/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.

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)

For Loops

Go's for is its only looping construct, unifying while, do-while, and C-style for:

// Condition-only (Go's "while")
for x > 0 {
    x = process(x)
}

// Infinite loop
for {
    if done() { break }
}

// C-style three-component
for i := 0; i < n; i++ { ... }

Range

range iterates over slices, maps, strings, and channels:

for i, v := range slice { ... }   // index + value
for k, v := range myMap { ... }   // key + value (non-deterministic order)
for i, r := range "héllo" { ... } // byte index + rune (not byte)
for v := range ch { ... }         // receives until channel closed

Key rules:

• Range over strings yields runes, not bytes — i is the byte offset
• Range over maps has non-deterministic order — don't rely on it
• Use _ to discard the index or value: for _, v := range slice

Parallel Assignment

Go has no comma operator. Use parallel assignment for multiple loop variables:

for i, j := 0, len(a)-1; i < j; i, j = i+1, j-1 {
    a[i], a[j] = a[j], a[i]
}

++ and -- are statements, not expressions — they cannot appear in parallel assignment.

Switch: Labeled Break

break inside a switch within a for loop only breaks the switch. Use a labeled break to exit the enclosing loop:

Loop:
    for _, v := range items {
        switch v.Type {
        case "done":
            break Loop  // breaks the for loop
        }
    }

For type switches, see go-interfaces: Type Switch.

The Blank Identifier

Never discard errors carelessly — a nil dereference panic may follow.

Route compile-time interface assertions to go-interfaces.

Quick Reference

Related Skills

• Error flow: See go-error-handling when structuring guard clauses, early returns, or error-first patterns
• Type switches: See go-interfaces when using type switches, the comma-ok idiom, or interface satisfaction checks
• Nesting reduction: See go-style-core when reducing nesting depth or resolving formatting questions
• Variable scoping: See go-declarations when using if-init, := redeclaration, or reducing variable scope