go-declarations

name	go-declarations
description	Use when declaring or initializing Go variables, constants, structs, or maps — including var vs :=, reducing scope with if-init, formatting composite literals, designing iota enums, and using any instead of interface{}. Also use when writing a new struct or const block, even if the user doesn't ask about declaration style. Does not cover naming conventions (see go-naming).
license	Apache-2.0
compatibility	Uses any keyword (Go 1.18+)
metadata	{"sources":"Google Style Guide, Uber Style Guide"}

Go Declarations and Initialization

---

Quick Reference: var vs :=

Context	Use	Example
Top-level	var (always)	var _s = F()
Local with value	:=	s := "foo"
Local zero-value (intentional)	var	var filtered []int
Type differs from expression	var with type	var _e error = F()

Read references/SCOPE.md when deciding between var and := in complex initialization patterns or multi-return assignments.

---

Group Similar Declarations

Group related var, const, type in parenthesized blocks. Separate unrelated declarations into distinct blocks.

// Bad
const a = 1
const b = 2

// Good
const (
    a = 1
    b = 2
)

Inside functions, group adjacent vars even if unrelated:

var (
    caller  = c.name
    format  = "json"
    timeout = 5 * time.Second
)

---

Constants and iota

Start enums at one so the zero value represents invalid/unset:

const (
    Add Operation = iota + 1
    Subtract
    Multiply
)

Use zero when the default behavior is desirable (e.g., LogToStdout).

Read references/IOTA.md when designing iota enums with bitmask patterns, byte-size constants, or String() methods.

---

Variable Scope

Use if-init to limit scope when the result is only needed for the error check:

if err := os.WriteFile(name, data, 0644); err != nil {
    return err
}

Don't reduce scope if it forces deeper nesting or you need the result outside the if. Move constants into functions when only used there.

Read references/SCOPE.md when working with top-level declarations or choosing between var and := for local variables.

---

Initializing Structs

• Always use field names (enforced by go vet). Exception: test tables with ≤3 fields.
• Omit zero-value fields — let Go set defaults.
• Use var for zero-value structs: var user User not user := User{}
• Use &T{} over new(T): sptr := &T{Name: "bar"}

Read references/STRUCTS.md when initializing structs with many fields, building slices of struct pointers, or choosing single-line vs multi-line format.

---

Composite Literal Formatting

Use field names for external package types. Match closing brace indentation with the opening line. Omit repeated type names in slice/map literals (gofmt -s).

Read references/INITIALIZATION.md when working with complex composite literals, cuddled braces, or zero-value field decisions.

Read references/LITERALS.md when formatting complex composite literals.

---

Initializing Maps

Scenario	Use	Example
Empty, populated later	make(map[K]V)	m := make(map[string]int)
Nil declaration	var	var m map[string]int
Fixed entries at init	Literal	m := map[string]int{"a": 1}

make() visually distinguishes empty-but-initialized from nil. Use size hints when the count is known.

---

Raw String Literals

Use backtick strings to avoid hand-escaped characters:

// Bad
wantError := "unknown name:\"test\""

// Good
wantError := `unknown name:"test"`

Ideal for regex, SQL, JSON, and multi-line text.

---

Prefer any Over interface{}

Go 1.18+: use any instead of interface{} in all new code.

---

Avoid Shadowing Built-In Names

Never use predeclared identifiers (error, string, len, cap, append, copy, new, make, close, delete, panic, recover, any, true, false, nil, iota) as names. Use go vet to detect.

// Bad — shadows the builtin
var error string

// Good
var errorMessage string

Read references/SHADOWING.md when debugging issues where := creates new variables that shadow outer scope.

---

Related Skills

• Naming conventions: See go-naming when choosing variable names, constant names, or deciding name length by scope
• Data structures: See go-data-structures when choosing between new and make, or initializing slices and maps
• Control flow scoping: See go-control-flow when using if-init, := redeclaration, or avoiding variable shadowing
• Capacity hints: See go-performance when pre-allocating maps or slices with known sizes