---
name: go-generics
description: 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).
---
# Go Generics and Type Parameters
> Compatibility: Generics require Go 1.18+.
## Resource Routing
- `references/CONSTRAINTS.md` - Read when composing constraints, using type sets, or choosing between generics and interfaces.
## When to Use Generics
Start with concrete types. Generalize only when a second type appears.
### Prefer Generics When
- Multiple types share identical logic (sorting, filtering, map/reduce)
- You would otherwise rely on `any` and excessive type switching
- You are building a reusable data structure (concurrent-safe set, ordered map)
### Avoid Generics When
- Only one type is being instantiated in practice
- Interfaces already model the shared behavior cleanly
- The generic code is harder to read than the type-specific alternative
> "Write code, don't design types." — Robert Griesemer and Ian Lance Taylor
### Decision Flow
```
Do multiple types share identical logic?
├─ No → Use concrete types
├─ Yes → Do they share a useful interface?
│ ├─ Yes → Use an interface
│ └─ No → Use generics
```
**Bad:**
```go
// 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
}
```
**Good:**
```go
func SumInts(vals []int) int {
var total int
for _, v := range vals {
total += v
}
return total
}
```
---
## Type Parameter Naming
| 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:
```go
func Marshal[Opts encoding.MarshalOptions](v any, opts Opts) ([]byte, error)
```
---
## Type Aliases vs Type Definitions
Type aliases (`type Old = new.Name`) are rare — use only for package migration
or gradual API refactoring.
---
## Constraint Composition
Combine constraints with `~` (underlying type) and `|` (union):
```go
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
}
```
Use the `constraints` package or `cmp` package (Go 1.21+) for standard constraints
like `cmp.Ordered` instead of writing your own.
---
## Common Pitfalls
### Don't Wrap Standard Library Types
```go
// 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{}{}
```
Generics justify their complexity when they eliminate duplication across
**multiple call sites**. A single-use generic is just indirection.
### Don't Use Generics for Interface Satisfaction
```go
// 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 { ... }
```
### Avoid Over-Constraining
```go
// 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 { ... }
```
---
## Quick Reference
| 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, `|` for unions; prefer `cmp.Ordered` over custom |
| Common pitfall | Don't genericize single-use code or when interfaces suffice |
---
## Related Skills
- **Interfaces vs generics**: See [go-interfaces](../go-interfaces/SKILL.md) when deciding whether an interface already models the shared behavior without generics
- **Type declarations**: See [go-declarations](../go-declarations/SKILL.md) when defining new types, type aliases, or choosing between type definitions and aliases
- **Documenting generic APIs**: See [go-documentation](../go-documentation/SKILL.md) when writing doc comments and runnable examples for generic functions
- **Naming type parameters**: See [go-naming](../go-naming/SKILL.md) when choosing names for type parameters or constraint interfaces