Sounds like would be more suitable for #C03S1KBA2.

you can just call meta on anything so that extra satisfies check is a little redundant

(or (satisfies? TheProtocol x) (contains? (meta x) 'my-qualified/impl-name))

I would say that calling satisfies? in the first place is probably a code smell and would question why you were doing it?

ah. I thought I had run into issues calling meta on things that don’t implement it but could just be my failing memory

user=> (meta 42)
nil
user=> (meta "s")
nil
user=>

It’s a generic data-structure viewer which allows customizing/overriding views for particular types, so we can’t make any assumptions about what is being passed in

@U050RLRRQ So this check is... where...? Protocols can be extended to Object so that would be the default case where no more specific protocol extension applies.

ah i know where I got that idea from, with-meta

but yes meta can be safely called

meta includes a check on the type (in Clojure) and returns nil otherwise.

we run values through a list of “viewers”, each of which can either handle the value or “pass” if it does not apply. the viewers can be dynamically rebound

so we can’t only rely on implementing a single protocol as the viewers can take more than just an object’s type to decide whether they handle it or not

Could it be a good use case for "Ask For Forgiveness, Not Permission"?

if you ask me this check is totally fine. you could skip it if you provide a default implementation, but that only makes sense if you control the full protocol

It was a general question, and I meant catching the exception that would be thrown if there's no implementation and treating it as "welp, seems like it doesn't satisfy the protocol after all".

I'd assume the common case will not have this, so the exception will probably be the common case. but that assumption comes from building shadow-cljs Inspect. which may or may not apply elsewhere 😛

yeah in the vast majority of cases a built-in viewer will be used, eg. inspecting common data structures (a generic map-viewer, list-viewer, etc), or a viewer for a custom type where we don’t need to use a protocol. But in certain cases we may want to view eg. a map differently without changing its value and so putting a viewer in its metadata can work nicely

thank you everyone for your help

but if there always is a default viewer then you can just call it? and let the metadata basically override it? so no need to check?

not sure I follow exactly. if I call the. protocol’s fn on something that hasn’t implemented it, it will throw?

brb

no it'll throw if its not implemented. Just thought you have a extend-protocol for the common types already, and letting the metadata override that

Some people have also used the technique of adding a special method to the protocol isFoo or so and extend the protocol to Object and return false there, to bypass satisfies?. I'm not sure if that applies here.