Enum Type

June 21, 2026 · View on GitHub

In Go, defining an enumeration requires two separate declarations: first a named type with an explicit underlying type, and then a const block whose values are typed as that named type:

type Color int

const (
    ColorRed   Color = iota
    ColorGreen
    ColorBlue
)

This pattern is verbose for a very common need, and it forces the developer to make an early, often arbitrary, decision about the underlying type (int, int8, string, bool, etc.) before any values have even been written down. In practice the underlying type is usually derived from the values themselves — if all values are integers, the underlying type is some integer type; if all values are strings, it's string; and so on.

XGo aims to reduce this boilerplate while remaining fully compatible with Go's type system, AST, and toolchain at the semantic level.

Proposal

Introduce a new declaration form:

type XXX const (
    XXXEnum1 = expr1
    XXXEnum2 = expr2
    ...
)

This declares XXX as a new named type whose underlying type is inferred from the constant expressions inside the block, and simultaneously declares XXXEnum1, XXXEnum2, ... as named constants of type XXX.

This is semantically equivalent to writing, in Go:

type XXX <inferred underlying type>

const (
    XXXEnum1 XXX = expr1
    XXXEnum2 XXX = expr2
    ...
)

but without requiring the author to spell out <inferred underlying type> or repeat XXX on every line.

Syntax

type Identifier const (
    Name1 [= Expr1]
    Name2 [= Expr2]
    ...
)

Identifier becomes a new named type.
Each Name becomes a constant of type Identifier.
Expr follows the same rules as ordinary const declarations, including iota support and the "repeat previous expression" shorthand:

type Weekday const (
    Sunday = iota
    Monday
    Tuesday
    Wednesday
    Thursday
    Friday
    Saturday
)

This expands to underlying type int (the default type of iota) and is equivalent to:

type Weekday int

const (
    Sunday Weekday = iota
    Monday
    Tuesday
    Wednesday
    Thursday
    Friday
    Saturday
)

Underlying Type Inference

The underlying type of XXX is determined by applying Go's normal constant type-inference rules to the expressions in the block, then taking the default type of the resulting untyped constant:

Constant expressions	Inferred underlying type
Untyped integer constants (`0`, `iota`)	`int`
Untyped string constants (`"a"`)	`string`
Untyped boolean constants (`true`)	`bool`
Untyped float constants (`1.5`)	`float64`
Untyped rune constants (`'a'`)	`rune` (`int32`)

Rules

All expressions must have the same default type. Mixing, e.g., string and integer constants in the same block is a compile-time error:
```
type Bad const (
    A = 1
    B = "x" // error: inconsistent constant types in enum declaration
)
```
iota defaults to int, matching Go's existing behavior.
Explicit type conversions are allowed for cases where a different underlying type is desired, e.g.:
```
type SmallEnum const (
    A = int8(1)
    B = int8(2)
)
// underlying type: int8
```
In this case all expressions must convert to (or default to) the same type, following normal Go conversion/typing rules.
The first expression establishes the type if there is any ambiguity; subsequent expressions are checked for consistency against it. This mirrors how iota-based blocks already behave when only the first entry carries an explicit type in Go.

Examples

Integer enum (most common case)

type Color const (
    Red   = iota
    Green
    Blue
)

Equivalent Go:

type Color int

const (
    Red   Color = iota
    Green
    Blue
)

String enum

type LogLevel const (
    Debug = "debug"
    Info  = "info"
    Warn  = "warn"
    Error = "error"
)

Equivalent Go:

type LogLevel string

const (
    Debug LogLevel = "debug"
    Info  LogLevel = "info"
    Warn  LogLevel = "warn"
    Error LogLevel = "error"
)

Boolean-backed enum

type Toggle const (
    Off = false
    On  = true
)

Equivalent Go:

type Toggle bool

const (
    Off Toggle = false
    On  Toggle = true
)

Bit-flag enum with explicit expressions

type Permission const (
    Read    = 1 << iota
    Write
    Execute
)

Equivalent Go:

type Permission int

const (
    Read    Permission = 1 << iota
    Write
    Execute
)

Compatibility

This is purely additive syntax sugar at the source level. After XGo's compiler expands type XXX const (...) into the equivalent Go type + const declarations, the resulting Go AST is indistinguishable from hand-written Go code.
All downstream tooling (type-checking, go vet, reflection, JSON marshaling, String()/Stringer codegen via go generate, etc.) works unchanged, since the generated type and constants are ordinary Go declarations.
Existing Go code using the traditional two-block form remains fully valid and is unaffected.