go-sumtype

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.

This work was inspired by our code at Diffeo.

Installation

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

For usage info, just run the command:

$ go-sumtype

Typical usage might look like this:

$ go-sumtype $(go list ./... | grep -v vendor)

Usage

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, running go-sumtype on this source file:

package main

//go-sumtype:decl MySumType

type MySumType interface {
        sealed()
}

type VariantA struct{}

func (*VariantA) sealed() {}

type VariantB struct{}

func (*VariantB) sealed() {}

func main() {
        switch MySumType(nil).(type) {
        case *VariantA:
        }
}

produces the following:

$ 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.

Details and motivation

Sum types are otherwise known as discriminated unions. That is, a sum type is a finite set of disjoint values. In type systems that support sum types, the language will guarantee that if one has a sum type T, then its value must be one of its variants.

Go's type system does not support sum types. A typical proxy for representing sum types in Go is to use an interface with an unexported method and define each variant of the sum type in the same package to satisfy said interface. This guarantees that the set of types that satisfy the interface is closed at compile time. Performing case analysis on these types is then done with a type switch statement, e.g., switch x.(type) { ... }. Each clause of the type switch corresponds to a variant of the sum type. The downside of this approach is that Go's type system is not aware of the set of variants, so it cannot tell you whether case analysis over a sum type is complete or not.

The go-sumtype command recognizes this pattern, but it needs a small amount of help to recognize which interfaces should be treated as sum types, which is why the //go-sumtype:decl ... annotation is required. go-sumtype will figure out all of the variants of a sum type by finding the set of types defined in the same package that satisfy the interface specified by the declaration.

The go-sumtype command will prove its worth when you need to add a variant to an existing sum type. Running go-sumtype will tell you immediately which case analyses need to be updated to account for the new variant.