How to define an `instanceof` operation in the FFI

[foretspaisibles]

While working on some FFI features prototypes I have tried to expose the instanceof operation but my understanding of the compiler internals is too limited for this.

I am trying to implement jscl:js-instance-of so that

(jscl:js-instanceof "String instance" #j:String)
=> T

My attempts were

;; Not good enough!
(define-builtin js-instanceof (x constructor)
  (convert-to-bool `(instanceof ,x ,constructor)))


;; Not good enough eithehr!
(define-compilation js-instanceof (x constructor)
  `(call-internal |js_to_lisp| (instanceof ,x ,constructor)))

Any idea, how I could implement this?

[davazp]

(jscl:js-instanceof "String instance" #j:String) => T

that is probably never going to be true because strings in JSCL are implemented as arrays. The reason is that strings in Common Lisp are mutable, but immutable in JS.

[foretspaisibles]

Oh I see, but then what about

(jscl:js-instanceof "String instance" #j:Object)
=> T

If I got my test rights, my definitions do not pass this either.

[davazp]

Ok, I found the problem! Additionally I'll share a cool trick to debug this kind of problems.

In the JSCL repl, evaluate #:Object. Then in the JS console, do jscl.CL['*'].value. That will show you the raw JS value of the last expression. You'll see that JSCL wrapped Object to be a CL function so it is not the constructor..

Using more the internals, you can get the raw value like this:

(js-instanceof "String instance" (%js-vref "Object" t))

That works for for first attempt! Note that 't' argument to %js-vref means raw. You can find the definition in compiler/compiler.lisp

[davazp]

Note how that relates to the problem 3 from the FFI design discussions you just added:

https://github.com/jscl-project/jscl/wiki/FFI-Design-Discussions#problem-3-conversion-between-lisp-and-js-values

We definitely need that. Or even better, if we could think of a representation of functions in CL so no conversion is needed.

[foretspaisibles]

I prototyped a higher-level specification for foreign class interface, see the wiki and the linked gist!

BTW, is there a list of non-implemented features? (Aside from the README.)

[davazp]

BTW, is there a list of non-implemented features? (Aside from the README.)

Not really, we used to have a file with all symbols from CLHS and their implementation status, but not anymore.