/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).

Go Declarations and Initialization

> Compatibility: Examples may use `any`, which requires Go 1.18+.

Resource Routing

• `references/SCOPE.md` - Read when deciding between `var`, `:=`, if-init, and narrow variable scope.
• `references/IOTA.md` - Read when designing constants or enum-like values.
• `references/INITIALIZATION.md` - Read when initializing structs, maps, zero values, or pointers.
• `references/LITERALS.md` - Read for composite literal formatting and keyed-field tradeoffs.
• `references/STRUCTS.md` - Read when designing or initializing structs.
• `references/SHADOWING.md` - Read when a declaration may shadow a builtin or outer variable.

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()` |

---

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`).

---

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.

---

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"}`

---

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`).

---

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

---

Related Skills

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