"Though Clojure's mixture of Arc 'if' and optional types is a whole new level of unholiness."
Is this what you meant instead? "Though Clojure's 'let', which mixes Arc's 'withs' with type hints, is a whole new level of unholiness."
Even so, I think Clojure's 'let doesn't actually have any[1] added complexity when it comes to type hints. It might look like the list is bunched into groups of either two or three depending on whether a type hint is present...
(let [^String x "x string"
y 2]
(body-goes-here))
...but that's not a quality of 'let. That's a quality of the ^ syntax. An occurrence of ^ consumes the next two s-expressions, just like ' consumes the next one s-expression:
So the bindings of a 'let are consistently bunched into groups of two s-expressions, just like Arc's 'withs.
[1] Of course, the type hints are actually used for optimization at some point, so the complexity of parsing them has to go somewhere. This is exactly as complex as destructuring: Arc's 'withs syntax supports destructuring, but it doesn't need special-case destructuring logic because it just translates down to 'fn. As it happens, Clojure's 'let syntax also supports destructuring, and it probably uses the same general technique.
Yes! Turns out I didn't notice the switch from if to let.
I have to say, though, I have no sympathy for the argument that it's still groups of two s-expressions. As a reader it's still more onerous to have to mentally group:
^a b
compared to:
'a
So the presence or absence of parsing complexity feels irrelevant.
