go-control-flow

Go Control Flow

Resource Routing

• references/SWITCH-PATTERNS.md - Read when using switch statements, type switches, fallthrough, or labeled breaks.
• references/BLANK-IDENTIFIER.md - Read when using _, blank imports, unused compile-time assertions, or intentional discards.

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If with Initialization

if and switch accept an optional initialization statement. Use it to scope variables to the conditional block:

if err := file.Chmod(0664); err != nil {
    log.Print(err)
    return err
}

If you need the variable beyond a few lines after the if, declare it separately and use a standard if instead:

x, err := f()
if err != nil {
    return err
}
// lots of code that uses x

Indent Error Flow (Guard Clauses)

When an if body ends with break, continue, goto, or return, omit the unnecessary else. Keep the success path unindented:

f, err := os.Open(name)
if err != nil {
    return err
}
d, err := f.Stat()
if err != nil {
    f.Close()
    return err
}
codeUsing(f, d)

Never bury normal flow inside an else when the if already returns.

---

Redeclaration and Reassignment

The := short declaration allows redeclaring variables in the same scope:

f, err := os.Open(name)  // declares f and err
d, err := f.Stat()       // declares d, reassigns err

A variable v may appear in a := declaration even if already declared, provided:

1. The declaration is in the same scope as the existing v
2. The value is assignable to v
3. At least one other variable is newly created by the declaration

Variable Shadowing

Warning: If v is declared in an outer scope, := creates a new variable that shadows it — a common source of bugs:

// Bug: ctx inside the if block shadows the outer ctx
if *shortenDeadlines {
    ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
    defer cancel()
}
// ctx here is still the original — the shadowed ctx didn't escape

// Fix: use = instead of :=
var cancel func()
ctx, cancel = context.WithTimeout(ctx, 3*time.Second)