Go: Error is Value

error_wrapper.go

package wrap

import (
	"reflect"
)

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

func NewErrorWrapper(vs ...interface{}) *ErrorWrapper {
	args := make([]reflect.Value, 0)
	// if no vs, nothing happened
	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)
	// here reflect.Type.NumOut promise:
	//  1. task is a function(else NumOut will panic)
	//  2. < 1 mean, task has at least return 1 value
	if rTask.NumOut() < 1 {
		panic("at least return error at the end")
	}
	if w.err == nil {
		vTask := reflect.ValueOf(task)
		lenOfReturn := rTask.NumOut()
		// reflect.Value.Call using []reflect.Value as it's arguments.
		res := vTask.Call(w.prevReturns)
		// res[lenOfReturn-1] mean error at here
		//
		// for example:
		//  Then(func() (string, error) { return "", nil })
		//
		// lenOfReturns = 2
		// res[0] is string
		// res[1] is error, 1 = lenOfReturns - 1
		//
		// this check is because when error = nil, it can't convert to type error
		if res[lenOfReturn-1].Interface() != nil {
			w.err = res[lenOfReturn-1].Interface().(error)
		}
		// prepare next task's arguments(when we want to passing some result to next task)
		w.prevReturns = res[:lenOfReturn-1]
	}
	return w
}

// Final using catch handle the error generated by a task.
func (w *ErrorWrapper) Final(catch func(error)) {
	if w.err != nil {
		catch(w.err)
	}
}