$ java -jar semi_arc.jar
arc> (let ((o x 2)) nil x)
2
arc> (let (a (o b 2)) '(1) b)
2
arc> (let default 2 ((fn ((o x default)) x)))
2
arc> (let default 2 (let ((o x default)) nil x))
2
arc> (let default 2 (let (a (o b default)) '(1) b))
2
arc> (let (x y) '(a b c) y)
*** 2 args expected for #<fn:2::(#((a b c)))>: (a b c)
0: (#<done> #<fn:2::(#((a b c)))> a b c)
arc> (let (x y z nothing) '(a b c) nothing)
*** 4 args expected for #<fn:4::(#((a b c)))>: (a b c)
0: (#<done> #<fn:4::(#((a b c)))> a b c)
arc>
I upvoted you 'cause Semi-Arc's doing what you want it to do (which is what really matters), but I'd like to mention that I actually kinda appreciate the way Arc accepts mismatched destructuring lengths.
I've seen other people's code use a (let (a b c) nil ...) idiom as a shortcut for (with (a nil b nil c nil) ...). As for me, I like being able to design multiple types so that they can be used in an ad-hoc polymorphic way like this:
Both these techniques could still be used in an Arc implementation with a stricter treatment of destructuring: The first could be done by defining a 'w/nil macro, and the second could be expressed as (let (x y . rest) rep.point ...). In your favor, the strict treatment makes it easier to write functions that do complain if the length is incorrect, encouraging fail-fast code. However, as far as portability is concerned, the "N args expected" errors are a quirk of Semi-Arc.
I'm going to try out Semi-Arc right now. ^_^ I'm sure I'll have some error reports for you in a moment. Hopefully not too many! :-p
Nice. Have you played around with multiple arc implementations in the process of making semi-arc? I'd love to read about your experience writing semi-arc, and the design decisions you made. (email in profile if you want to chat further)
No, I have not played the original Arc yet and had hardly known both implementations in Java until recently. My version 1 implementation was a little Lisp written in ISO Standard Pascal:
Little Lambda Lisp in Standard Pascal
http://www.oki-osk.jp/esc/llsp/v1.html
You find the notable(?) feature of Semi-Arc, macro expansion without free symbol capture, even in the version 2 of it:
Little Lambda Lisp 2 in Standard Pascal
http://www.oki-osk.jp/esc/llsp/v2.html
I had translated it to Ruby, Python, C# and Java. Recently I noticed that its design is near to the core of Arc. I made it up so that it may look Arc:-). The result is Semi-Arc.
As for argument destructuring, I implemented it in a simple, general and recursive (but not so efficient) way. Track the variable 'nestedArgs' in the method 'compile(Cell j, Cell env)' in the source:
