Re: [patch] continue statement
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- Subject: Re: [patch] continue statement
- From: jrs@...
- Date: 2005年9月20日 21:38:39 -0500 (CDT)
A little late, but ...
The semantics of continue is available in Lua. No patch required.
while something do
repeat
...
if expr then break end
...
until true
end
Here 'break' is read as 'goto end of block' ('continue', if you
prefer :-)), and 'repeat ... until true' is read as 'block ... end'.
Some other control structures produced by 'misusing' repeat:
perl's while { ... next ... } continue { ... }
while something do
repeat
...
if expr then do break end end
...
until true
... -- the continue block
-- is always executed just before the
-- conditional is to be evaluated again.
end
perl's redo and more:
while something do
... -- only runs if not redo
local redo = false
repeat
if redo then
... -- runs only if redo
redo = false
end
repeat
...
-- next line is pronounced 'redo if expr;' :-)
if expr then redo = true break end
...
until true
... -- runs whether or not redo
until not redo
... -- continue block doesn't run if redo
end
Whew! That's enough of that.
The 'goto end of block' can be used to reduce 'if nests' and 'else
thickets', the problem the patch was to solve. For example,
if a then
...
else
...
if b then
...
else
... -- code 1 if not a and not b independent of c or d
if c then
...
else
...
if d then
...
end
end
... -- code 2 if not a and not b perhaps dependent on c or d
end -- some elses in here would make this code even harder to grasp
end
becomes
repeat
if a then
...
else break end
...
if b then
...
else break
end -- the traditional placement, easier to read perhaps.
... -- code 1 if not a and not b
if c then
...
else -- code 2 may prevent use of break here. perhaps it could
-- be here and else could be replaced by 'else break end'
...
if d then
...
end
end
... -- code 2 here if code under 'if c' or 'if d' does
-- influence
until true
where 'repeat ... until true' is read as 'block ... end fnord:'
and 'else break' is read as 'goto fnord'. :-)
--
jrs