Related
For a given custom error type:
type CustomError struct {
// Err optionally wraps the original error.
Err error `json:"-"`
// Human readable message.
Message string `json:"message" validate:"required,gte=3"`
// StatusCode is a valid HTTP status code, e.g.: 404.
StatusCode int `json:"-"`
}
Which implements both Error() string, and the Unwrap() error interface, also have a factory:
func NewCustomError(m string, s int, e error) *CustomError {}
How to create another "type", based on CustomType - let's call it FailedTo for errors such as "Failed to created X" which will have by default:
A prefixed Message with Failed to
Status code of 500
and on top of that, another one, such as FailedToCreateSomething in a way where....
func createSomething() error {
return FailedToCreateSomething(errors.New("File is busy"))
}
errCreateSomething := createSomething()
...errCreateSomething is type of FailedToCreateSomething, also FailedTo, and CustomError?
Let's distill the example down to it's essence.
package customerror
import (
"errors"
)
type CustomError struct {
Aux string
Err error
}
func (cE *CustomError) Error() string { /*...*/ }
func (err *CustomError) Unwrap() error { return err.Err }
func NewFailedToError(aux string, err error) error {
return &CustomError{ Aux: aux, Err: err }
}
var FailedToWriteToFile = NewFailedToError("write to file", nil)
We can now get to the question in the gist:
// Some function just for demonstration.
func WriteToFile() error {
// Something caused some error..
errSome := errors.New("Failed to open file")
// How can I set the `err` value of `FailedToWriteToFile` to `errSome`
// without setting that for all `FailedToWriteToFile` instances (pointer)?
// while still making it of `FailedToWriteToFile` type (errors.Is(errSome, FailedToWriteToFil))?
return FailedToWriteToFile
}
Let's reframe this question to how do I make an errSome with a new message where errors.Is(errSome, FailedToWriteToFil) holds.
We can checkout the documentation of errors.Is:
Is reports whether any error in err's chain matches target.
The chain consists of err itself followed by the sequence of errors obtained by repeatedly calling Unwrap.
An error is considered to match a target if it is equal to that target or if it implements a method Is(error) bool such that Is(target) returns true.
An error type might provide an Is method so it can be treated as equivalent to an existing error. For example, if MyError defines
func (m MyError) Is(target error) bool { return target == fs.ErrExist }
then Is(MyError{}, fs.ErrExist) returns true. See syscall.Errno.Is for an example in the standard library.
This gives us two paths. One of the paths is to put FailedToWriteToFile on the Unwrap chain. CustomError has enough fields for this if we use the Err field to point to FailedToWriteToFile, e.g.
&CustomError{Aux: "Failed to open file", Err: FailedToWriteToFile}
Unwrap() on this is == to FailedToWriteToFile. FWIW Aux could be field of type error if you are trying to capture an error value coming from another source. As long as the Unwrap() chain leads to the FailedToWriteToFile eventually.
The other path is to define an Is predicate on CustomError. There are a lot of possible predicates to choose from. A simple one is that all *CustomError are considered equivalent. That would be:
func (e *CustomError) Is(target error) bool {
_, ok := target.(*CustomError)
return ok
}
Code:
import (
"fmt"
logger "log"
"os"
"testing"
)
var log = logger.Default()
func TestErrorWrapping(t *testing.T) {
_, err := os.ReadFile("wrong file path")
if err != nil {
// custom error with wrapped a source error
err = fmt.Errorf("can't read file, %w", err)
// print it
log.Printf("something went wrong, %s", err.Error())
}
}
Output:
something went wrong, can't read file, open wrong file path: The system cannot find the file specified.
I would like to pass a function pointer to a function to "anything".
It's easy to print something that gets passed in from just about anything (as in https://play.golang.org/p/gmOy6JWxGm0):
func printStuff(stuff interface{}) {
fmt.Printf("Testing : %v", stuff)
}
Let's say, though, that I want to do this:
Have multiple structs
Have data loaded from various functions
Have a generic print that calls the function for me
I tried this in a Play (https://play.golang.org/p/l3-OkL6tsMW) and I get the following errors:
./prog.go:35:12: cannot use getStuff1 (type func() SomeObject) as type FuncType in argument to printStuff
./prog.go:36:12: cannot use getStuff2 (type func() SomeOtherObject) as type FuncType in argument to printStuff
In case the Play stuff gets deleted, here's the code I'm trying to figure out how to get to work:
package main
import (
"fmt"
)
type SomeObject struct {
Value string
}
type SomeOtherObject struct {
Value string
}
type FuncType func() interface{}
func getStuff1() SomeObject {
return SomeObject{
Value: "Hello, world!",
}
}
func getStuff2() SomeOtherObject {
return SomeOtherObject{
Value: "Another, hello!",
}
}
func printStuff(toCall FuncType) {
stuff := toCall()
fmt.Printf("Testing : %v", stuff)
}
func main() {
printStuff(getStuff1)
printStuff(getStuff2)
}
What is the secret sauce to get this stuff passed in properly?
Larger Goal Explanation
So what I am trying to accomplish here is reduction of boilerplate code that lives inside a gigantic file. Unfortunately I cannot refactor it further at this point due to other restrictions and I was wondering if this were possible at all considering the error messages and what I had read seemed to dictate otherwise.
There's a large amount of copy-and-paste code that looks like this:
func resendContraDevice(trap *TrapLapse, operation *TrapOperation) {
loaded := contra.Load()
err := trap.SnapBack(operation).send(loaded);
// default error handling
// logging
// boilerplate post-process
}
func resendPolicyDevice(trap *TrapLapse, operation *TrapOperation) {
loaded := policy.Load()
err := trap.SnapBack(operation).send(loaded);
// default error handling
// logging
// boilerplate post-process
}
// etc.
In these, the Load() functions all return a different struct type and they are used elsewhere throughout the application.
I want hoping to get something where I could have:
loaded := fn()
err := trap.SnapBack(operation).send(loaded);
// default error handling
// logging
// boilerplate post-process
Signature for send is, which accepts an interface{} argument:
func (s SnapBack) send(data interface{}) error
I don't know if you have control over the return values of contra.Load() and policy.Load(), for instance, so there may be a better approach, but assuming those cannot be modified, this would allow you to eliminate a lot of boilerplate, without any fancy manipulation:
func boilerplate(tram *TrapLapse, operation *TrapOperation, loader func() interface{}) {
loaded := loader()
err := trap.SnapBack(operation).send(loaded);
// default error handling
// logging
// boilerplate post-process
}
func resendContraDevice(trap *TrapLapse, operation *TrapOperation) {
boilerplate(trap, operation, func() interface{} { return contra.Load() })
}
func resendPolicyDevice(trap *TrapLapse, operation *TrapOperation) {
boilerplate(trap, operation, func() interface{} { return policy.Load() })
}
If there's nothing more complex, you can also simplify this even further:
func boilerplate(tram *TrapLapse, operation *TrapOperation, loaded interface{}) {
err := trap.SnapBack(operation).send(loaded);
// default error handling
// logging
// boilerplate post-process
}
func resendContraDevice(trap *TrapLapse, operation *TrapOperation) {
boilerplate(trap, operation, contra.Load())
}
func resendPolicyDevice(trap *TrapLapse, operation *TrapOperation) {
boilerplate(trap, operation, policy.Load())
}
May I know how to pass imagick.MagickWand struct to functions and apply it methods on it? It seems that the imagick.NewMagickWand return the type of *imagick.MagickWand, isn't it?
I can't get it done, keep getting the error message: ERROR_WAND: ContainsNoImages `MagickWand-0'.
How to pass proper struct reference to the function so that the mw can be continuously used in those functions?
func generateImage() error {
// skip error handling
var err error
mw := imagick.NewMagickWand()
defer mw.Destroy()
err = createCanvas(mw, "red") // create canvas
err = compositeIcon(mw, "c:/icon.png") // add icon
err = addText(mw, "Hello world") // add text
err = mw.WriteImage("c:/output") // get the output
}
func createCanvas(mw *imagick.MagickWand, color string) error {
// skip error handling
var err error
pw := imagick.NewPixelWand()
defer pw.Destroy()
pw.SetColor("blue")
err = mw.NewImage(200, 100, pw)
return nil
}
May please help? Newbie here. :)
Update:
The example that I gave is correct. Passing by reference is done correctly in the example above, I received the error because I overlook my code and debug the wrong lines. Sorry for confusion.
Thanks.
If mw has type *imagick.MagickWand then *mw has type imagick.MagickWand.
That is to say, mw is a pointer to that type and the * operator dereferences the pointer to the type itself.
mw := imagick.NewMagickWand() // Now mw is a *imagick.MagickWand
*mw // Is a imagick.MagickWand
I am new to golang, my application needs to return multiple errors in a loop, later requires to be combined and returned as a single error string. I am not able to use the string functions to combine the error messages. What methods can be use to combine these errors into a single error before returning ?
package main
import (
"fmt"
"strings"
)
func Servreturn() (err error) {
err1 = fmt.Errorf("Something else occured")
err2 = fmt.Errorf("Something else occured again")
// concatenate both the error
return err3
}
UPDATE for Go 1.13:
As of Go version 1.13, the language's errors package now supports error wrapping directly.
You can wrap an error by using the %w verb in fmt.Errorf:
err := errors.New("Original error")
err = fmt.Errorf("%w; Second error", err)
Use Unwrap to remove the last error added, and return what remains: previousErrors := errors.Unwrap(err)
Playground Example for errors.Unwrap
Two more functions, errors.Is and errors.As provide ways to check for and retrieve a specific type of error.
Playground Example for errors.As and errors.Is
Dave Cheney's excellent errors package (https://github.com/pkg/errors) include a Wrap function for this purpose:
package main
import "fmt"
import "github.com/pkg/errors"
func main() {
err := errors.New("error")
err = errors.Wrap(err, "open failed")
err = errors.Wrap(err, "read config failed")
fmt.Println(err) // "read config failed: open failed: error"
}
This also allows additional functionality, such as unpacking the cause of the error:
package main
import "fmt"
import "github.com/pkg/errors"
func main() {
err := errors.New("original error")
err = errors.Wrap(err, "now this")
fmt.Println(errors.Cause(err)) // "original error"
}
As well as the option to output a stack trace when specifying fmt.Printf("%+v\n", err).
You can find additional information about the package on his blog: here and here.
String functions don't work on errors because error is really an interface that implements the function Error() string.
You can use string functions on err1.Error() and err2.Error()
but not on the "err1" reference itself.
Some errors are structs, like the ones you get from database drivers.
So there's no natural way to use string functions on errors since they may not actually be strings underneath.
As for combining two errors:
Easy, just use fmt.Errorf again.
fmt.Errorf("Combined error: %v %v", err1, err2)
Alternatively:
errors.New(err1.Error() + err2.Error())
You could use the strings.Join() and append() function to acheive this slice.
example: golang playgorund
package main
import (
"fmt"
"strings"
"syscall"
)
func main() {
// create a slice for the errors
var errstrings []string
// first error
err1 := fmt.Errorf("First error:server error")
errstrings = append(errstrings, err1.Error())
// do something
err2 := fmt.Errorf("Second error:%s", syscall.ENOPKG.Error())
errstrings = append(errstrings, err2.Error())
// do something else
err3 := fmt.Errorf("Third error:%s", syscall.ENOTCONN.Error())
errstrings = append(errstrings, err3.Error())
// combine and print all the error
fmt.Println(fmt.Errorf(strings.Join(errstrings, "\n")))
}
This would output a single string which you can send back to the client.
First error:server1
Second error:Package not installed
Third error:Socket is not connected
hope this helps!
To expand on what #WillC had mentioned in a comment it is possible to define your own error type as error is an interface type. Any type that implements a Error() string function implements the error interface. Therefore, you could create a CollectionError which aggregates errors and returns a concatenated error string.
type ErrorCollector []error
func (c *ErrorCollector) Collect(e error) { *c = append(*c, e) }
func (c *ErrorCollector) Error() (err string) {
err = "Collected errors:\n"
for i, e := range *c {
err += fmt.Sprintf("\tError %d: %s\n", i, e.Error())
}
return err
}
This provides a collection function that appends a given error to a slice. Upon calling Error() string it iterates over the slice and creates a concatenated error string.
func main() {
collector := new(ErrorCollector)
for i := 0; i < 10; i++ {
collector.Collect(errors.New(fmt.Sprintf("%d Error", i)))
}
fmt.Println(collector)
}
There is a great golang.org blog post going over errors in more detail. A full example of the example is available on The Go Playground.
Uber has a multierr package for this use case:
return multierr.Combine(err1, err2)
People may be interested in https://github.com/hashicorp/go-multierror which describes itself as "A Go (golang) package for representing a list of errors as a single error.".
As of Go 1.20, we'll be able to wrap multiple errors using errors.Join.
See this proposal for more details.
First Option
You can print the errors; they will be separated by a newline character when you do that.
var (
ErrIncorrectUsername = errors.New("incorrect username")
ErrIncorrectPassword = errors.New("incorrect password")
)
func main() {
err := validate("ruster", "4321")
// You can print multi-line errors
// Each will be separated by a newline character (\n).
if err != nil {
fmt.Println(err)
// incorrect username
// incorrect password
}
}
func validate(username, password string) error {
var errs []error
// errors.Join the errors into a single error
if username != "gopher" {
errs = append(errs, ErrIncorrectUsername)
}
if password != "1234" {
errs = append(errs, ErrIncorrectPassword)
}
// Join returns a single `error`.
// Underlying, the error contains all the errors we add.
return errors.Join(errs...)
}
Second Option
errors.Join returns an error that contains each error you add. So you can use errors.Is and errors.As to check for individual errors for finer granularity.
// ...
func main() {
err := validate("ruster", "4321")
// You can detect each one:
if errors.Is(err, ErrIncorrectUsername) {
// handle the error here
}
// Or detect the other one:
if errors.Is(err, ErrIncorrectPassword) {
// handle the error here
}
}
func validate(username, password string) error {
// ...
}
Note: This naive validate example is here to convey the idea. Instead of chaining errors, think errors like a tree. Join allows you to do that when combined with other Joins.
Run both on Go Playground.
This seems to work well for me (space separated errors):
Put all your errors in a error slice/list/array, ie: var errors [] error
fmt.Sprintf("%s", errors)
var errors []error
errors = append(errors, fmt.Errorf("error 1"))
errors = append(errors, fmt.Errorf("error 2"))
errors = append(errors, fmt.Errorf("and yet another error"))
s := fmt.Sprintf("%s", errors)
fmt.Println(s)
Sometimes i need the way to detect if there some error in the chain. the standard way, provided with https://pkg.go.dev/errors is pretty convenient:
someErr:=errors.New("my error")
fmt.Errorf("can't process request: %w",someErr)
...
err:=f()
if errors.Is(err,someErr){...}
But it could be applied only in the case of detection of the last error in the chain. You can't wrap someErr1 and then someErr2 and then get true from both of checks: errors.Is(err,someErr1) and errors.Is(err,someErr2)
I solved this problem with following type:
func NewJoinedErrors(err1 error, err2 error) JoinedErrors {
return JoinedErrors{err1: err1, err2: err2}
}
type JoinedErrors struct {
err1 error
err2 error
}
func (e JoinedErrors) Error() string {
return fmt.Sprintf("%s: %s", e.err1, e.err2)
}
func (e JoinedErrors) Unwrap() error {
return e.err2
}
func (e JoinedErrors) Is(target error) bool {
return errors.Is(e.err1, target)
}
It uses the fact, that
An error is considered to match a target if it is equal to that target
or if it implements a method Is(error) bool such that Is(target)
returns true.
So you can join two errors and get positive result on both checks:
someErr1:=errors.New("my error 1")
someErr2:=errors.New("my error 2")
err:=NewJoinedErrors(someErr1, someErr2)
// this will be true because
// (e JoinedErrors) Is(target error)
// will return true
if errors.Is(err, someErr1){...}
// this will be true because
// (e JoinedErrors) Unwrap() error
// will return err2
if errors.Is(err, someErr2){...}
you can check it out here: https://play.golang.org/p/W7NGyfvr0v_N
func condenseErrors(errs []error) error {
switch len(errs) {
case 0:
return nil
case 1:
return errs[0]
}
err := errs[0]
for _, e := range errs[1:] {
err = errors.Wrap(err, e.Error())
}
return err
}
Use this function:
func JoinErrs(errs ...error) error {
var joinErrsR func(string, int, ...error) error
joinErrsR = func(soFar string, count int, errs ...error) error {
if len(errs) == 0 {
if count == 0 {
return nil
}
return fmt.Errorf(soFar)
}
current := errs[0]
next := errs[1:]
if current == nil {
return joinErrsR(soFar, count, next...)
}
count++
if count == 1 {
return joinErrsR(fmt.Sprintf("%s", current), count, next...)
} else if count == 2 {
return joinErrsR(fmt.Sprintf("1: %s\n2: %s", soFar, current), count, next...)
}
return joinErrsR(fmt.Sprintf("%s\n%d: %s", soFar, count, current), count, next...)
}
return joinErrsR("", 0, errs...)
}
It will give you nil when all errors are nil, will give you the same error when just one non-nil error, will give you numbered list of non-nil errors when multiple non-nil errors
Try it out here: https://play.golang.org/p/WttztCr-xHG
Due to error handling in Go, I often end up with multiple values functions. So far, the way I have managed this has been very messy and I am looking for best practices to write cleaner code.
Let's say I have the following function:
type Item struct {
Value int
Name string
}
func Get(value int) (Item, error) {
// some code
return item, nil
}
How can I assign a new variable to item.Value elegantly. Before introducing the error handling, my function just returned item and I could simply do this:
val := Get(1).Value
Now I do this:
item, _ := Get(1)
val := item.Value
Isn't there a way to access directly the first returned variable?
In case of a multi-value return function you can't refer to fields or methods of a specific value of the result when calling the function.
And if one of them is an error, it's there for a reason (which is the function might fail) and you should not bypass it because if you do, your subsequent code might also fail miserably (e.g. resulting in runtime panic).
However there might be situations where you know the code will not fail in any circumstances. In these cases you can provide a helper function (or method) which will discard the error (or raise a runtime panic if it still occurs).
This can be the case if you provide the input values for a function from code, and you know they work.
Great examples of this are the template and regexp packages: if you provide a valid template or regexp at compile time, you can be sure they can always be parsed without errors at runtime. For this reason the template package provides the Must(t *Template, err error) *Template function and the regexp package provides the MustCompile(str string) *Regexp function: they don't return errors because their intended use is where the input is guaranteed to be valid.
Examples:
// "text" is a valid template, parsing it will not fail
var t = template.Must(template.New("name").Parse("text"))
// `^[a-z]+\[[0-9]+\]$` is a valid regexp, always compiles
var validID = regexp.MustCompile(`^[a-z]+\[[0-9]+\]$`)
Back to your case
IF you can be certain Get() will not produce error for certain input values, you can create a helper Must() function which would not return the error but raise a runtime panic if it still occurs:
func Must(i Item, err error) Item {
if err != nil {
panic(err)
}
return i
}
But you should not use this in all cases, just when you're sure it succeeds. Usage:
val := Must(Get(1)).Value
Go 1.18 generics update: Go 1.18 adds generics support, it is now possible to write a generic Must() function:
func Must[T any](v T, err error) T {
if err != nil {
panic(err)
}
return v
}
This is available in github.com/icza/gog, as gog.Must() (disclosure: I'm the author).
Alternative / Simplification
You can even simplify it further if you incorporate the Get() call into your helper function, let's call it MustGet:
func MustGet(value int) Item {
i, err := Get(value)
if err != nil {
panic(err)
}
return i
}
Usage:
val := MustGet(1).Value
See some interesting / related questions:
How to pass multiple return values to a variadic function?
Return map like 'ok' in Golang on normal functions
Yes, there is.
Surprising, huh? You can get a specific value from a multiple return using a simple mute function:
package main
import "fmt"
import "strings"
func µ(a ...interface{}) []interface{} {
return a
}
type A struct {
B string
C func()(string)
}
func main() {
a := A {
B:strings.TrimSpace(µ(E())[1].(string)),
C:µ(G())[0].(func()(string)),
}
fmt.Printf ("%s says %s\n", a.B, a.C())
}
func E() (bool, string) {
return false, "F"
}
func G() (func()(string), bool) {
return func() string { return "Hello" }, true
}
https://play.golang.org/p/IwqmoKwVm-
Notice how you select the value number just like you would from a slice/array and then the type to get the actual value.
You can read more about the science behind that from this article. Credits to the author.
No, but that is a good thing since you should always handle your errors.
There are techniques that you can employ to defer error handling, see Errors are values by Rob Pike.
ew := &errWriter{w: fd}
ew.write(p0[a:b])
ew.write(p1[c:d])
ew.write(p2[e:f])
// and so on
if ew.err != nil {
return ew.err
}
In this example from the blog post he illustrates how you could create an errWriter type that defers error handling till you are done calling write.
No, you cannot directly access the first value.
I suppose a hack for this would be to return an array of values instead of "item" and "err", and then just do
item, _ := Get(1)[0]
but I would not recommend this.
How about this way?
package main
import (
"fmt"
"errors"
)
type Item struct {
Value int
Name string
}
var items []Item = []Item{{Value:0, Name:"zero"},
{Value:1, Name:"one"},
{Value:2, Name:"two"}}
func main() {
var err error
v := Get(3, &err).Value
if err != nil {
fmt.Println(err)
return
}
fmt.Println(v)
}
func Get(value int, err *error) Item {
if value > (len(items) - 1) {
*err = errors.New("error")
return Item{}
} else {
return items[value]
}
}
Here's a generic helper function with assumption checking:
func assumeNoError(value interface{}, err error) interface{} {
if err != nil {
panic("error encountered when none assumed:" + err.Error())
}
return value
}
Since this returns as an interface{}, you'll generally need to cast it back to your function's return type.
For example, the OP's example called Get(1), which returns (Item, error).
item := assumeNoError(Get(1)).(Item)
The trick that makes this possible: Multi-values returned from one function call can be passed in as multi-variable arguments to another function.
As a special case, if the return values of a function or method g are equal in number and individually assignable to the parameters of another function or method f, then the call f(g(parameters_of_g)) will invoke f after binding the return values of g to the parameters of f in order.
This answer borrows heavily from existing answers, but none had provided a simple, generic solution of this form.