

wow, so this is a new one

apparently trying to run emacs with /dev/null as the working directory was the problem

I guess this is why we have a /var/run/ folder