exhaustive

README

exhaustive

Package exhaustive defines an analyzer that checks exhaustiveness of switch statements of enum-like constants in Go source code.

For supported flags, the definition of enum, and the definition of exhaustiveness used by this package, see pkg.go.dev. For a changelog, see CHANGELOG in the GitHub wiki.

The analyzer can be configured to additionally check exhaustiveness of map literals whose key type is enum-like.

Usage

Command line program:

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

exhaustive [flags] [packages]

Package:

go get github.com/nishanths/exhaustive

The exhaustive.Analyzer variable follows the guidelines of the golang.org/x/tools/go/analysis package. This should make it possible to integrate exhaustive in your own analysis driver program.

Example

Given an enum:

package token // import "example.org/token"

type Token int

const (
	Add Token = iota
	Subtract
	Multiply
	Quotient
	Remainder
)

And code that switches on the enum:

package calc // import "example.org/calc"

import "example.org/token"

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

var m = map[token.Token]string{
	token.Add:      "add",
	token.Subtract: "subtract",
	token.Multiply: "multiply",
}

Running exhaustive with default options will report:

% exhaustive example.org/calc
calc.go:6:2: missing cases in switch of type token.Token: token.Quotient, token.Remainder

Specify the flag -check=switch,map to additionally check exhaustiveness of map literal keys:

% exhaustive -check=switch,map example.org/calc
calc.go:6:2: missing cases in switch of type token.Token: token.Quotient, token.Remainder
calc.go:14:9: missing keys in map of key type token.Token: token.Quotient, token.Remainder

Contributing

Issues and changes are welcome. Please discuss substantial changes in an issue first.

Documentation

Overview

Package exhaustive defines an analyzer that checks exhaustiveness of switch statements of enum-like constants in Go source code. The analyzer can be configured to additionally check exhaustiveness of map literals whose key type is enum-like.

Definition of enum

The Go language spec does not provide an explicit definition for enums. For the purpose of this analyzer, and by convention, an enum type is any named type that has:

• underlying type float, string, or integer (includes byte and rune, which are aliases for uint8 and int32, respectively); and
• at least one constant of the type defined in the same scope.

In the example below, Biome is an enum type. The three constants are its enum members.

package eco

type Biome int

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

Enum member constants for a particular enum type do not necessarily all have to be declared in the same const block. The constant values may be specified using iota, using literal values, or using any valid means for declaring a Go constant. It is allowed for multiple enum member constants for a particular 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 enum members, by constant value, are listed in the switch statement's cases. If multiple members 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. In a switch statement's cases, only identifiers (e.g. Tundra) and qualified identifiers (e.g. somepkg.Grassland) that name constants may contribute towards satisfying exhaustiveness; other expressions such as literal values and function calls will not.

By default, the existence of a default case in a switch statement does not unconditionally make a switch statement exhaustive. Use the -default-signifies-exhaustive flag to adjust this behavior.

A similar definition of exhaustiveness applies to a map literal whose key type is an enum type. For the map literal to be considered exhaustive, all enum members, by constant value, must be listed as keys. Empty map literals are not checked. For the analyzer to check map literals, the -check flag must include the value "map".

Type parameters

A switch statement that switches on a value whose type is a type parameter is checked for exhaustiveness if each type element in the type constraint is an enum type and shares the same underlying basic type kind.

In the following example, the switch statement on the value of type parameter T will be checked, because each type element of T—namely M, N, and O—is an enum type and shares the same underlying basic type kind (i.e. int8). To satisfy exhaustiveness, all enum members, by constant value, for each of the enum types M, N, and O—namely A, B, C, and D—must be listed in the switch statement's cases.

func bar[T M | I](v T) {
	switch v {
		case T(A):
		case T(B):
		case T(C):
		case T(D):
	}
}

type I interface{ N | J }
type J interface{ O }

type M int8
const A M = 1

type N int8
const B N = 2
const C N = 3

type O int8
const D O = 4

Type aliases

The analyzer handles type aliases as shown in the example below. Here T2 is a enum type. T1 is an alias for T2. Note that T1 itself isn't considered an enum type; T1 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
)

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, by constant value, are listed in the switch statement's cases. (Recall that only constants declared in the same scope as type T2's scope can be T2's enum members.)

The following switch statements are exhaustive.

// Note: the type of v is effectively newpkg.T2, due to type aliasing.
func f(v pkg.T1) {
	switch v {
	case newpkg.A:
	case newpkg.B:
	case newpkg.Y:
	}
}

func g(v pkg.T1) {
	switch v {
	case pkg.A:
	case pkg.B:
	}
}

The analyzer guarantees that introducing a type alias (such as type T1 = newpkg.T2) will not result in new diagnostics from the analyzer, as long as the set of enum member constant values of the alias RHS type is a subset of the set of enum member constant values of the LHS type.

Flags

Summary:

flag                           type                     default value
----                           ----                     -------------
-check                         comma-separated string   switch
-explicit-exhaustive-switch    bool                     false
-explicit-exhaustive-map       bool                     false
-check-generated               bool                     false
-default-signifies-exhaustive  bool                     false
-ignore-enum-members           regexp pattern           (none)
-ignore-enum-types             regexp pattern           (none)
-package-scope-only            bool                     false

Flag descriptions:

• The -check flag specifies a comma-separated list of program elements that should be checked for exhaustiveness; supported program elements are "switch" and "map". The default flag value is "switch", which means that only switch statements are checked. Specify the flag value "switch,map" to check both switch statements and map literals.

• If -explicit-exhaustive-switch is enabled, the analyzer checks a switch statement only if it is associated with a comment beginning with "//exhaustive:enforce". Otherwise, the analyzer checks every enum switch statement not associated with a comment beginning with "//exhaustive:ignore".

• The -explicit-exhaustive-map flag is the map literal counterpart for the -explicit-exhaustive-switch flag.

• If -check-generated is enabled, switch statements and map literals in generated Go source files are checked. By default, the analyzer does not check generated files. Refer to https://golang.org/s/generatedcode for the definition of generated files.

• If -default-signifies-exhaustive is enabled, the presence of a default case in a switch statement unconditionally satisfies exhaustiveness (all enum members do not have to be listed). Enabling this flag usually tends to counter the purpose of exhaustiveness checking, so it is not recommended that you enable this flag.

• The -ignore-enum-members flag specifies a regular expression in Go package regexp syntax. Constants matching the regular expression do not have to be listed in switch statement cases or map literals in order to satisfy exhaustiveness. The specified regular expression is matched against the constant name inclusive of the enum package import path. For example, if the package import path of the constant is "example.org/eco" and the constant name is "Tundra", the specified regular expression will be matched against the string "example.org/eco.Tundra".

• The -ignore-enum-types flag is similar to the -ignore-enum-members flag, except that it applies to types.

• If -package-scope-only is enabled, the analyzer only finds enums defined in package scope but not in inner scopes such as functions; consequently only switch statements and map literals that use such enums are checked for exhaustiveness. By default, the analyzer finds enums defined in all scopes, including in inner scopes such as functions.

Skip analysis

To skip analysis of a switch statement or a map literal, associate it with a comment that begins with "//exhaustive:ignore". For example:

//exhaustive:ignore
switch v {
case A:
case B:
}

To ignore specific constants in exhaustiveness checks, use the -ignore-enum-members flag:

exhaustive -ignore-enum-members '^example\.org/eco.Tundra$'

To ignore specific types, use the -ignore-enum-types flag:

exhaustive -ignore-enum-types '^time\.Duration$|^example\.org/measure\.Unit$'

Index

• Constants
• Variables

Constants

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

	IgnorePatternFlag    = "ignore-pattern"    // Deprecated: use IgnoreEnumMembersFlag.
	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{}},
}