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Module: stdgo.errors

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Overview

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].

Index

Examples

Variables

import stdgo.errors.Errors
var _errorType:stdgo.internal.reflectlite.Type_
var errUnsupported:stdgo.Error

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.

Functions

import stdgo.errors.Errors

function as

function as(_err:stdgo.Error, _target:stdgo.AnyInterface):Bool

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.

exampleAs

function exampleAs():Void {
        {
            var __tmp__ = stdgo.os.Os.open(("non-existing" : stdgo.GoString)), __0:stdgo.Ref<stdgo.os.Os.File> = __tmp__._0, _err:stdgo.Error = __tmp__._1;
            if (_err != null) {
                var _pathError:stdgo.Ref<stdgo.io.fs.Fs.PathError> = (null : stdgo.Ref<stdgo.io.fs.Fs.PathError>);
                if (stdgo.errors.Errors.as(_err, stdgo.Go.toInterface((stdgo.Go.setRef(_pathError) : stdgo.Ref<stdgo.Ref<stdgo.io.fs.Fs.PathError>>)))) {
                    stdgo.fmt.Fmt.println(stdgo.Go.toInterface(("Failed at path:" : stdgo.GoString)), stdgo.Go.toInterface(_pathError.path));
                } else {
                    stdgo.fmt.Fmt.println(stdgo.Go.toInterface(_err));
                };
            };
        };
    }

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function is_

function is_(_err:stdgo.Error, _target:stdgo.Error):Bool

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.

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function join

function join(_errs:haxe.Rest<stdgo.Error>):stdgo.Error

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.

exampleJoin

function exampleJoin():Void {
        var _err1:stdgo.Error = stdgo.errors.Errors.new_(("err1" : stdgo.GoString));
        var _err2:stdgo.Error = stdgo.errors.Errors.new_(("err2" : stdgo.GoString));
        var _err:stdgo.Error = stdgo.errors.Errors.join(_err1, _err2);
        stdgo.fmt.Fmt.println(stdgo.Go.toInterface(_err));
        if (stdgo.errors.Errors.is_(_err, _err1)) {
            stdgo.fmt.Fmt.println(stdgo.Go.toInterface(("err is err1" : stdgo.GoString)));
        };
        if (stdgo.errors.Errors.is_(_err, _err2)) {
            stdgo.fmt.Fmt.println(stdgo.Go.toInterface(("err is err2" : stdgo.GoString)));
        };
    }

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function new_

function new_(_text:stdgo.GoString):stdgo.Error

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.

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function unwrap

function unwrap(_err:stdgo.Error):stdgo.Error

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].

exampleUnwrap

function exampleUnwrap():Void {
        var _err1:stdgo.Error = stdgo.errors.Errors.new_(("error1" : stdgo.GoString));
        var _err2:stdgo.Error = stdgo.fmt.Fmt.errorf(("error2: [%w]" : stdgo.GoString), stdgo.Go.toInterface(_err1));
        stdgo.fmt.Fmt.println(stdgo.Go.toInterface(_err2));
        stdgo.fmt.Fmt.println(stdgo.Go.toInterface(stdgo.errors.Errors.unwrap(_err2)));
    }

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Typedefs

import stdgo.errors.*

typedef T__interface_0

typedef T__interface_0 = {
	public function unwrap():stdgo.Error;
};

typedef T__interface_1

typedef T__interface_1 = {
	public function is_(_0:stdgo.Error):Bool;
};

typedef T__interface_2

typedef T__interface_2 = {
	public function unwrap():stdgo.Slice<stdgo.Error>;
};

typedef T__interface_3

typedef T__interface_3 = {
	public function as(_0:stdgo.AnyInterface):Bool;
};