In Go it’s idiomatic to communicate errors via an
explicit, separate return value. This contrasts with
the exceptions used in languages like Java and Ruby and
the overloaded single result / error value sometimes
used in C. Go’s approach makes it easy to see which
functions return errors and to handle them using the
same language constructs employed for other,
non-error tasks. See the documentation of the
errors package
and
this blog post
for additional
details. | |
|  package main
|
| import (
"errors"
"fmt"
)
|
By convention, errors are the last return value and
have type
error
, a built-in interface. | func f(arg int) (int, error) {
if arg == 42 {
|
errors.New
constructs a basic
error
value
with the given error message.
| return -1, errors.New("can't work with 42")
}
|
A
nil
value in the error position indicates that
there was no error. | return arg + 3, nil
}
|
A sentinel error is a predeclared variable that is used to
signify a specific error condition. | var ErrOutOfTea = fmt.Errorf("no more tea available")
var ErrPower = fmt.Errorf("can't boil water")
|
| func makeTea(arg int) error {
if arg == 2 {
return ErrOutOfTea
} else if arg == 4 {
|
We can wrap errors with higher-level errors to add
context. The simplest way to do this is with the
%w
verb in
fmt.Errorf
. Wrapped errors
create a logical chain (A wraps B, which wraps C, etc.)
that can be queried with functions like
errors.Is
and
errors.As
. | return fmt.Errorf("making tea: %w", ErrPower)
}
return nil
}
|
| func main() {
for _, i := range []int{7, 42} {
|
It’s common to use an inline error check in the
if
line. | if r, e := f(i); e != nil {
fmt.Println("f failed:", e)
} else {
fmt.Println("f worked:", r)
}
}
|
| for i := range 5 {
if err := makeTea(i); err != nil {
|
errors.Is
checks that a given error (or any error in its chain)
matches a specific error value. This is especially useful with wrapped or
nested errors, allowing you to identify specific error types or sentinel
errors in a chain of errors.
| if errors.Is(err, ErrOutOfTea) {
fmt.Println("We should buy new tea!")
} else if errors.Is(err, ErrPower) {
fmt.Println("Now it is dark.")
} else {
fmt.Printf("unknown error: %s\n", err)
}
continue
}
|
| fmt.Println("Tea is ready!")
}
}
|