Go control flow

Go Control Flow

Go gives you if, for, and switch — and one looping construct that covers them all. The idioms are small but strict: scope variables tightly, return early, keep the happy path unindented.

Core Rules

1. Scope variables with if-init when they live only for the check. if x, err := f(); err != nil { ... }.
2. Guard clauses over nested else. When the if body returns/breaks/continues, drop the else.
3. := redeclares only in the same scope. In an inner scope it shadows — a frequent bug source.
4. One for, three forms. Condition-only (while), three-clause (C-style), and infinite (for {}).
5. range over string yields runes, over map yields non-deterministic order, over channel drains until closed.
6. break inside switch only breaks the switch. Use a labeled break to exit the enclosing for.
7. The blank identifier _ discards, but never errors. Silent error dropping is a bug.

Decision: var vs := vs = at a glance

Situation	Use
New variable, scoped to the check	if v, err := f(); err != nil
Reusing an outer variable	plain = (avoid := to prevent shadowing)
At least one new + reuse of outer	:= is fine (same scope only)
Wanted zero value	var x T

If with Initialization

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

If err is needed past the if, declare it separately:

x, err := f()
if err != nil {
    return err
}
// use x freely

Guard Clauses

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 the success path inside else.

The Shadowing Trap

// Bug: inner ctx never escapes the if block
if *shortenDeadlines {
    ctx, cancel := context.WithTimeout(ctx, 3*time.Second)
    defer cancel()
}

// Fix: assign with =, declaring cancel separately
var cancel func()
ctx, cancel = context.WithTimeout(ctx, 3*time.Second)
defer cancel()

Read references/blank-identifier.md for _ use cases (interface checks, side-effect imports, multi-return discards).

For Loops

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

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

// Three-clause
for i := 0; i < n; i++ { ... }

Range

for i, v := range slice { ... }   // index + value
for k, v := range myMap  { ... }  // non-deterministic order
for i, r := range "héllo" { ... } // i is byte offset; r is rune
for v := range ch { ... }         // drains until closed

Parallel Assignment

Go has no comma operator. Use parallel assignment instead:

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 inside a parallel assignment.

Switch and Labeled Break

Loop:
    for _, v := range items {
        switch v.Type {
        case "done":
            break Loop // breaks the for, not just the switch
        }
    }

Read references/switch-patterns.md for expression-less switches, comma cases, fallthrough, and type switches.

Anti-Patterns

Anti-pattern	Why it hurts	Do this instead
} else { return ... } after a returning if	Pointless nesting	Drop else; let the happy path stay flat
if _, err := f(); err == nil { ... } then use the value	Value is out of scope	Move the := outside the if
:= in inner scope reassigning outer var	Silently shadows	Use = (declare the new locals separately)
Iterating a map and relying on order	Order is randomized	Sort keys explicitly
break inside switch expecting to exit for	Only exits switch	Use labeled break Label
_ = doSomething() to silence an error	Real failures vanish	Handle it or document why

Verification Checklist

• No else branch after an if that returns/breaks/continues
• := in inner scopes does not shadow important outer variables
• Loops use the simplest of the three forms that fits
• range over strings treats the index as a byte offset, not a rune index
• Map iteration does not assume an order
• Errors are never silently discarded with _
• Labeled break is used when a switch needs to exit a surrounding loop

References

• references/switch-patterns.md — expression-less, comma cases, fallthrough, type switches
• references/blank-identifier.md — _ for multi-return, interface compliance, side-effect imports