package errors

Import Path
	errors (on go.dev)

Dependency Relation
	imports one package, and imported by 82 packages


Involved Source Files

	
		Package errors implements functions to manipulate errors.
		
		The [New] function creates errors whose only content is a text message.
		
		An error e wraps another error if e's type has one of the methods
		
			Unwrap() error
			Unwrap() []error
		
		If e.Unwrap() returns a non-nil error w or a slice containing w,
		then we say that e wraps w. A nil error returned from e.Unwrap()
		indicates that e does not wrap any error. It is invalid for an
		Unwrap method to return an []error containing a nil error value.
		
		An easy way to create wrapped errors is to call [fmt.Errorf] and apply
		the %w verb to the error argument:
		
			wrapsErr := fmt.Errorf("... %w ...", ..., err, ...)
		
		Successive unwrapping of an error creates a tree. The [Is] and [As]
		functions inspect an error's tree by examining first the error
		itself followed by the tree of each of its children in turn
		(pre-order, depth-first traversal).
		
		Is examines the tree of its first argument looking for an error that
		matches the second. It reports whether it finds a match. It should be
		used in preference to simple equality checks:
		
			if errors.Is(err, fs.ErrExist)
		
		is preferable to
		
			if err == fs.ErrExist
		
		because the former will succeed if err wraps [io/fs.ErrExist].
		
		As examines the tree of its first argument looking for an error that can be
		assigned to its second argument, which must be a pointer. If it succeeds, it
		performs the assignment and returns true. Otherwise, it returns false. The form
		
			var perr *fs.PathError
			if errors.As(err, &perr) {
				fmt.Println(perr.Path)
			}
		
		is preferable to
		
			if perr, ok := err.(*fs.PathError); ok {
				fmt.Println(perr.Path)
			}
		
		because the former will succeed if err wraps an [*io/fs.PathError].

	    join.go
	    wrap.go


Code Examples

	
		package main
		
		import (
			"fmt"
			"time"
		)
		
		// MyError is an error implementation that includes a time and message.
		type MyError struct {
			When time.Time
			What string
		}
		
		func (e MyError) Error() string {
			return fmt.Sprintf("%v: %v", e.When, e.What)
		}
		
		func oops() error {
			return MyError{
				time.Date(1989, 3, 15, 22, 30, 0, 0, time.UTC),
				"the file system has gone away",
			}
		}
		
		func main() {
			if err := oops(); err != nil {
				fmt.Println(err)
			}
		}

	
		package main
		
		import (
			"errors"
			"fmt"
			"io/fs"
			"os"
		)
		
		func main() {
			if _, err := os.Open("non-existing"); err != nil {
				var pathError *fs.PathError
				if errors.As(err, &pathError) {
					fmt.Println("Failed at path:", pathError.Path)
				} else {
					fmt.Println(err)
				}
			}
		
		}

	
		package main
		
		import (
			"errors"
			"fmt"
			"io/fs"
			"os"
		)
		
		func main() {
			if _, err := os.Open("non-existing"); err != nil {
				if errors.Is(err, fs.ErrNotExist) {
					fmt.Println("file does not exist")
				} else {
					fmt.Println(err)
				}
			}
		
		}

	
		package main
		
		import (
			"errors"
			"fmt"
		)
		
		func main() {
			err1 := errors.New("err1")
			err2 := errors.New("err2")
			err := errors.Join(err1, err2)
			fmt.Println(err)
			if errors.Is(err, err1) {
				fmt.Println("err is err1")
			}
			if errors.Is(err, err2) {
				fmt.Println("err is err2")
			}
		}

	
		package main
		
		import (
			"errors"
			"fmt"
		)
		
		func main() {
			err := errors.New("emit macho dwarf: elf header corrupted")
			if err != nil {
				fmt.Print(err)
			}
		}

	
		package main
		
		import (
			"fmt"
		)
		
		func main() {
			const name, id = "bimmler", 17
			err := fmt.Errorf("user %q (id %d) not found", name, id)
			if err != nil {
				fmt.Print(err)
			}
		}

	
		package main
		
		import (
			"errors"
			"fmt"
		)
		
		func main() {
			err1 := errors.New("error1")
			err2 := fmt.Errorf("error2: [%w]", err1)
			fmt.Println(err2)
			fmt.Println(errors.Unwrap(err2))
			// Output
			// error2: [error1]
			// error1
		}




Package-Level Type Names (total 2, neither is exported)


	/* sort exporteds by:  |  */
	


Package-Level Functions (total 5, all are exported)


	
		As finds the first error in err's tree that matches target, and if one is found, sets
		target to that error value and returns true. Otherwise, it returns false.
		
		The tree consists of err itself, followed by the errors obtained by repeatedly
		calling Unwrap. When err wraps multiple errors, As examines err followed by a
		depth-first traversal of its children.
		
		An error matches target if the error's concrete value is assignable to the value
		pointed to by target, or if the error has a method As(interface{}) bool such that
		As(target) returns true. In the latter case, the As method is responsible for
		setting target.
		
		An error type might provide an As method so it can be treated as if it were a
		different error type.
		
		As panics if target is not a non-nil pointer to either a type that implements
		error, or to any interface type.


	
		Is reports whether any error in err's tree matches target.
		
		The tree consists of err itself, followed by the errors obtained by repeatedly
		calling Unwrap. When err wraps multiple errors, Is examines err followed by a
		depth-first traversal of its children.
		
		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. An Is method should only shallowly
		compare err and the target and not call Unwrap on either.


	
		Join returns an error that wraps the given errors.
		Any nil error values are discarded.
		Join returns nil if every value in errs is nil.
		The error formats as the concatenation of the strings obtained
		by calling the Error method of each element of errs, with a newline
		between each string.
		
		A non-nil error returned by Join implements the Unwrap() []error method.


	
		New returns an error that formats as the given text.
		Each call to New returns a distinct error value even if the text is identical.


	
		Unwrap returns the result of calling the Unwrap method on err, if err's
		type contains an Unwrap method returning error.
		Otherwise, Unwrap returns nil.
		
		Unwrap only calls a method of the form "Unwrap() error".
		In particular Unwrap does not unwrap errors returned by [Join].




Package-Level Variables (total 2, in which 1 is exported)


	
		ErrUnsupported indicates that a requested operation cannot be performed,
		because it is unsupported. For example, a call to [os.Link] when using a
		file system that does not support hard links.
		
		Functions and methods should not return this error but should instead
		return an error including appropriate context that satisfies
		
			errors.Is(err, errors.ErrUnsupported)
		
		either by directly wrapping ErrUnsupported or by implementing an Is method.
		
		Functions and methods should document the cases in which an error
		wrapping this will be returned.