90 lines
2.4 KiB
Markdown
90 lines
2.4 KiB
Markdown
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# errwrap
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`errwrap` is a package for Go that formalizes the pattern of wrapping errors
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and checking if an error contains another error.
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There is a common pattern in Go of taking a returned `error` value and
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then wrapping it (such as with `fmt.Errorf`) before returning it. The problem
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with this pattern is that you completely lose the original `error` structure.
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Arguably the _correct_ approach is that you should make a custom structure
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implementing the `error` interface, and have the original error as a field
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on that structure, such [as this example](http://golang.org/pkg/os/#PathError).
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This is a good approach, but you have to know the entire chain of possible
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rewrapping that happens, when you might just care about one.
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`errwrap` formalizes this pattern (it doesn't matter what approach you use
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above) by giving a single interface for wrapping errors, checking if a specific
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error is wrapped, and extracting that error.
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## Installation and Docs
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Install using `go get github.com/hashicorp/errwrap`.
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Full documentation is available at
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http://godoc.org/github.com/hashicorp/errwrap
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## Usage
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#### Basic Usage
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Below is a very basic example of its usage:
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```go
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// A function that always returns an error, but wraps it, like a real
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// function might.
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func tryOpen() error {
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_, err := os.Open("/i/dont/exist")
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if err != nil {
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return errwrap.Wrapf("Doesn't exist: {{err}}", err)
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}
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return nil
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}
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func main() {
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err := tryOpen()
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// We can use the Contains helpers to check if an error contains
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// another error. It is safe to do this with a nil error, or with
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// an error that doesn't even use the errwrap package.
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if errwrap.Contains(err, ErrNotExist) {
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// Do something
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}
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if errwrap.ContainsType(err, new(os.PathError)) {
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// Do something
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}
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// Or we can use the associated `Get` functions to just extract
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// a specific error. This would return nil if that specific error doesn't
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// exist.
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perr := errwrap.GetType(err, new(os.PathError))
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}
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```
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#### Custom Types
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If you're already making custom types that properly wrap errors, then
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you can get all the functionality of `errwraps.Contains` and such by
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implementing the `Wrapper` interface with just one function. Example:
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```go
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type AppError {
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Code ErrorCode
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Err error
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}
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func (e *AppError) WrappedErrors() []error {
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return []error{e.Err}
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}
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```
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Now this works:
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```go
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err := &AppError{Err: fmt.Errorf("an error")}
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if errwrap.ContainsType(err, fmt.Errorf("")) {
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// This will work!
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}
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```
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