Arc Forum | It could be, if you had an old function that was using optional arguments, and t...

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2 points by almkglor 6466 days ago | link | parent

It could be, if you had an old function that was using optional arguments, and then eventually had to add even more arguments, which you finally decide to make keyworded; without breaking existing code, you can support both optional and keyword args.

What I would like to see is optional, keyword, and rest arguments. Imagine something like this:

 (def foo (k v)
 'type (k int)
 (+ k v))

3 points by kennytilton 6466 days ago | link

The syntax to add rest args, if it followed the CL example, would be:

 (dsb (r1 &o o1 &r rest &k k1) data ....)

If data was (1 2 'k1 3) then most params would be bound as expected and then rest would be bound to (k1 3).

But now the data (1 2 'k1 3 4) causes an error "Key list is not even", ie, once you say &k you undertake certain obligations as the caller. Even if you even up the list:

 (1 2 'k1 2 3 5)

...you get an error "3 is an invalid keyword", because 3 appears in a keyword position. This can be avoided by announcing your intention to have undeclared keywords:

 (a b &rest rest &key k1 &allow-other-keys)

That of course is CL, and it kinda makes my day that if I were crazy enough to extend dsb in Arc I would end up with:

 (a b &r rest &k k1 &aok)

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1 point by kennytilton 6465 days ago | link

"...if I were crazy enough to extend dsb..."

Was there ever any doubt? :)

Still requiring extensions from earlier posts:

 (def dsb-params-parse (params)
 (withs (reqs nil key? nil opt? nil keys nil opts nil
 rest (mem '&r params)
 rest-var (cadr rest)
 aok? (find '&aok cddr.rest)
 resting? nil
 no-mas nil)
 (each p params
 (if no-mas (assert nil "No params &aok, OK?" ',params)
 (is p '&o) (do (assert ~opt? "Duplicate &o:" ',params)
 (assert ~key? "&k cannot precede &o:" ',params)
 (= opt? t))
 (is p '&k) (do (assert ~key? "Duplicate &k:" ',params)
 (= key? t))
 (is p '&r) (= resting? t)
 (is p '&aok) (= no-mas t)
 key? (push-end p keys)
 (and opt? (no resting?)) (push-end p opts)
 (no resting?) (do (assert (~acons p) "Reqd parameters need not be defaulted:" p)
 (push-end p reqs))))
 (prt 're-obj!!!!! reqs opts rest-var keys aok?)
 (obj reqs reqs opts opts rst rest-var keys keys aok? aok?)))

And man was I happy to have the above as a breakout from the macro itself:

 (mac dsb (params data . body)
 (w/uniq (tree kvs valid-keys aok?)
 `(withs (,tree ,data
 ,@(let plist (dsb-params-parse params)
 (prn `(reqs ,plist!reqs))
 (prn `(rst ,plist!rst))
 (prn `(keys ,plist!keys))
 (prn `(&aok ,plist!aok?))
 (with (n -1)
 (+ (mappend [list _ `(nth ,(++ n) ,tree)] plist!reqs)
 (mappend [list (carif _) `(if (< ,(++ n) (len ,tree))
 (nth ,n ,tree)
 ,(cadrif _))] plist!opts)
 `(,plist!rst (nthcdr ,(++ n) ,tree))
 `(,valid-keys ',plist!keys)
 `(,aok? ',plist!aok?)
 `(,kvs (do (prt 'foing ,valid-keys)
 (when (and ,plist!rst ,valid-keys)
 (assert (even (len ,plist!rst)) "Keyword list not even" ,plist!rst)
 (let ,kvs (pair ,plist!rst)
 (prt 'vetting ,valid-keys 'againt ,kvs)
 (unless ,aok?
 (assert (all [find (car _) ,valid-keys] ,kvs)
 "Invalid key in" (map car ,kvs)))
 (prt 'kvs!!!!! ,kvs)
 ,kvs))))
 (mappend [list (carif _)
 `(do (prt 'kvs!!! ',(carif _) ,kvs)
 (aif (assoc ',(carif _) ,kvs)
 cadr.it
 ,(cadrif _)))] plist!keys)))))
 ,@body)))

What is missing (largely) is graceful handling of invalid use.

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1 point by almkglor 6465 days ago | link

> What is missing (largely) is graceful handling of invalid use.

(err:tostring:write ...) works fine for me for reporting errors and aborting

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2 points by kennytilton 6466 days ago | link

Ok, and then going forward only the new code has the burden of supplying optionals... hmmm, refactoring at 7am with the demo to the CEO scheduled for 9am?...

Lock and load! Add the keywords!! :)

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