terr

terr (short for traced error) is a minimalistic library for adding error tracing to Go 1.20+.

The error representation primitives introduced in Go 1.13 are quite sufficient, but the lack of tracing capabilities makes it hard to confidently debug errors across layers in complex applications.

To help with that, terr fully embraces the native Go error handling paradigms, but it adds two features:

• file and line information for tracing errors;
• the ability to print error tracing trees using the fmt package with the %@ verb.

This package introduces the concept of traced errors: a wrapper for errors that includes the location where they were created (errors.New), passed along (return err), wrapped (%w) or masked (%v). Traced errors keep track of children traced errors that relate to them. An error is a traced error if it was returned by one of the functions of this package.

Most importantly, traced errors work seamlessly with errors.Is, errors.As and errors.Unwrap just as if terr were not being used.

This blog post explains more about good practices for error handling in Go and the reasons for terr being created.

Using terr

Without terr	With terr
errors.New("error")	terr.Newf("error")
fmt.Errorf("error: %w", err)	terr.Newf("error: %w", err)
[return] err	terr.Trace(err)
[return] &CustomError{}	terr.TraceSkip(&CustomError{}, 1)

terr.Newf can receive multiple errors. In fact, it is just a very slim wrapper around fmt.Errorf. Any traced error passed to terr.Newf will be included in the error tracing tree, regardless of the fmt verb used for it.

terr.Trace and terr.TraceSkip on the other hand do nothing with the error they receive (no wrapping and no masking), but they add one level to the error tracing tree. terr.TraceSkip lets custom errors constructors return a traced error with the location defined by skipping a number of stack frames.

To obtain the full trace, terr functions must be used consistently. If fmt.Errorf is used at one point, the error tracing information will be reset at that point, but Go's wrapped error tree will be preserved even in that case.

To see these functions in use, check out the package examples.

In the glorious day error tracing is added to Go, and assuming it gets done in a way that respects error handling as defined in Go 1.13+, removing terr from a code base should be a matter of replacing the terr function calls with equivalent expressions.

Printing errors

An error tracing tree can be printed with the special %@ formatting verb (example).

%@ prints the tree in a tab-indented, multi-line representation. If a custom format is needed (e.g., JSON), it is possible to implement a function that walks the error tracing tree and outputs it in the desired format. See how to walk the error tracing tree.

Tracing custom errors

Constructor functions for custom error types and wrapped sentinel errors can use terr.TraceSkip(err, skip) (example).

Walking the error tracing tree

Starting with Go 1.20, wrapped errors are kept as a n-ary tree. terr works by building a tree containing tracing information in parallel, leaving the Go error tree untouched, as if terr were not being used. Each traced error is thus a node in this parallel error tracing tree.

terr.TraceTree(err) ErrorTracer can be used to obtain the root of an n-ary error tracing tree, which can be navigated using the following methods:

type ErrorTracer interface {
	Error() string
	Location() (string, int)
	Children() []ErrorTracer
}

Note that this is not the tree of wrapped errors built by the Go standard library, because:

• if non-traced errors are provided to terr.Newf, even if wrapped, they will not be a part of the error tracing tree;
• even masked (%v) errors will be part of the error tracing tree if terr.Newf was used to mask them.

Methods provided by the by the Go standard library should be used to walk Go's wrapped error tree, which would include non-traced errors and ignore masked errors (e.g., errors.Unwrap).

An example is available.

Adopting terr

Adopting terr requires some thought about how errors are being constructed and which errors are worth tracing. Usage of terr may vary greatly for different code bases, and it is easiest to adopt terr when just the vanilla Go error handling practices are in use, without any third-party libraries.

The terr examples, particularly the example for terr.TraceSkip, provide a good illustration of how to use terr while following Go's recommended error guidelines.

In larger code bases, using gofmt -r might help, but it might also produce unwanted results if not used carefully. Applying the reverse of the rewrite rules for getting rid of terr may be helpful when introducing terr to a code base.

Getting rid of terr

While adding terr to a large code base can take some effort, removing it is very easy. Run the following commands in a directory tree to get rid of terr in all its files (make sure goimports is installed first):

$ gofmt -w -r 'terr.Newf(a) -> errors.New(a)' .
$ gofmt -w -r 'terr.Newf -> fmt.Errorf' .
$ gofmt -w -r 'terr.Trace(a) -> a' .
$ gofmt -w -r 'terr.TraceSkip(a, b) -> a' .
$ goimports -w .
$ go mod tidy

Also make sure to remove any uses of the %@ fmt verb, since this verb only works with traced errors.