Go Function Design

A function's surface is read more often than its body. Optimize for the reader: predictable ordering in the file, signatures that scan, no hidden bool flags.

Core Rules

1. Order by use, not alphabet. Types → constructors → exported methods → unexported → utilities.
2. Keep signatures on one line when reasonable. When wrapping, every parameter on its own line with a trailing comma.
3. *Never pass `Interface`.** Pass the interface value; the underlying data can already be a pointer.
4. Replace naked `bool`/`int` parameters with named types or add /* name */ comments at call sites.
5. Printf-style functions end in `f` so go vet can check the format.
6. Prefer `%q` over `%s` plus manual quoting when formatting strings for errors and logs.

File Ordering

type Server struct{ ... }

func NewServer(...) *Server { ... }     // constructor next to type

func (s *Server) Start(ctx context.Context) error { ... } // exported
func (s *Server) Stop() error           { ... }

func (s *Server) acceptLoop() { ... }   // unexported

func parseAddr(s string) (string, error) { ... } // file-local helper

Rules:

1. Types and their constructors sit together at the top.
2. Exported methods come before unexported ones.
3. File-local helpers go at the bottom.
4. Within a section, follow rough call order.

Signature Formatting

// Fits on one line — keep it on one line
func Sum(xs []int) int

// Too long — break with every param on its own line
func (r *Repo) SaveTransaction(
    ctx context.Context,
    userID string,
    tx Transaction,
    opts ...SaveOption,
) (string, error) {
    ...
}

The trailing comma is required and gofmt-stable.

Avoid Naked Bool/Int Parameters

// Bad — what does `true` mean?
NewServer(":8080", true, 30, false)

// Better — call-site comments
NewServer(":8080", true /* tls */, 30 /* maxConn */, false /* readonly */)

// Best — named types or options
NewServer(":8080", WithTLS(), WithMaxConn(30))

When a single bool is genuinely binary and obvious from the function name (SetVerbose(true)), it's fine.

Read references/signatures.md for return-value styles, naked returns, function-as-parameter formatting, and the variadic-options call-site shape.

Pointers to Interfaces

// Bad
func process(r *io.Reader) { ... }

// Good
func process(r io.Reader) { ... }

An interface value already carries a pointer-sized data word. *io.Reader is a pointer to an interface — almost always a mistake.

Printf and Stringer

Functions that accept a format string should end in f:

func Logf(format string, args ...any)

go vet then checks that %s, %d, etc. match the argument types.

When formatting strings into errors or logs, prefer %q:

return fmt.Errorf("unknown key %q", key) // unknown key "foo\nbar"

%q quotes and escapes; %s plus manual quoting ("key \"" + key + "\"") is fragile.

Read references/printf-and-stringer.md for %v vs %s vs %q, implementing fmt.Stringer safely, avoiding String() infinite recursion, and fmt.Formatter.

Variadic Options at the Call Site

db.Open(addr,
    db.WithCache(false),
    db.WithLogger(log),
    db.WithRetries(3),
)

Each option on its own line, trailing comma. Use this layout whenever the call doesn't fit on a single line.

Constructors

A constructor immediately follows its type. Use the short form when no error is possible:

type Counter struct{ n int }

func NewCounter() *Counter { return &Counter{} }

Return an error when construction can fail:

func NewClient(addr string) (*Client, error) { ... }

Don't expose a half-built type through a constructor that "always succeeds" but requires Init() afterward.

Anti-Patterns

Verification Checklist

• [ ] Types appear above their constructors; exported methods above unexported
• [ ] Long signatures wrap with one parameter per line and a trailing comma
• [ ] No pointer-to-interface parameters
• [ ] Bool/int parameters are either obvious from the function name or named with /* */ comments
• [ ] Functions taking a format string end in f
• [ ] Errors and logs use %q when formatting arbitrary strings
• [ ] Constructors return (*T, error) when construction can fail — no half-built objects

References

• references/signatures.md — multi-line wrapping, named results, function-typed parameters
• references/printf-and-stringer.md — format verbs, fmt.Stringer, recursion traps, fmt.Formatter