exhaustive

README

exhaustive

Check exhaustiveness of enum switch statements in Go source code.

go install github.com/nishanths/exhaustive/cmd/exhaustive@latest

For docs on the flags, the definition of enum, and the definition of exhaustiveness, see godocs.io.

For the changelog, see CHANGELOG in the wiki.

The package provides an Analyzer that follows the guidelines in the go/analysis package; this should make it possible to integrate exhaustive with your own analysis driver program.

Bugs

exhaustive does not report missing cases if the switch statement switches on a type parameterized type. See this issue for details.

Example

Given the enum

package token

type Token int

const (
	Add Token = iota
	Subtract
	Multiply
	Quotient
	Remainder
)

and the switch statement

package calc

import "token"

func f(t token.Token) {
	switch t {
	case token.Add:
	case token.Subtract:
	case token.Multiply:
	default:
	}
}

running exhaustive will print

calc.go:6:2: missing cases in switch of type token.Token: Quotient, Remainder

Contributing

Issues and pull requests are welcome. Before making a substantial change, please discuss it in an issue.

Documentation

Overview

Package exhaustive provides an analyzer that checks exhaustiveness of enum switch statements in Go source code.

Definition of enum

The Go language spec does not provide an explicit definition for an enum. For the purpose of this analyzer, an enum type is any named type (a.k.a. defined type) whose underlying type is an integer (includes byte and rune), a float, or a string type. An enum type has associated with it constants of this named type; these constants constitute the enum members.

In the example below, Biome is an enum type with 3 members.

type Biome int

const (
    Tundra  Biome = 1
    Savanna Biome = 2
    Desert  Biome = 3
)

For a constant to be an enum member for an enum type, the constant must be declared in the same scope as the enum type. Note that the scope requirement implies that only constants declared in the same package as the enum type's package can constitute the enum members for the enum type.

Enum member constants for a given enum type don't necessarily have to all be declared in the same const block. Constant values may be specified using iota, using explicit values, or by any means of declaring a valid Go const. It is allowed for multiple enum member constants for a given enum type to have the same constant value.

Definition of exhaustiveness

A switch statement that switches on a value of an enum type is exhaustive if all of the enum type's members are listed in the switch statement's cases. If multiple enum member constants have the same constant value, it is sufficient for any one of these same-valued members to be listed.

For an enum type defined in the same package as the switch statement, both exported and unexported enum members must be listed to satisfy exhaustiveness. For an enum type defined in an external package, it is sufficient that only exported enum members are listed.

Only identifiers denoting constants (e.g. Tundra) and qualified identifiers denoting constants (e.g. somepkg.Grassland) listed in a switch statement's cases can contribute towards satisfying exhaustiveness. Literal values, struct fields, re-assignable variables, etc. will not.

The analyzer will produce a diagnostic about unhandled enum members if the required memebers are not listed in a switch statement's cases (this applies even if the switch statement has a 'default' case).

Type aliases

The analyzer handles type aliases for an enum type in the following manner. Consider the example below. T2 is a enum type, and T1 is an alias for T2. Note that we don't term T1 itself an enum type; it is only an alias for an enum type.

package pkg
type T1 = newpkg.T2
const (
    A = newpkg.A
    B = newpkg.B
)

package newpkg
type T2 int
const (
    A T2 = 1
    B T2 = 2
)

Then a switch statement that switches on a value of type T1 (which, in reality, is just an alternate spelling for type T2) is exhaustive if all of T2's enum members are listed in the switch statement's cases. The same conditions described in the previous section for same-valued enum members and for exported/unexported enum members apply here too.

It is worth noting that, though T1 and T2 are identical types, only constants declared in the same scope as type T2's scope can be T2's enum members. In the example, newpkg.A and newpkg.B are T2's enum members.

The analyzer guarantees that introducing a type alias (such as type T1 = newpkg.T2) will never result in new diagnostics from the analyzer, as long as the set of enum member constant values of the new RHS type (newpkg.T2) is a subset of the set of enum member constant values of the old LHS type (T1).

Advanced notes

Non-enum member constants in a switch statement's cases: Recall from an earlier section that a constant must be declared in the same scope as the enum type to be an enum member. It is valid, however, both to the Go type checker and to this analyzer, for any constant of the right type to be listed in the cases of an enum switch statement (it does not necessarily have to be an enum member constant declared in the same scope/package as the enum type's scope/package). This is particularly useful when a type alias is involved: A forwarding constant declaration (such as pkg.A, in type T1's package) can take the place of the actual enum member constant (newpkg.A, in type T2's package) in the switch statement's cases to satisfy exhaustiveness.

var v pkg.T1 = pkg.ReturnsT1() // v is effectively of type newpkg.T2 due to alias
switch v {
case pkg.A: // valid substitute for newpkg.A (same constant value)
case pkg.B: // valid substitute for newpkg.B (same constant value)
}

Flags

Notable flags supported by the analyzer are described below. All of these flags are optional.

flag                            type    default value

-explicit-exhaustive-switch     bool    false
-check-generated                bool    false
-default-signifies-exhaustive   bool    false
-ignore-enum-members            string  (none)
-package-scope-only             bool    false

If the -explicit-exhaustive-switch flag is enabled, the analyzer only runs on switch statements explicitly marked with the comment text ("exhaustive:enforce"). Otherwise, it runs on every enum switch statement not marked with the comment text ("exhaustive:ignore").

If the -check-generated flag is enabled, switch statements in generated Go source files are also checked. Otherwise, by default, switch statements in generated files are not checked. See https://golang.org/s/generatedcode for the definition of generated file.

If the -default-signifies-exhaustive flag is enabled, the presence of a 'default' case in a switch statement always satisfies exhaustiveness, even if all enum members are not listed. It is not recommended that you enable this flag; enabling it generally defeats the purpose of exhaustiveness checking.

The -ignore-enum-members flag specifies a regular expression in Go syntax. Enum members matching the regular expression don't have to be listed in switch statement cases to satisfy exhaustiveness. The specified regular expression is matched against an enum member name inclusive of the enum package import path: for example, if the enum package import path is "example.com/pkg" and the member name is "Tundra", the specified regular expression will be matched against the string "example.com/pkg.Tundra".

If the -package-scope-only flag is enabled, the analyzer only finds enums defined in package scopes, and consequently only switch statements that switch on package-scoped enums will be checked for exhaustiveness. By default, the analyzer finds enums defined in all scopes, and checks switch statements that switch on all these enums.

Skip analysis

In implicitly exhaustive switch mode (-explicit-exhaustive-switch=false), skip checking of a specific switch statement by associating the comment shown in the example below with the switch statement. Note the lack of whitespace between the comment marker ("//") and the comment text ("exhaustive:ignore").

//exhaustive:ignore
switch v { ... }

In explicitly exhaustive switch mode (-explicit-exhaustive-switch=true), run exhaustiveness checks on a specific switch statement by associating the comment shown in the example below with the switch statement.

//exhaustive:enforce
switch v { ... }

To ignore specific enum members, see the -ignore-enum-members flag.

Switch statements in generated Go source files are not checked by default. Use the -check-generated flag to change this behavior.

Index

• Constants
• Variables

Constants

const (
	ExplicitExhaustiveSwitchFlag   = "explicit-exhaustive-switch"
	CheckGeneratedFlag             = "check-generated"
	DefaultSignifiesExhaustiveFlag = "default-signifies-exhaustive"
	IgnoreEnumMembersFlag          = "ignore-enum-members"
	PackageScopeOnlyFlag           = "package-scope-only"

	IgnorePatternFlag    = "ignore-pattern"    // Deprecated: see IgnoreEnumMembersFlag instead.
	CheckingStrategyFlag = "checking-strategy" // Deprecated.
)

Flag names used by the analyzer. They are exported for use by analyzer driver programs.

Variables

var Analyzer = &analysis.Analyzer{
	Name:      "exhaustive",
	Doc:       "check exhaustiveness of enum switch statements",
	Run:       run,
	Requires:  []*analysis.Analyzer{inspect.Analyzer},
	FactTypes: []analysis.Fact{&enumMembersFact{}},
}