Involved Source Files Package errors provides simple error handling primitives.
The traditional error handling idiom in Go is roughly akin to
if err != nil {
return err
}
which when applied recursively up the call stack results in error reports
without context or debugging information. The errors package allows
programmers to add context to the failure path in their code in a way
that does not destroy the original value of the error.
Adding context to an error
The errors.Wrap function returns a new error that adds context to the
original error by recording a stack trace at the point Wrap is called,
together with the supplied message. For example
_, err := ioutil.ReadAll(r)
if err != nil {
return errors.Wrap(err, "read failed")
}
If additional control is required, the errors.WithStack and
errors.WithMessage functions destructure errors.Wrap into its component
operations: annotating an error with a stack trace and with a message,
respectively.
Retrieving the cause of an error
Using errors.Wrap constructs a stack of errors, adding context to the
preceding error. Depending on the nature of the error it may be necessary
to reverse the operation of errors.Wrap to retrieve the original error
for inspection. Any error value which implements this interface
type causer interface {
Cause() error
}
can be inspected by errors.Cause. errors.Cause will recursively retrieve
the topmost error that does not implement causer, which is assumed to be
the original cause. For example:
switch err := errors.Cause(err).(type) {
case *MyError:
// handle specifically
default:
// unknown error
}
Although the causer interface is not exported by this package, it is
considered a part of its stable public interface.
Formatted printing of errors
All error values returned from this package implement fmt.Formatter and can
be formatted by the fmt package. The following verbs are supported:
%s print the error. If the error has a Cause it will be
printed recursively.
%v see %s
%+v extended format. Each Frame of the error's StackTrace will
be printed in detail.
Retrieving the stack trace of an error or wrapper
New, Errorf, Wrap, and Wrapf record a stack trace at the point they are
invoked. This information can be retrieved with the following interface:
type stackTracer interface {
StackTrace() errors.StackTrace
}
The returned errors.StackTrace type is defined as
type StackTrace []Frame
The Frame type represents a call site in the stack trace. Frame supports
the fmt.Formatter interface that can be used for printing information about
the stack trace of this error. For example:
if err, ok := err.(stackTracer); ok {
for _, f := range err.StackTrace() {
fmt.Printf("%+s:%d\n", f, f)
}
}
Although the stackTracer interface is not exported by this package, it is
considered a part of its stable public interface.
See the documentation for Frame.Format for more details.go113.gostack.go
Frame represents a program counter inside a stack frame.
For historical reasons if Frame is interpreted as a uintptr
its value represents the program counter + 1. Format formats the frame according to the fmt.Formatter interface.
%s source file
%d source line
%n function name
%v equivalent to %s:%d
Format accepts flags that alter the printing of some verbs, as follows:
%+s function name and path of source file relative to the compile time
GOPATH separated by \n\t (<funcname>\n\t<path>)
%+v equivalent to %+s:%d MarshalText formats a stacktrace Frame as a text string. The output is the
same as that of fmt.Sprintf("%+v", f), but without newlines or tabs.
Frame : encoding.TextMarshaler
Frame : fmt.Formatter
StackTrace is stack of Frames from innermost (newest) to outermost (oldest). Format formats the stack of Frames according to the fmt.Formatter interface.
%s lists source files for each Frame in the stack
%v lists the source file and line number for each Frame in the stack
Format accepts flags that alter the printing of some verbs, as follows:
%+v Prints filename, function, and line number for each Frame in the stack.
StackTrace : fmt.Formatter
Package-Level Functions (total 11)
As finds the first error in err's chain that matches target, and if so, sets
target to that error value and returns true.
The chain consists of err itself followed by the sequence of errors obtained by
repeatedly calling Unwrap.
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.
As will panic if target is not a non-nil pointer to either a type that implements
error, or to any interface type. As returns false if err is nil.
Cause returns the underlying cause of the error, if possible.
An error value has a cause if it implements the following
interface:
type causer interface {
Cause() error
}
If the error does not implement Cause, the original error will
be returned. If the error is nil, nil will be returned without further
investigation.
Errorf formats according to a format specifier and returns the string
as a value that satisfies error.
Errorf also records the stack trace at the point it was called.
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.
New returns an error with the supplied message.
New also records the stack trace at the point it was called.
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.
WithMessage annotates err with a new message.
If err is nil, WithMessage returns nil.
WithMessagef annotates err with the format specifier.
If err is nil, WithMessagef returns nil.
WithStack annotates err with a stack trace at the point WithStack was called.
If err is nil, WithStack returns nil.
Wrap returns an error annotating err with a stack trace
at the point Wrap is called, and the supplied message.
If err is nil, Wrap returns nil.
Wrapf returns an error annotating err with a stack trace
at the point Wrapf is called, and the format specifier.
If err is nil, Wrapf returns nil.
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