A CL-style keyword argument is like an optional argument in that you need to give it a default value for when it's omitted. But in addition, with a keyword argument at function-call-time, you can switch argument order if you specify the key.

This means that Arc could treat keyword arguments as a special case of optional arguments.

This is the optional-arg example from the Arc tutorial:

arc> (def greet (name (o punc (case name who #\? #\!)))

       (string "hello " name punc)) 

#<procedure: greet>

arc> (greet 'who)

"hello who?"

But let's say you allow both "o" and "k" for optional arguments, and when there is a "k", you allow the argument symbol to be used as a key when calling the function, like this:

arc> (def greet (name (k punc (case name who #\? #\!)))

       (string "hello " name punc)) 

#<procedure: greet>

arc> (greet 'who)

"hello who?"

arc> (greet 'john)

"hello john!"

arc> (greet :punc "?!?" 'jane)

"hello jane?!?"

Of course, once you allow an argument symbol to be used as an argument key when calling, you could actually extend this opportunity to every single argument, optional or not. Then you would no longer need the "k" syntax, since every argument is "keyable":

arc> (def greet (name (o punc (case name who #\? #\!)))

       (string "hello " name punc))

#<procedure: greet>

arc> (greet 'who)

"hello who?"

arc> (greet :punc "?!?" :name 'jane)