On 10/17/19 1:33 PM, Lassi Kortela wrote:
>> Apart from efficiency concers, classic Common Lisp-style keyword args
>>> are also hard to type-check statically.
>>
>> Actually they aren't, as long as the keywords are apparent at the point of
>> call.
>
> That seems true, but there has to be a reason why Kawa and Racket both have keywords as syntax markers instead of actual keyword objects. The reason was discussed in that big keywords thread in July but escapes me.

Consider:
     (f x y z)
Obvious a call to f with 3 positional (non-keyword) arguments, right?

But what about:
     (define y k1:)
     (f x y z)

Ooops. Turns out if was 1 positional and 1 keyword argument.
But only if f allows keyword arguments.

Unless you have some kind of type-checking/data-flow you can't guard
against this.  Even with that type of analysis you can't always *prove*
whether y is a keyword or not, so you have to compile conservative code
and check at run-time.
--
	--Per Bothner
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