Joseph Ferano
fedaec3e18
There is no TCO here and recur is not a cheaper substitute for one: the compiler verifies the call is in the loop body's tail position, so the mistake is a compile error where it was written rather than a stack overflow somewhere else. A loop is a let, a While whose condition is true, and two jumps — emit.ml is untouched, and the barrier question recur asks is the one labelled break already answered. Tail position is a permission that is withdrawn at the top of check, the same read-and-withdraw defer_ok does, handed back only by a block's last form, both arms of an if and a match arm. So nothing enumerates the forms that are not tails, which a pre-pass over the Ast would have had to, and would have had to keep doing. loop is also a barrier for break and continue, which is added rather than inherited: a loop answers with the value of its body and a jump out has no value to give. That is also why it takes no label. A while inside a loop keeps its own break. Two things the shape forced. A loop binding is a plain name, because destructuring would make recur's argument count unreadable off the binding vector. And in_loop's "moves a value bound outside the loop" rule had to be told about the loop's own names, or (loop [v (vec-new i32)] ...) would have been refused for doing the ordinary thing.
