I was thinking that if you have an even simpler way to bind specific variables in the function call, you can nest these anonymous functions, and not have the arguments shadow each other.
> In that case having different delimiters might be the way to go. So your brace notation would be used for anonymous functions with full lambda list capability.
Yeah, I think you're right. The square-brace anonymous function could have optionally a curly-braced first argument. If that argument isn't there, it works as the square-braced anonymous function currently does. If that argument is there, it's the arguments to that function. Since curly braces aren't used anywhere else in Arc, it can only be parsed in that one way.
But perhaps this is all a lot of work to avoid writing `fn (arg1 arg2)`. Other than golfing, I don't know if you really want to nest functions this way. And this seems like a waste of the only paired characters Arc doesn't use.
"And this seems like a waste of the only paired characters Arc doesn't use."
I don't think even the  syntax really pulls its weight. Between (foo [bar baz _]) and (foo:fn_:bar baz _), the latter is already more convenient in some ways, and one advantage is that we can define variations of (fn_ ...) under different names without feeling like these variations are second-class.
(Arc calls it "make-br-fn" rather than "fn_", but I wanted the example to look good. :-p )
Personally, I've never gotten comfortable with the colon intrasymbol syntax. Even in your example, I'm having trouble parsing it right now. I really don't like how it makes "bar" look like part of the first part, not the second.