From e6e5910cb432f0fb25a8af3dc6cb9950fd9a05f5 Mon Sep 17 00:00:00 2001 From: Graham Northup Date: Fri, 29 Jan 2021 11:34:00 -0500 Subject: Clarify key_value_swap's edge case (#10799) In compiler design especially, leaving behavior as "undefined" is a _strong_ condition that basically states that all possible integrity is violated; it's the kind of thing that happens when, say, dereferencing a pointer with unknown provenance, and most typically leads to a crash, but can result in all sorts of spectacular errors--thus, "it is undefined" how your program will melt down. The pure-Lua implementation of `key_value_swap` does not permit UB _per se_ (assuming the implementation of Lua itself is sound), but does deterministically choose the value to which a key is mapped (the last in visitation order wins--since visitation order is arbitrary, _some_ value _will_ be chosen). Most importantly, the program won't do something wildly unexpected. --- doc/lua_api.txt | 3 ++- 1 file changed, 2 insertions(+), 1 deletion(-) diff --git a/doc/lua_api.txt b/doc/lua_api.txt index 12ea85345..cfc150edc 100644 --- a/doc/lua_api.txt +++ b/doc/lua_api.txt @@ -3275,7 +3275,8 @@ Helper functions * Appends all values in `other_table` to `table` - uses `#table + 1` to find new indices. * `table.key_value_swap(t)`: returns a table with keys and values swapped - * If multiple keys in `t` map to the same value, the result is undefined. + * If multiple keys in `t` map to the same value, it is unspecified which + value maps to that key. * `table.shuffle(table, [from], [to], [random_func])`: * Shuffles elements `from` to `to` in `table` in place * `from` defaults to `1` -- cgit v1.2.3