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SKILL.md
npx skills add cxuu/golang-skills --skill go-generics
Use when deciding whether to use Go generics, writing generic functions or types, choosing constraints, or picking between type aliases and type definitions. Also use when a user is writing a utility function that could work with multiple types, even if they don't mention generics explicitly. Does not cover interface design without generics (see go-interfaces).
npx skills add cxuu/golang-skills --skill go-generics
Compatibility: Generics require Go 1.18+.
references/CONSTRAINTS.md - Read when composing constraints, using type sets, or choosing between generics and interfaces.Start with concrete types. Generalize only when a second type appears.
any and excessive type switching"Write code, don't design types." — Robert Griesemer and Ian Lance Taylor
Bad:
Good:
| Name | Typical Use |
|---|---|
T | General type parameter |
K | Map key type |
V | Map value type |
E | Element/item type |
For complex constraints, a short descriptive name is acceptable:
Type aliases (type Old = new.Name) are rare — use only for package migration
or gradual API refactoring.
Combine constraints with ~ (underlying type) and | (union):
Use the constraints package or cmp package (Go 1.21+) for standard constraints
like cmp.Ordered instead of writing your own.
Generics justify their complexity when they eliminate duplication across multiple call sites. A single-use generic is just indirection.
| Topic | Guidance |
|---|---|
| When to use generics | Only when multiple types share identical logic and interfaces don't suffice |
| Starting point | Write concrete code first; generalize later |
| Naming | Single uppercase letter (T, K, V, E) |
| Type aliases | Same type, alternate name; use only for migration |
| Constraint composition | Use ~ for underlying types, ` |
| Common pitfall | Don't genericize single-use code or when interfaces suffice |
func SumInts(vals []int) int {
var total int
for _, v := range vals {
total += v
}
return total
}func Marshal[Opts encoding.MarshalOptions](v any, opts Opts) ([]byte, error)type Numeric interface {
~int | ~int8 | ~int16 | ~int32 | ~int64 |
~float32 | ~float64
}
func Sum[T Numeric](vals []T) T {
var total T
for _, v := range vals {
total += v
}
return total
}// Bad: generic wrapper adds complexity without value
type Set[T comparable] struct {
m map[T]struct{}
}
// Better: use map[T]struct{} directly when the usage is simple
seen := map[string]struct{}{}// Bad: T is only used to satisfy an interface — just use the interface
func Process[T io.Reader](r T) error { ... }
// Good: accept the interface directly
func Process(r io.Reader) error { ... }// Bad: constraint is more restrictive than needed
func Contains[T interface{ ~int | ~string }](slice []T, target T) bool { ... }
// Good: comparable is sufficient
func Contains[T comparable](slice []T, target T) bool { ... }