Writing Generic Data Structures (Go 1.18+)

Type Constraints

Use the tightest constraint that satisfies your needs:

Constraint	What It Allows	Use For
`any`	All types	Containers that only store/retrieve
`comparable`	Types supporting `==` and `!=`	Map keys, set membership, dedup
`cmp.Ordered`	Numeric types + `string`	Sorting, min/max, binary search
Custom interface	Domain-specific operations	Specialized containers

Custom Constraints

// Union constraint — restrict to specific types
type Number interface {
    ~int | ~int64 | ~float64
}

// Method constraint — require specific behavior
type Stringer interface {
    comparable
    String() string
}

The ~ prefix includes all types whose underlying type matches (e.g., ~int matches type UserID int).

Generic Set Example

type Set[T comparable] map[T]struct{}

func NewSet[T comparable](vals ...T) Set[T] {
    s := make(Set[T], len(vals))
    for _, v := range vals {
        s[v] = struct{}{}
    }
    return s
}

func (s Set[T]) Add(v T)             { s[v] = struct{}{} }
func (s Set[T]) Remove(v T)          { delete(s, v) }
func (s Set[T]) Contains(v T) bool   { _, ok := s[v]; return ok }
func (s Set[T]) Len() int            { return len(s) }

func (s Set[T]) Union(other Set[T]) Set[T] {
    result := NewSet[T]()
    for v := range s {
        result.Add(v)
    }
    for v := range other {
        result.Add(v)
    }
    return result
}

Generic Sorted Slice

func InsertSorted[T cmp.Ordered](s []T, v T) []T {
    i, _ := slices.BinarySearch(s, v)
    return slices.Insert(s, i, v)
}

Constraint Composition

Combine multiple constraints with embedded interfaces:

type OrderedStringer interface {
    cmp.Ordered
    fmt.Stringer
}

When NOT to Use Generics

Single concrete type — generics add complexity for no benefit
any constraint with type switches — you're just reimplementing interface{} with extra syntax
Two or fewer instantiations — the abstraction overhead isn't justified
Complex type relationships — Go's type system doesn't support higher-kinded types; if the constraints become convoluted, use interfaces instead

Generics shine for data structures (containers, sets, trees), algorithms (sort, search, transform), and utility functions (min, max, clamp) where the logic is identical across types.

→ See samber/cc-skills-golang@golang-structs-interfaces skill for generics vs any guidance and interface design.

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