Properly handing errors?

[dnaeon]

In Go it is a common practice to have functions, which return errors in case something went wrong, e.g. func Foo() (FooResult, error).

Doing so it is up to the caller to decide whether this is a fatal error or something that can be handled properly.

Considering the following Go code, which registers a function in Lua, what would be the best way to handle errors? Should we just panic, which would raise an exception in Lua or is there a better way to do that?

package main

import (
        "log"
        "os/user"

        "github.com/layeh/gopher-luar"
        "github.com/yuin/gopher-lua"
)

type Person struct {
        Name string
        Home string
}

func main() {
        L := lua.NewState()
        defer L.Close()

        person := L.NewTable()
        person.RawSetH(lua.LString("new"), luar.New(L, NewPerson))
        L.SetGlobal("person", person)

        if err := L.DoFile("create-person.lua"); err != nil {
                log.Fatal(err)
        }
}

func NewPerson(name string) *Person {
        user, err := user.Lookup(name)
        if err != nil {
                // How to propagate the error to Lua ??
                // panic ??
        }

        p := &Person{
                Name: name,
                Home: user.HomeDir,
        }

        return p
}

And the following example Lua code.

p = person.new("nonexisting-user")
print("username: " .. p.Name)
print("homedir: " .. p.Home)

Thanks,
Marin

[ghost]

You could panic. That would mean you have to use pcall to capture the error if you want to handle it gracefully:

ok, ret = pcall(person.new, "nonexisting-user")
if not ok then
    print(ret)
else
   print("username: " .. ret.Name)
   print("homedir: " .. ret.Home)
end

Another solution is similar to how io.open operates: return the value on success, or nil and an error string on failure:

func NewPerson(name string) (*Person, string) {
        user, err := user.Lookup(name)
        if err != nil {
                return nil, err.Error()
        }

        p := &Person{
                Name: name,
                Home: user.HomeDir,
        }

        return p, ""
}

person, err = person.new("nonexisting-user")
if not person then
    print(err)
else
   print("username: " .. person.Name)
   print("homedir: " .. person.Home)
end

I prefer the latter.

[dnaeon]

Thanks, I like the latter solution as well :)