I can't think of a single non-lisp that allows adjacent condition and action without an intervening token.
I mean, isn't this requirement mostly because those languages are infix anyway? Parsing gets easier with explicit ways of separating things. I could easily imagine shift/reduce conflicts or what-have-you cropping up when you try to eliminate the requirement for, say, parentheses around conditionals in Java.
For example, in some hypothetical infix language that doesn't require conditional separators (parens, braces, then, etc.), would
if x==10-x ...
be
if (x == 10) -x ... // maybe this language doesn't use the "return" keyword,
// so the conditional is returning negative ten.
or
if (x == (10 - x)) ...
?
Because Lisps use s-expressions, "intervening tokens" (per se) are unnecessary. As you say, Arc's choices don't seem so unreasonable, considering that.
Yeah, that's a good point. Non-lisps use keywords and punctuation for readability and to make parsing tractable, and the two reasons are often hard to separate in any single design decision.
To summarize my position: I have some sympathy for the specific argument that multi-branch 'if's are harder to read in lisp than in traditional languages[1]. But this affects any arrangement of parens, whether traditional 'cond' or arc 'if'.
