I've been working on a prototype implementation of the binary data spec in pure
JS (implemented via typed arrays) and I've been bitten by the lack of a
standard mechanism for subclassing Function.
I'm using proxies for the implementation, and Proxy.createFunction doesn't let
me specify a custom prototype. Now, I can understand that this preserves the
existing property of the language that the only callable things are either
regexps or descendants of Function. But we could extend the proxy API to allow
custom functions with user-specified prototypes and still preserve this
property:
Proxy.createFunction(handler, callTrap[, constructTrap[, proto]])
The proto argument would default to the original value of Function.prototype
[1]. But if the user provides a prototype, the library could enforce that proto
instanceof Function [2]. This way we would preserve the invariant that for any
callable value v, either v is a regexp or (typeof v === "function" && v
instanceof Function).
Maciej has also suggested a Function.create(...) API for more lightweight
creation of function subtypes. This would strengthen the argument for allowing
Proxy.createFunction to specify a prototype, since Proxy.createFunction() ought
to be able to do anything Function.create() can do.
I'm curious to know if there are reasons I've missed why Proxy.createFunction()
doesn't support a custom prototype. It seems to me like a nice additional
expressiveness without any great loss of language invariants. But I may have
missed something.
Thanks,
Dave
[1] In the face of multiple globals, this would be the Function associated with
the same global as Proxy. (BTW, I intend to work with Andreas on speccing out
multiple globals.)
[2] Again, the Function associated with the same global as Proxy.
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