Is it idiomatic to define an “abstract struct” that provides a interface, then define struct as subtypes of it?

For incrementally building strings, string output ports are the usual solution.

Can I think that the second form of the begin (https://docs.racket-lang.org/reference/begin.html) is semantically equivalent to a function call? > (begin expr …+) That is (define (beginf . exps) (if (null? exps) (void) (last exps))) (+ 1 (beginf (displayln "111") (displayln "222") 333)) ;;111 ;;222 ;;334 = (+ 1 (begin (displayln "111") (displayln "222") 333)) ;;111 ;;222 ;;334 Because in Racket (https://docs.racket-lang.org/guide/application.html) > a function call is evaluated by first evaluating the proc-expr and all arg-exprs in order (left to right).

Yes, that’s correct

Thanks.

Say that I have structure types A and B that implement a generic interface. Is it possible to have a method implementation for type B call the implementation for type A?

I have tried to do this, but it appears that calling the method does not dispatch to the implementation for A and instead recurses.

iirc for generics you define like #:methods gen-id [(define proc-id proc-expr)] well you could have a proc like (define (A-impl self . args) (void)) and in the struct A #:methods gen-id [(define proc-id A-impl)] so then in B #:methods gen-id [(define (proc-id b . args) (apply A-impl b args))] now i dont really remember if this is alright, also this is the naive approach

Hm, that won’t work in my case… I accidentally oversimplified the problem. In fact, the method implementations are mutually recursive.

hmph mind giving a gist with a more fleshed out example of the problem?

Yeah, inside the place where you define the B method, any call to the method will just directly call the B method you defined instead of dispatching on the value. The way around that is to define another function with a different name that’s an alias of the dispatching version, and then you can call that one. The define/generic utility is a recommended way to define that alias, and its documentation has a full example: https://docs.racket-lang.org/reference/struct-generics.html?q=define-generic#%28form._%28%28lib._racket%2Fgeneric..rkt%29._define%2Fgeneric%29%29\|https://docs.racket-lang.org/reference/struct-generics.html?q=define-generic#%28form._%28%28lib._racket%2Fgeneric..rkt%29._define%2Fgeneric%29%29

That works! Thank-you.

I tried @rokitna’s suggestion, and that solved my problem.