errwrap

Wrap and fix Go errors with the new %w verb directive. This tool analyzes fmt.Errorf() calls and reports calls that contain a verb directive that is different than the new %w verb directive introduced in Go v1.13. It's also capable of rewriting calls to use the new %w wrap verb directive.

Install

# minimum v1.16 is required
go install github.com/fatih/errwrap@latest

or download one of the pre-compiled binaries from the releases page and copy to the desired location.

Usage

By default, errwrap prints the output of the analyzer to stdout. You can pass a file, directory or a Go package:

$ errwrap foo.go # pass a file
$ errwrap ./...  # recursively analyze all files
$ errwrap github.com/fatih/gomodifytags # or pass a package

When called it displays the error with the line and column:

[email protected]/main.go:200:16: call could wrap the error with error-wrapping directive %w
[email protected]/main.go:641:17: call could wrap the error with error-wrapping directive %w
[email protected]/main.go:749:15: call could wrap the error with error-wrapping directive %w

errwrap is also able to rewrite your source code to replace any verb directive used for an error type with the %w verb directive. Assume we have the following source code:

$ cat demo.go
package main

import (
        "errors"
        "fmt"
)

func main() {
        _ = foo()
}

func foo() error {
        err := errors.New("bar!")
        return fmt.Errorf("foo failed: %s: %w bar ...", "foo", err)
}

Calling errwrap with the -fix flag will rewrite the source code:

$ errwrap -fix main.go
main.go:14:9: call could wrap the error with error-wrapping directive %w

diff --git a/main.go b/main.go
index 41d1c42..6cb42b8 100644
--- a/main.go
+++ b/main.go
@@ -11,5 +11,5 @@ func main() {

 func foo() error {
        err := errors.New("bar!")
-       return fmt.Errorf("failed for %s with error: %s", "foo", err)
+       return fmt.Errorf("failed for %s with error: %w", "foo", err)
 }

Whether to Wrap or not?

Wrapping an error is not always the best approach. Wrapping exposes the underlying error and makes it part of your public API. This means clients who rely on them could see breaking changes if you change your underlying implementation or don't wrap anymore.

The blog post Working with Errors in Go 1.13 contains a section called Whether to Wrap that explains this in more detail

Credits

This tool is built on top of the excellent go/analysis package that makes it easy to write custom analyzers in Go. If you're interested in writing a tool, check out my Using go/analysis to write a custom linter blog post.

Also part of the code that parses the verb directives is from the go/analysis/passes/printf analyzer. It's a simplified version and might contain discrepancies.