Go Data Idioms

Slices are (ptr, len, cap) headers over a shared backing array; aliasing is the default. defer evaluates args at the defer site, not at return. Design types so the zero value is usable.

new vs make

new([]int) yields *[]int pointing to a nil slice — rarely useful. Use make([]int, n).

Slices: aliasing and append

s := []int{1, 2, 3, 4}
t := s[1:3]            // shares backing; t[0]=9 mutates s[1]
u := append(s, 5)      // may or may not reallocate

s = append(s, x)       // MUST reassign
append(s, x)           // lost — returned header discarded

Reallocation when len + added > cap. Before realloc, original and returned share memory; after, they don't. Treat the return as authoritative.

Maps: comma-ok for absence

v, ok := m[k]    // ok=false means absent
_, ok := m[k]    // presence-only

Missing key silently returns zero value; without ok you cannot tell "absent" from "present and zero".

Defer: LIFO and argument capture

for i := 0; i < 3; i++ {
    defer fmt.Println(i)    // args evaluated NOW → 2, 1, 0
}

x := 1
defer func() { fmt.Println(x) }()
x = 2                       // prints 2 — closure reads at call time

Direct-call defer captures args at the defer statement. A func(){} literal reads variables when it runs.

Named returns + defer = post-return mutation

func parse() (out T, err error) {
    defer func() {
        if err != nil { err = fmt.Errorf("parse: %w", err) }
    }()
    ...
    return   // defer sees and rewrites err
}

Works only with named returns. Defer runs after return operands evaluate, before control leaves the frame.

Zero-value-useful

type Buf struct {
    mu  sync.Mutex       // zero: unlocked
    buf bytes.Buffer     // zero: empty, usable
}
var b Buf                // no constructor needed

Pitfalls

• Forgetting to reassign append: silent element loss when cap is exceeded.
• Slice retention: big[0:1] keeps the whole backing array alive. copy into a fresh slice when keeping a small prefix.
• defer in a loop: all defers run at function exit, not per iteration — handles accumulate. Wrap iteration in a helper.
• Closure vs argument capture: defer f(x) captures x now; defer func(){ f(x) }() captures x at execution.
• Nil map write panics: var m map[string]int; m["k"] = 1 panics. Always make before writing.
• Two-dim slice allocation: one contiguous make([]T, x*y) sliced into rows beats per-row make when rows have fixed size.