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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 skill4agent add cxuu/golang-skills go-genericsany"Write code, don't design types." — Robert Griesemer and Ian Lance Taylor
Do multiple types share identical logic?
├─ No → Use concrete types
├─ Yes → Do they share a useful interface?
│ ├─ Yes → Use an interface
│ └─ No → Use generics// Premature generics: only ever called with int
func Sum[T constraints.Integer | constraints.Float](vals []T) T {
var total T
for _, v := range vals {
total += v
}
return total
}func SumInts(vals []int) int {
var total int
for _, v := range vals {
total += v
}
return total
}| Name | Typical Use |
|---|---|
| General type parameter |
| Map key type |
| Map value type |
| Element/item type |
func Marshal[Opts encoding.MarshalOptions](v any, opts Opts) ([]byte, error)type Old = new.Name~|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
}constraintscmpcmp.OrderedRead references/CONSTRAINTS.md when writing custom type constraints, composing constraints with ~ and |, or debugging type inference issues.
// 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 { ... }| 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 ( |
| Type aliases | Same type, alternate name; use only for migration |
| Constraint composition | Use |
| Common pitfall | Don't genericize single-use code or when interfaces suffice |