go-functions

# Go Function Design

> **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

```go
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](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:

```go
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.

```go
// 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](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:

```go
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](../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