Re: New scoping rule for "repeat" and "continue" patch
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- Subject: Re: New scoping rule for "repeat" and "continue" patch
- From: Rici Lake <lua@...>
- Date: 2005年9月26日 11:38:46 -0500
On 26-Sep-05, at 10:38 AM, Roberto Ierusalimschy wrote:
The compiler could detect such cases and signal an error ("jumping
across local declaration"), but that would make the mechanism more
complex (to implement, to explain, etc.).
Indeed, but that would arbitrarily prohibit "valid" code, since the
problem isn't jumping over a local declaration but rather *using* an
uninitialised value.
To be more precise, the first two of the following are presumably
correct whereas the third is incorrect, but unless the compiler
actually did use analysis, all three would be rejected as "jumping
across local declaration":
Example 1:
repeat
local done = false
local w = f()
if w == "skip" then continue end
local ok, msg = handle(w)
if msg then print(msg) end
done = not ok
until done
Example 2:
repeat
local w = f()
if w == "skip" then continue end
local ok, msg = handle(w)
if msg then print(msg) end
if not ok then break end
until false
Example 3: (The error)
repeat
local w = f()
if w == "skip" then continue end
local ok, err = handle(w)
if not ok and err then print(err) end
until not ok
----------------------
As I've said, I have no particular object to "continue" but the above
seems to indicate that it does not work with the new semantics of
repeat...until. It could, of course, be restricted to "for" and "while"
loops, or it could be allowed in "repeat" loops with the proviso that
the "repeat" loop is syntactic sugar for a while loop:
repeat <body> until <expr>
===>
while true do <body>; if <expr> then break end end
in which case, the "continue" statement would effectively skip the
termination test.
However, both of these possibilities increase the "cognitive load", and
that needs to be balanced against the utility of the construct.
As a postscript, in a private mail to Glenn Maynard (which he suggested
I could have posted publicly), I mentioned that his example of
continue:
if (cond1) {
action1;
continue;
}
if (cond2) {
action2;
continue;
}
if (cond3) {
action3;
continue;
}
// repeated many more times
could be more "naturally" (to me) expressed in either C or Lua by using
else if/elseif:
if (cond1) {
action1;
}
else if (cond2) {
action2;
}
else if (cond3) {
action3;
}
-- in Lua:
if cond1 then
action1
elseif cond2 then
action2
elseif cond3 then
action3
end
This idiom is independent of inclusion in a looping construct.