Control Flow Highlights Summary

Every highlight bullet from the 9 pages in this section, gathered on one page and grouped by the page it came from.

Go Control Flow: Expression-Oriented Design

• Go has one loop keyword (for) that covers C-style, while, and range forms
• if and switch can carry init statements that scope locals to the block
• switch does not fall through by default - explicit fallthrough is rare
• Control flow favors early returns and flat structure over deep nesting
• defer, panic, and recover are control-flow tools with strict runtime rules

Control Flow Basics

• If conditions require braces but no parentheses, enforced at compile time
• For loops handle all iteration in Go, no while keyword exists
• Range values don't update slices directly, use index to modify
• Switch cases require explicit break, no automatic fallthrough
• Defer runs in LIFO order on exit, even during panic unwinding
• If init statements scope variables to conditional blocks only

for Loop Variants & range Semantics

• for range copies collection headers, not array elements, affecting mutations
• Map iteration order randomizes by design, never depend on it
• String range decodes UTF-8 runes, advancing index by byte width
• Channel for range blocks until ready, ends when closed and drained
• Mutate slices with index access s[i], not the range value copy
• Three-part for header counts loops, condition-only form acts like while

if with Initialization & switch Patterns

• Init statements in if and switch scope temporary variables to that block
• Expression switches stop after the first matching case without fallthrough
• Tagless switches chain boolean expressions sequentially like if-else-if
• Each type switch case binds variables with the concrete matched type
• fallthrough keyword explicitly continues to the next case unconditionally

defer, panic & recover

• Use defer to schedule cleanup when a function returns
• recover stops panic unwind but only works inside a deferred call
• Deferred function arguments evaluate immediately, not when called
• Avoid defer in loops; stack grows until the surrounding function returns
• Panic is per-goroutine; other goroutines continue running
• Use error returns for expected failures, not panic

break, continue & Labeled Control Flow

• break exits innermost for, switch, or select; continue skips to next loop iteration
• Label a loop or select to target break and continue across nested constructs
• break in select only exits select, not surrounding loop unless labeled
• Unlabeled break in select inside for loops forever, requiring labeled break
• continue only targets loops, not switch statements; restructure if needed
• Helper functions often replace labeled breaks for clearer, more maintainable code

range Variable Reuse Pitfalls

• Go 1.22+ fresh variables per iteration; three-part for loops still reuse
• Goroutines and closures see final loop value before Go 1.22 without copy
• Pointer to loop variable aliases same slot; pass by parameter or copy
• Copy loop variable before goroutines: v := v or pass as parameter
• Defer captures final loop values; pass arguments to defer block
• Parameter passing and explicit copies work on all Go versions

Control Flow in HTTP Middleware

• Wrap HTTP handlers in defer recover to catch panics and prevent process crashes
• Use defer for request-scoped cleanup like timers, body close, and span cancel
• Middleware returns early by not calling next or by writing a response and returning
• Place panic recovery at the outermost useful layer like server or router middleware
• Each request goroutine panics independently so recovery prevents process-wide crashes
• Pair defer recover with structured logging and request IDs from context

Control Flow Best Practices

• Return early to keep the happy path at left margin
• Default to for range for slices, maps, and strings
• Never depend on map iteration order for stable output
• Install panic recovery middleware on production servers
• Place defer right after resource acquisition succeeds
• Mutate slices by index, not the range value copy