Arc Forumnew | comments | leaders | submitlogin
2 points by are 3939 days ago | link | parent

Very nice work on supporting both opt and key args.

Although if you had something like this:

(defun fn (a &o (b 'b) (c 'c) &k (d 'd))

with a usage like this:

(fn 1 'd 'e)

... how would you know whether:

1) 'd is the value of the first opt arg and 'e is the value of the second (the key arg unsupplied)

or

2) 'd is the key for the key arg, and 'e is its supplied value (the 2 opt args unsupplied)

?

Maybe I'm missing something here, but it seems to me that unless you have special syntax for keywords, you will get into trouble.

And if you have to introduce special syntax for keys anyway, it is just as well to make every single argument keyable on its symbol (even vanilla ones), and just worry about combining &o and &rest (which should then be doable).



3 points by kennytilton 3938 days ago | link

"with a usage like this: (fn 1 'd 'e) how would you know..."

The interpretation is that d and e are the two optional args, so any caller wanting to supply a keyword arg has to supply the optionals. Recall that I said it was possible, not sensible. :) But in tool design I think we should let users hang themselves rather than guess (perhaps wrongly) that no one would ever come up with a good use for such a thing.

A second, lesser motivation is that CL works that way.

-----

2 points by kennytilton 3938 days ago | link

The gang on comp.lang.lisp reminded me of (in effect):

  (def read-from-string (s &o eof-error-p eof-value 
                           &k start end preserve-whitespace)
     ...)
Which does make me think the guess about keywords being added as an afterthought might be spot on.

-----

2 points by kennytilton 3938 days ago | link

c.l.lisp just offered a much better observation: the optional args above are standard for the various "read" functions, and the start and end keywords are standard for string functions. Read-from-string then is inheriting consistently from both families.

-----

2 points by eds 3938 days ago | link

Even CL's special keyword syntax doesn't save you from optional and keyword confusion:

  [1]> (defun test (a &optional (b 'b) (c 'c) &key (d 'd))
         (list a b c d))
  TEST
  [2]> (test 1 :d :e)
  (1 :D :E D)
Optional parameters always bind first in CL, and I believe dsb is written to mimic that behavior.

Having all parameters be keyword arguments as well might be interesting, but it wouldn't avoid optional/keyword confusion.

-----

1 point by are 3938 days ago | link

> Having all parameters be keyword arguments as well might be interesting, but it wouldn't avoid optional/keyword confusion.

Why not?

Let's say you have a function with 3 standard args followed by 2 opt args. So you have 5 args, all keyable on the symbol you give them in the def.

Let's further say that in a call to this function, I key the middle standard arg (#2 in the def) plus the first opt arg (#4 in the def) and also, I omit the second opt arg (#5 in the def). So, I'm supplying two standard args apart from the two args I'm keying. Then the function call parser would know, after identifying the 2 keyed args and matching them to positions #2 and #4 in the def, that the first non-key arg supplied corresponds to position #1 in the def, the second non-key arg supplied corresponds to position #3 in the def, and that an arg for position #5 in the def is missing, leading to the usage of the default value for this opt arg.

This would even work when you want to raise an error for a non-supplied, non-opt arg.

Wouldn't this work quite intuitively (keying an arg when calling a function "lifts it out" of the normal "vanillas then optionals" argument sequence, shortening that sequence, put keeping a well-defined order for it)? (You would need special syntax for keys in this proposal. My suggestion is a colon appended to the arg symbol, rather than prepended, like in CL.)

Can someone give a counterexample if they think this somehow wouldn't work?

&rest args are left as an exercise for the reader :-)

-----