phant-app/go-control-flow

Go Control Flow

Source: Effective Go. Go's control structures are related to C but differ in important ways. Understanding these differences is essential for writing idiomatic Go code.

Go has no do or while loop—only a generalized for. There are no parentheses around conditions, and bodies must always be brace-delimited.

---

If Statements

Basic Form

Go's if requires braces and has no parentheses around the condition:

if x > 0 {
    return y
}

If with Initialization

if and switch accept an optional initialization statement. This is common for scoping variables to the conditional block:

// Good: err scoped to if block
if err := file.Chmod(0664); err != nil {
    log.Print(err)
    return err
}

Omit Else for Early Returns

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

// Good: no else, success path at left margin
f, err := os.Open(name)
if err != nil {
    return err
}
codeUsing(f)

// Bad: else clause buries normal flow
f, err := os.Open(name)
if err != nil {
    return err
} else {
    codeUsing(f)  // unnecessarily indented
}

Guard Clauses for Error Handling

Code reads well when the success path flows down the page, eliminating errors as they arise:

// Good: guard clauses eliminate errors early
f, err := os.Open(name)
if err != nil {
    return err
}
d, err := f.Stat()
if err != nil {
    f.Close()
    return err
}
codeUsing(f, d)

---

Redeclaration and Reassignment

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

f, err := os.Open(name)  // declares f and err
// ...
d, err := f.Stat()       // declares d, reassigns err (not a new 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

This pragmatic rule makes it easy to reuse a single err variable through a chain of operations.

// Good: err reused across multiple calls
data, err := fetchData()
if err != nil {
    return err
}
result, err := processData(data)  // err reassigned, result declared
if err != nil {
    return err
}

Warning: If v is declared in an outer scope, := creates a new variable that shadows it:

// Bad: accidental shadowing
var err error
if condition {
    x, err := someFunc()  // this err shadows the outer err!
    // outer err remains nil
}

---

For Loops

Go unifies for and while into a single construct with three forms:

// C-style for (only form with semicolons)
for init; condition; post { }

// While-style (condition only)
for condition { }

// Infinite loop
for { }

Range Clause

Use range to iterate over arrays, slices, strings, maps, and channels:

// Iterate with key and value
for key, value := range oldMap {
    newMap[key] = value
}

// Key/index only (drop the second variable)
for key := range m {
    if key.expired() {
        delete(m, key)
    }
}

// Value only (use blank identifier for index)
for _, value := range array {
    sum += value
}

Range Over Strings

For strings, range iterates over UTF-8 encoded runes (not bytes), handling multi-byte characters automatically.

Parallel Assignment in For

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

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

Note: ++ and -- are statements, not expressions, so they cannot be used in parallel assignment.

---

Switch

Go's switch is more flexible than C's:

• Expressions need not be constants or integers
• Cases are evaluated top to bottom until a match
• No automatic fall through (no need for break in each case)

Expression-less Switch

If the switch has no expression, it switches on true. This is idiomatic for writing clean if-else-if chains:

// Good: expression-less switch for ranges
func unhex(c byte) byte {
    switch {
    case '0' <= c && c <= '9':
        return c - '0'
    case 'a' <= c && c <= 'f':
        return c - 'a' + 10
    case 'A' <= c && c <= 'F':
        return c - 'A' + 10
    }
    return 0
}

Comma-Separated Cases

Multiple cases can be combined with commas (no fall through needed):

func shouldEscape(c byte) bool {
    switch c {
    case ' ', '?', '&', '=', '#', '+', '%':
        return true
    }
    return false
}

Break with Labels

break terminates the switch by default. To break out of an enclosing loop, use a label:

Loop:
    for n := 0; n < len(src); n += size {
        switch {
        case src[n] < sizeOne:
            break        // breaks switch only
        case src[n] < sizeTwo:
            if n+1 >= len(src) {
                break Loop   // breaks out of for loop
            }
        }
    }

---

Type Switch

A type switch discovers the dynamic type of an interface value using .(type):

switch v := value.(type) {
case nil:
    fmt.Println("value is nil")
case int:
    fmt.Printf("integer: %d\n", v)      // v is int
case string:
    fmt.Printf("string: %q\n", v)       // v is string
case bool:
    fmt.Printf("boolean: %t\n", v)      // v is bool
default:
    fmt.Printf("unexpected type %T\n", v)
}

It's idiomatic to reuse the variable name (v := value.(type)) since the variable has a different type in each case clause.

When a case lists multiple types (case int, int64:), the variable has the interface type.

---

The Blank Identifier

The blank identifier _ discards values. It's like writing to /dev/null.

Multiple Assignment

Discard unwanted values from multi-value expressions:

// Only need the error
if _, err := os.Stat(path); os.IsNotExist(err) {
    fmt.Printf("%s does not exist\n", path)
}

// Only need the value (discard ok)
value := cache[key]  // simpler: just use single-value form
_, present := cache[key]  // when you only need presence check

Never discard errors carelessly:

// Bad: ignoring error will crash if path doesn't exist
fi, _ := os.Stat(path)
if fi.IsDir() {  // nil pointer dereference if path doesn't exist
    // ...
}

Unused Imports and Variables During Development

Silence compiler errors temporarily during active development:

import (
    "fmt"
    "io"
)

var _ = fmt.Printf  // silence unused import (remove before committing)
var _ io.Reader

func main() {
    fd, _ := os.Open("test.go")
    _ = fd  // silence unused variable
}

Import for Side Effect

Import a package only for its init() side effects:

import _ "net/http/pprof"  // registers HTTP handlers
import _ "image/png"       // registers PNG decoder

This makes clear the package is imported only for side effects—it has no usable name in this file.

Interface Compliance Check

Verify at compile time that a type implements an interface:

// Verify that *MyType implements io.Writer
var _ io.Writer = (*MyType)(nil)

// Verify that MyHandler implements http.Handler
var _ http.Handler = MyHandler{}

This fails at compile time if the type doesn't implement the interface, catching errors early.

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Quick Reference

| Pattern | Go Idiom | |---------|----------| | If initialization | if err := f(); err != nil { } | | Early return | Omit else when if body returns | | Redeclaration | := reassigns if same scope + new var | | C-style for | for i := 0; i < n; i++ { } | | While-style | for condition { } | | Infinite loop | for { } | | Range with key+value | for k, v := range m { } | | Range value only | for _, v := range slice { } | | Range key only | for k := range m { } | | Parallel assignment | i, j = i+1, j-1 | | Expression-less switch | switch { case cond: } | | Comma cases | case 'a', 'b', 'c': | | No fallthrough | Default behavior (explicit fallthrough if needed) | | Break from loop in switch | break Label | | Type switch | switch v := x.(type) { } | | Discard value | _, err := f() | | Side-effect import | import _ "pkg" | | Interface check | var _ Interface = (*Type)(nil) |

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See Also

• go-style-core: Core Go style principles and formatting
• go-error-handling: Error handling patterns including guard clauses
• go-naming: Naming conventions for loop variables and labels
• go-concurrency: Goroutines, channels, and select statements
• go-defensive: Defensive programming patterns