/go-functions

name: go-functions
description: Use when organizing functions within a Go file, formatting function signatures, designing return values, or following Printf-style naming conventions. Also use when a user is adding or refactoring any Go function, even if they don't mention function design or signature formatting. Does not cover functional options constructors (see go-functional-options).

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).

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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.

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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 |

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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