Would anyone be willing to give some advice on how #:fallbacks is intended to be used? It seems someone else was similarly confused, but nobody answered: https://stackoverflow.com/questions/52287226/why-cant-code-in-fallbacks-refer-to-the-generic-methods

The issue is attempting to refer to methods defined in #:defaults when defining a #:fallback. I was attempting to add a default method to Lexi Lamba’s generic collections library: (define-generics indexable (ref indexable . _) (set-ref indexable . _) (update-ref indexable . _) #:defaults ([hash? (define ref hash-ref) (define set-ref hash-set) (define update-ref hash-update)] [dict? (define ref dict-ref) (define set-ref dict-set) (define update-ref dict-update)] [sequence? (define ref nth) (define set-ref set-nth)] ) #:fallbacks [(define/generic reference ref) (define/generic set-reference set-ref) (define (update-ref indexable index proc) (let ([old (reference indexable index)]) (set-reference indexable index (proc old))))]) but without the extra (define/generic …) forms, it claims ref and set-ref are not defined. Surely this isn’t how it’s supposed to be done (the linked OP mentions it is in fact a syntax error to use (define/generic) outside of #:methods), but I’d argue that it’s not immediately obvious how this works.

Cheers.

As I understand within the #:methods the interface names (ref/set-ref/update-ref) are from the scope of the #:methods block. So within the block a function call doesn’t have to go through the generics machinery, but if you want to go through the machinery (like you have wrapped a generic value) you can get to it with define/generic

Edit: indeed, it is mentioned in the documentation for define/generics: “When used inside the method definitions associated with the #:methods keyword, binds local-id to the generic for method-id. This form is useful for method specializations to use generic methods (as opposed to the local specialization) on other values.” in fairness, the documentation on #:fallbacks mentions it has the exact same form as #:methods. Honestly, though, I don’t think I’d have had any chance of coming across/working that out for myself. Oh well, now I know! Thank you :slightly_smiling_face: