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errors: add example showing a custom error with Unwrap
Change-Id: I2bddee9b460d3875911859b49528a00d318f37fc Reviewed-on: https://go-review.googlesource.com/c/go/+/184237 Reviewed-by: Russ Cox <rsc@golang.org> Reviewed-by: Emmanuel Odeke <emm.odeke@gmail.com>
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// Copyright 2019 The Go Authors. All rights reserved. | ||
// Use of this source code is governed by a BSD-style | ||
// license that can be found in the LICENSE file. | ||
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package errors_test | ||
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import ( | ||
"errors" | ||
"fmt" | ||
"os" | ||
"time" | ||
) | ||
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// MyError2 is an error implementation that includes a time, a message, and an | ||
// underlying error. | ||
type MyError2 struct { | ||
When time.Time | ||
What string | ||
err error | ||
} | ||
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func (e MyError2) Error() string { | ||
return fmt.Sprintf("%v at %v: %v", e.What, e.When, e.err) | ||
} | ||
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// Unwrap returns e's underlying error, or nil if there is none. | ||
func (e MyError2) Unwrap() error { | ||
return e.err | ||
} | ||
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func readConfig() error { | ||
if _, err := os.Open("non-existing"); err != nil { | ||
return MyError2{ | ||
time.Date(1989, 3, 15, 22, 30, 0, 0, time.UTC), | ||
"reading config file", | ||
err, | ||
} | ||
} | ||
return nil | ||
} | ||
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func Example_unwrap() { | ||
if err := readConfig(); err != nil { | ||
// Display the error. | ||
fmt.Println(err) | ||
// If we can retrieve the path, try to recover | ||
// by taking another action. | ||
var pe *os.PathError | ||
if errors.As(err, &pe) { | ||
restoreFile(pe.Path) | ||
} | ||
} | ||
// Output: reading config file at 1989-03-15 22:30:00 +0000 UTC: open non-existing: no such file or directory | ||
} | ||
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func restoreFile(path string) {} |