A linter that detects when reassigning a top-level variable in another package.

go install github.com/curioswitch/go-reassign

reassign ./...

Change the pattern to match against variables being reassigned. By default, only EOF and Err* variables are checked.

reassign -pattern ".*" ./...

Package variables are commonly used to define sentinel errors which callers can use with errors.Is to determine the type of a returned error. Some examples exist in the standard os library.

Unfortunately, as with any variable, these are mutable, and it is possible to write this very dangerous code.

package main
import "io"
func bad() {
	// breaks file reading
	io.EOF = nil
}

This caused a new pattern for constant errors to gain popularity, but they don't work well with improvements to the errors package in recent versions of Go and may be considered to be non-idiomatic compared to normal errors.New. If we can catch reassignment of sentinel errors, we gain much of the safety of constant errors.

This linter will catch reassignment of variables in other packages. By default it intends to apply to as many codebases as possible and only checks a restricted set of variable names, EOF and ErrFoo, to restrict to sentinel errors. Package variable reassignment is generally confusing, though, and we recommend avoiding it for all variables, not just errors. The pattern flag can be set to a regular expression to define what variables cannot be reassigned, and .* is recommended if it works with your code.

mage is used for development. Run go run mage.go -l to see available targets.

For example, to run checks before sending a PR, run go run mage.go check.