Go Function Design

Resource Routing

• references/SIGNATURES.md - Read when designing parameters, return values, named results, or signature readability.
• references/PRINTF-STRINGER.md - Read when using fmt verbs, Stringer, GoStringer, Formatter, or Printf-style function naming.

When this skill does NOT apply: For functional options constructors (WithTimeout, WithLogger), see [go-functional-options](../go-functional-options/SKILL.md). For error return conventions, see [go-error-handling](../go-error-handling/SKILL.md). For naming functions and methods, see [go-naming](../go-naming/SKILL.md).

Function Grouping and Ordering

Organize functions in a file by these rules:

1. Functions sorted in rough call order
2. Functions grouped by receiver
3. Exported functions appear first, after struct/const/var definitions
4. NewXxx/newXxx constructors appear right after the type definition
5. Plain utility functions appear toward the end of the file

type something struct{ ... }

func newSomething() *something { return &something{} }

func (s *something) Cost() int { return calcCost(s.weights) }

func (s *something) Stop() { ... }

func calcCost(n []int) int { ... }

Function Signatures

Keep the signature on a single line when possible. When it must wrap, put all arguments on their own lines with a trailing comma:

func (r *SomeType) SomeLongFunctionName(
    foo1, foo2, foo3 string,
    foo4, foo5, foo6 int,
) {
    foo7 := bar(foo1)
}

Add / name / comments for ambiguous arguments, or better yet, replace naked bool parameters with custom types.

Pointers to Interfaces

You almost never need a pointer to an interface. Pass interfaces as values — the underlying data can still be a pointer.

// Bad: pointer to interface
func process(r *io.Reader) { ... }

// Good: pass the interface value
func process(r io.Reader) { ... }

Printf and Stringer

Printf-style Function Names

Functions that accept a format string should end in f for go vet support. Declare format strings as const when used outside Printf calls.

Prefer %q over %s with manual quoting when formatting strings for logging or error messages — it safely escapes special characters and wraps in quotes:

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

See go-functional-options when designing a constructor with 3+ optional parameters.

Quick Reference

Related Skills

• Error returns: See [go-error-handling](../go-error-handling/SKILL.md) when designing error return patterns or wrapping errors in multi-return functions
• Naming conventions: See [go-naming](../go-naming/SKILL.md) when naming functions, methods, or choosing getter/setter patterns
• Functional options: See [go-functional-options](../go-functional-options/SKILL.md) when designing a constructor with 3+ optional parameters
• Formatting principles: See [go-style-core](../go-style-core/SKILL.md) when deciding line length, naked returns, or signature formatting