Good question. Here are a few things that tripped me up:
1) An easy way to call differing functions based on the data type of an argument (polymorphism). Ruby has a nice approach as shown here (see the section marked 'Duck Typing'): http://www.ibm.com/developerworks/java/library/j-ruby/
2) I spent a while chasing a bug in an 'if' statement because I neglected to surround multiple statements with a 'do' statement.
3) It wasn't obvious to me how to build a bitmap array. I naively created an array filled with zeros for each scanline. I would then push this scanline onto another var. But it turned out all of the scanlines pushed this way were pointing to the same scanline. This was a problem until I found out how to copy a var in Arc (use copy!).
4) It's not clear to me how you modularize code, so everything is in one file.
5) Coding style. I have none when it comes to Arc, and there are few examples. For example, I didn't realize that an Arc convention is to append an asterisk to global variables (This follows the Lisp convention I think).
6) Spent time figuring out how to make my vector operations take list arguments without quotes (This was my initiation into the world of macros). Then realized that this was not necessary since these functions would have variables passed in arguments, not literal lists.
7) Errors were not specific (understatement).
8) No raw file output, or low level bit operators. So I added these to the base language.
1) Hmm. Currently I've been experimenting around with Arc's type system. The type system is "not bad" but there's a definite problem: making a user-defined type quack like a built-in type is very difficult. My "Create your own collection" series is probably of interest in this research.
If however you are using only user types (not masquerading existing types), then nex3's defm macro is your friend: http://arclanguage.com/item?id=4644
2) LOL. However, if you need just the "then" branch, or just the "else" branch, you can use the 'when and 'unless macros:
(from "arc.arc")
[mac] (when test . body)
When `test' is true, do `body'.
See also [[unless]] [[if]] [[awhen]]
nil
arc> (help unless)
(from "arc.arc")
[mac] (unless test . body)
When `test' is not true, do `body'.
See also [[when]] [[if]] [[no]]
nil
4) Definite problem. In fact, pg's own code doesn't seem very modularized: bits and pieces of the various parts of the webserver are in srv.arc, html.arc, and app.arc. Fortunately Arkani's 'help command also includes which file the function is defined in, so at least you can track files a little (although in practice I often find myself looking for definitions using grep).
5) Arkani has a "CONVENTIONS" file for some conventions. However it's incomplete IMO, so probably we need to fill this in somewhat.
7) True, true. Sure, Arc code is fun to write, until it encounters a problem. Then it's hard to debug/optimize/whatever
