林子篆

Posted on Jun 21, 2018 • Originally published at dannypsnl.github.io on Jun 22, 2018

Error is Value

#go #errors

I think most of Gopher had read error-handling-and-go

Has anyone had watched Go Lift?

Let’s getting start from Go Lift!

The point of Go Lift is: Error is Value.

Of course, we know this fact. But do you really understand what that means?

In Go Lift , John Cinnamond mentions a trick about wrapping the error by command executor.

For example, we create a connection to server:6666 by TCP.

conn := net.Dial("tcp", "server:6666")

Can we? Ah…, No!

Correct code is

conn, err := net.Dial("tcp", "server:6666")
if err != nil {
    panic(err)
}

Then we writing something to the connection.

nBtye := conn.Write([]byte{`command`})

We want that, but the real code is

nBtye, err := conn.Write([]byte{`command`})
if err != nil {
    panic(err)
}
// using nByte

Next, we read something from server:6666, so we create a reader.

reader := bufio.NewReader(conn)
response := reader.ReadString('\n')

No! We have to handle the error.

response, err := reader.ReadString('\n')
if err != nil {
    panic(err)
}
// using response

But the thing hasn’t ended yet if we have to rewrite the command if response tells us the command fail?

If we are working for a server, we can’t just panic?

So Go Lift has a solution:

func newSafeConn(network, host string) *safeConn {
    conn, err := net.Dail(network, host)
    return &safeConn{
        err: err,
        conn: conn, // It's fine even conn is nil
    }
}

type safeConn struct {
    err error

    conn net.Conn
}

func (conn *safeConn) Write(bs []byte) {
    if conn.err != nil {
    // if contains error, do nothing
        return
    }
    _, err := conn.Write(bs)
    conn.err = err // update error
}

func (conn *safeConn) ReadString(delim byte) string {
    if conn.err != nil {
        return ""
    }
    reader := bufio.NewReader(conn.conn)
    response, err := reader.ReadString("\n")
    conn.err = err
    return response
}

Then usage will become

conn := newSafeConn("tcp", "server:6666")
conn.Write([]byte{`command`})
response := conn.ReadString('\n')

if conn.err != nil {
    panic(conn.err)
}
// else, do following logic

But can we do much more than this?

Yes! We can have an error wrapper for executing the task.

type ErrorWrapper struct {
    err error
}

func (wrapper *ErrorWrapper) Then(task func() error) *ErrorWrapper {
    if wrapper.err == nil {
        wrapper.err = task()
    }
    return wrapper
}

Then you can put anything you want into it.

w := &ErrorWrapper{err: nil}
var conn net.Conn
w.Then(func() error {
    conn, err := net.Dial("tcp", "server:6666")
    return err
}).Then(func() error {
    _, err := conn.Write([]byte{`command`})
})

Wait! But we need to send the connection to next task without an outer scope variable. But how to?

Now let’s get into reflect magic.

type ErrorWrapper struct {
    err error
    prevReturns []reflect.Value
}

func NewErrorWrapper(vs ...interface{}) *ErrorWrapper {
    args := make([]reflect.Value, 0)
    for _, v := range vs {
        args = append(args, reflect.ValueOf(v))
    }
    return &ErrorWrapper{
        err: nil,
        prevReturns: args,
    }
}

func (w *ErrorWrapper) Then(task interface{}) *ErrorWrapper {
    rTask := reflect.TypeOf(task)
    if rTask.NumOut() < 1 {
        panic("at least return error at the end")
    }
    if w.err == nil {
        lenOfReturn := rTask.NumOut()
        vTask := reflect.ValueOf(task)
        res := vTask.Call(w.prevReturns)
        if res[lenOfReturn-1].Interface() != nil {
            w.err = res[lenOfReturn-1].Interface().(error)
        }
        w.prevReturns = res[:lenOfReturn-1]
    }
    return w
}

func (w *ErrorWrapper) Final(catch func(error)) {
    if w.err != nil {
        catch(w.err)
    }
}

Now, we coding like

w := NewErrorWrapper("tcp", "server:6666")

w.Then(func(network, host string) (net.Conn, error) {
    conn, err := net.Dail(network, host)
    return conn, err
}).Then(func(conn net.Conn) error {
    _, err := conn.Write([]byte{`command`})
    return err
}).Final(func(e error) {
    panic(e)
})