Go declarations

Go Declarations and Initialization

Pick the simplest declaration form that expresses your intent: scope variables tightly, group related declarations, and let the zero value do its job.

Core Rules

1. := for locals with values; var for intentional zero values or top-level declarations.
2. Group related declarations in parenthesized blocks. Separate unrelated ones into distinct blocks.
3. Start enums at iota + 1 so the zero value is "invalid/unset" — unless zero is genuinely meaningful.
4. Initialize structs with field names. Omit zero-value fields; let defaults speak.
5. Use any, not interface{}, in all new code.
6. Never shadow builtins (len, cap, error, new, make, copy, any, nil, ...).

Decision: var vs :=

Context	Use	Example
Package-level	var (always)	var startTime = time.Now()
Local with computed value	:=	s := "foo"
Local zero-value, intentional	var	var filtered []int
Declared type differs from RHS	var T = expr	var e error = f()

Read references/scope-and-shadowing.md when fighting subtle bugs caused by := redeclaring an outer variable.

Group Related Declarations

// Bad
const a = 1
const b = 2

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

Inside functions, group adjacent vars even if loosely related:

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

Constants and iota

Zero is the default; reserve it for "uninitialized" by starting enums at iota + 1:

type Operation int

const (
    Add      Operation = iota + 1 // 1
    Subtract                      // 2
    Multiply                      // 3
)

Use plain iota only when the zero value is the sensible default (e.g., LogToStdout = iota).

Read references/iota-and-literals.md for bitmask enums, String() methods, raw strings, and composite-literal formatting.

Initializing Structs

• Always use field names. Positional struct literals break on field reordering and are caught by go vet.
• Omit zero-value fields — clarity beats explicit zeros.
• var u User for a zero-value struct (not u := User{}).
• &T{...} over new(T) when you want a pointer.

u := User{Name: "Ada", Email: "[email protected]"}
sptr := &Config{Timeout: 5 * time.Second}
var empty Buffer // zero value, ready to use

Test tables with ≤3 fields may use positional literals when the meaning is obvious.

Initializing Maps

Scenario	Use	Example
Empty, will be populated	make(map[K]V)	m := make(map[string]int)
Nil, lazily allocated	var	var m map[string]int
Known entries up front	Literal	m := map[string]int{"a": 1}

make signals "initialized but empty" — different from a nil map (which panics on write). Provide a size hint when the count is known: make(map[K]V, n).

Raw String Literals

Use backticks to avoid escape gymnastics:

// Bad
re := "^\\s*name:\\s*\"(.*)\""

// Good
re := `^\s*name:\s*"(.*)"`

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

any, not interface{}

// Old
func Print(v interface{}) { ... }

// New
func Print(v any) { ... }

any is an alias for interface{} since Go 1.18 — same type, less noise.

Don't Shadow Builtins

The predeclared identifiers (error, string, len, cap, append, copy, new, make, close, delete, panic, recover, any, true, false, nil, iota) are not reserved words — Go lets you shadow them. Don't.

// Bad — shadows the builtin error type
var error string

// Good
var errorMessage string

go vet catches the most common cases.

Read references/structs-and-tags.md when designing struct fields that cross a serialization boundary (JSON, YAML, protobuf), embedding types, or formatting many-field literals.

Anti-Patterns

Anti-pattern	Why it hurts	Do this instead
u := User{} for a zero value	Misleads readers into expecting non-defaults	var u User
new(T) then assign fields	Two-step where one works	&T{Field: v}
Positional struct literals (>3 fields)	Silent breakage on field reordering	Use field names
iota starting at 0 for an enum	Zero value collides with a real case	iota + 1
var m map[string]int then m[k] = v	Panic on nil map write	m := make(map[string]int)
Hand-escaped JSON or regex strings	Hard to read, easy to mistype	Raw string literal
interface{} in new code	Verbose, outdated	any

Verification Checklist

• Top-level declarations use var/const; locals use := unless zero-value is intended
• Related const/var/type are in grouped blocks
• Enums start at iota + 1 (or the zero value is explicitly meaningful)
• Struct literals use field names; zero-value fields are omitted
• Maps that will be written to are constructed with make
• No builtins shadowed (error, len, cap, ...)
• any used instead of interface{}

References

• references/scope-and-shadowing.md — variable scope, := redeclaration rules, shadowing traps
• references/iota-and-literals.md — iota patterns, bitmasks, raw strings, composite literals
• references/structs-and-tags.md — struct initialization, field tags, embedding