A simple utility for running exhaustiveness checks on type switch statements. Exhaustiveness checks are only run on interfaces that are declared to be sum types.

Dual-licensed under MIT or the UNLICENSE.

$ go get github.com/BurntSushi/go-sumtype

For usage info, just run the command:

$ go-sumtype

go-sumtype takes a list of Go package paths or files and looks for sum type declarations in each package/file provided. Exhaustiveness checks are then performed for each use of a declared sum type in a type switch statement. Namely, go-sumtype will report an error for any type switch statement that either lacks a default clause or does not account for all possible variants.

Declarations are provided in comments like so:

//go-sumtype:decl MySumType

MySumType must satisfy the following:

1. It is a type defined in the same package.
2. It is an interface.
3. It is sealed. That is, part of its interface definition contains an unexported method.

go-sumtype will produce an error if any of the above is not true.

For valid declarations, go-sumtype will look for all occurrences in which a value of type MySumType participates in a type switch statement. In those occurrences, it will attempt to detect whether the type switch is exhaustive or not. If it's not, go-sumtype will report an error. For example:

$ cat mysumtype.go
package main

//go-sumtype:decl MySumType

type MySumType interface {
        sealed()
}

type VariantA struct{}

func (a *VariantA) sealed() {}

type VariantB struct{}

func (b *VariantB) sealed() {}

func main() {
        switch MySumType(nil).(type) {
        case *VariantA:
        }
}
$ go-sumtype mysumtype.go
mysumtype.go:18:2: exhaustiveness check failed for sum type 'MySumType': missing cases for VariantB

Adding either a default clause or a clause to handle *VariantB will cause exhaustive checks to pass.

As a special case, if the type switch statement contains a default clause that always panics, then exhaustiveness checks are still performed.