I'll admit I'm not a Ruby developer, but wow it allows return conditionally inside an expression? What were the language designers thinking?! I can't think of any other language that allows return inside an expression (or break/continue). Let's see: C, C++, C#, Python, Javascript, Object Pascal, Java, Rust; all nope. That is indefensible.
You've listed languages that separate statements from expressions in their syntax. There is a whole other family of languages where choose consists entirely of expressions - starting with LISP in the 50s, and including virtually all FP languages today (F#, Ocaml, Haskell, SML, Clojure). Ruby happens to be one a member of the latter group. To be fair, most of the languages in that group don't have an equivalent of return/break/continue either - I think only Ruby and some Lisps do.
Regardless, all of the languages above except C and maybe Object Pascal can still have control stop in the middle of an expression, since an expression can throw an Exception (indirectly). Returning is not significantly different from that.
> I can't think of any other language that allows return inside an expression ... Rust
Er, what?
You understand almost everything in Rust is an expression right?
let x = if yeah_nah() { return 5; } else { "X" };
That return is an expression, obviously we mostly care about the expression's side effect which is a change of control flow to leave the function with some sort of integer 5 as the return value (the function signature tells Rust which kind of integer this 5 is) - but the expression "return 5" itself does have a type, it's ! aka Never, an Empty Type because no values of this type can exist - because the side effect will change the control flow before we need a value.
Rust's type inference engine is fine with this because under type arithmetic all the values fit in a single type, the type of "X" - &'static str - there are no values on the left to disrupt that.
It's not doing "return conditionally inside an expression". The above statement is equivalent to:
if !condition
return
end
In other words, the return in question has no argument. The unusual (but not unique) aspect of Ruby here is that if/unless can suffix the "then" block and not just prefix it.
This isn't exactly true. It's a syntax error to put a return anywhere where the grammar expects a value expression, because it's one of few constructs in Ruby that doesn't return a value (contrast with e.g. "if" and "def" and "class", all of which return a value).
E.g. "42 == (return true)" is a syntax error while "42 == if false; true; end" is syntactically valid.
You can return in the expression in the comment above because the return is at statement level - nothing above it in the grammar requires a value (but obviously allows it).
Off the top of my head I can't remember which other keywords fall in that category. Obviously "end", "then", "alias" and there'll be a few more, but for most keywords in Ruby you're right they can be treated as expressions.
