Go Function Design Skill

Go Function Design

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

Function Grouping and Ordering

Organize functions in a file by these rules:

Functions sorted in rough call order
Functions grouped by receiver
Exported functions appear first, after struct/const/var definitions
NewXxx/newXxx constructors appear right after the type definition
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

Read references/SIGNATURES.md when formatting multi-line signatures, wrapping return values, shortening call sites, or replacing naked bool parameters with custom types.

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

Read references/PRINTF-STRINGER.md when using Printf verbs beyond %v/%s/%d, implementing fmt.Stringer or fmt.GoStringer, writing custom Format() methods, or debugging infinite recursion in String() methods.

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

| Topic | Rule | |-------|------| | File ordering | Type -> constructor -> exported -> unexported -> utils | | Signature wrapping | All args on own lines with trailing comma | | Naked parameters | Add /* name */ comments or use custom types | | Pointers to interfaces | Almost never needed; pass interfaces by value | | Printf function names | End with f for go vet support |

Related Skills

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

Agent Skills: Go Function Design

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