CODEWITHSUNDEEP
lua
MengCheng Wei
MengCheng Wei

Reputation: 727

How do Lua return an error without end of the program?

I have a simple lua code looks like below.

local function my_fun(x)
    return nil, error("oops", 2)
end

local res, err = my_fun("foo")
print(res)
print(err)
print("finish")

What I expected is that the program can print until "finish", but I got the program exit. How should I do for just return the error instead of exit?

lua: test.lua:5: oops
stack traceback:
        [C]: in function 'error'
        test.lua:2: in local 'my_fun'
        test.lua:5: in main chunk
        [C]: in ?

Upvotes: 0

Views: 1020

Answers (1)

Lucas S.
Lucas S.

Reputation: 2401

Lua does not have a runtime error/exception value. error does not return anything, instead it triggers a panic that unwinds the stack until caught.

You can catch such a panic with protected calls, using pcall(). pcall will return a boolean that is true when no error occurred and either the error or return value:

local function my_fun(x)
    if x == "foo" then
       error("oops")
       -- notice the lack of return, anything after `error()` will never be reached
       print("you will never see me")
    end
    return x
end

local ok, value = pcall(my_fun, "foo")
print(ok, value) -- prints "false, oops"

ok, value = pcall(my_fun, "bar")
print(ok, value) -- prints "true, bar"

Alternatively, you can define your own runtime error type. This could be as simple as just a string or as complex as an elaborate metatable-based class.

local function my_fun(x)
    return nil, "oops" -- a simple string as 'error type'
end
-- alternatively
local function my_fun2(x)
    return nil, debug.traceback("oops") -- also just a string, but includes a strack trace.
    -- Note that generating a trace is expensive
end

local res, err = my_fun("foo")
print(res)
print(err)
print("finish")

Programming in Lua also has multiple chapters on error handling: https://www.lua.org/pil/8.3.html.

Upvotes: 3

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