How to make sure some operations in ANY situation? (aka: with statement in Python)

[Xavier Wang <weasley.wx@...>]

hi all :)

I just found that the error/pcall/xpcall in lua is just like exceptions, you can report a error, and catch it like this:

function foo()

   ...

   if notReady then error"not ready" end

   ...

end

if pcall(foo) then

  ...

end

but I still have some question about this Idiom:

1. how can I make sure some operations MUST happen?

xpcall may be can do this, but the errfunc may not called if f call success.

some pseudo-code:

local f = io.open("file")

-- do something maybe call error, but I don't want to catch the error, just want error spread outside of me, but I want f:close tobe called:

do

   foo()

finally

   f:close()

end

all I want is if (whatever) error in foo is throwing, the f:close will be called, but the error will still throw out of here.

2. how to re-throw err to outside?

if you call error in err function in xpcall, the err function will called again, and i can't spread err outside.

xpcall(function()

    error"abc"

end, function(e)

    print(e)

    error(e)

end)

will call err function a lot of times. and the stack overflow...

3. how to handle multi return value of f?

how can I do this (pseudo-code):

function f() return 1, 2, 3 end

local status, retv... = pcall(f)

if status then

   -- do sth with retv...

   print(retv...)

else

   print("error: ", retv...)

end

(to assign multiret into a "tuple", and extract it in other place)?

if error/pcall/xpcall can:

- make exception safe,

- rethow error outside, or process error but not touch it,

- handle function return value safely (don't miss any values).

I think we can use it to split normal process code and error process code, just like exceptions does.