Re: fallbacks
[
Date Prev][
Date Next][
Thread Prev][
Thread Next]
[
Date Index]
[
Thread Index]
- Subject: Re: fallbacks
- From: Roberto Ierusalimschy <roberto@...>
- Date: 1998年4月06日 10:04:18 -0300
> Lua 3.1 doesn't have fallbacks. How do I get fallback like behavior
> out of tags? I want to get tags to apply to *all* tables, not just ones
> I specifically add them too.
All tables have a default tag (you can get its value with "tag{}").
If you set a tag method for this tag, it will aplly
to all "regular" tables (those you did not change the tag).
For a complete and exact mapping of fallbacks to tag methods, see the
following code. But I don't think you will need all that.
-- Roberto
--------------------------------------------------------------
-- Definition of "setfallback" using tag methods
-- (for compatibility with old code)
--------------------------------------------------------------
-- fallbacks defaults for each event:
local defaults = {
gettable = function () error('indexed expression not a table') end,
settable = function () error('indexed expression not a table') end,
index = function () return nil end,
getglobal = function () return nil end,
arith = function () error('number expected in arithmetic operation') end,
order = function () error('incompatible types in comparison') end,
concat = function () error('sting expected in concatenation') end,
gc = function () return nil end,
['function'] = function () error('called expression not a function') end,
error = function (s) write(_STDERR, s, '\n') end,
}
function setfallback (name, func)
-- set the given function as the tag method for all "standard" tags
-- (since some combinations may cause errors, use call to avoid messages)
local fillvalids = function (n, func)
call(settagmethod, {0, n, func}, 'x', nil)
call(settagmethod, {tag(0), n, func}, 'x', nil)
call(settagmethod, {tag(''), n, func}, 'x', nil)
call(settagmethod, {tag{}, n, func}, 'x', nil)
call(settagmethod, {tag(function () end), n, func}, 'x', nil)
call(settagmethod, {tag(settagmethod), n, func}, 'x', nil)
call(settagmethod, {tag(nil), n, func}, 'x', nil)
end
assert(type(func) == 'function')
local oldfunc
if name == 'error' then
oldfunc = seterrormethod(func)
elseif name == 'getglobal' then
oldfunc = settagmethod(tag(nil), 'getglobal', func)
elseif name == 'arith' then
oldfunc = gettagmethod(tag(0), 'pow')
foreach({"add", "sub", "mul", "div", "unm", "pow"},
function(_, n) %fillvalids(n, %func) end)
elseif name == 'order' then
oldfunc = gettagmethod(tag(nil), 'lt')
foreach({"lt", "gt", "le", "ge"},
function(_, n) %fillvalids(n, %func) end)
else
oldfunc = gettagmethod(tag(nil), name)
fillvalids(name, func)
end
return oldfunc or rawgettable(%defaults, name)
end