go-generics

Go Generics and Type Parameters

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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:

// 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:

func SumInts(vals []int) int {
    var total int
    for _, v := range vals {
        total += v
    }
    return total
}

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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:

func Marshal[Opts encoding.MarshalOptions](v any, opts Opts) ([]byte, error)

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Type Aliases vs Type Definitions

Type aliases (type Old = new.Name) are rare — use only for package migration or gradual API refactoring.

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Constraint Composition

Combine constraints with ~ (underlying type) and | (union):

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.

Read references/CONSTRAINTS.md when writing custom type constraints, composing constraints with ~ and |, or debugging type inference issues.

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Common Pitfalls

Don't Wrap Standard Library Types

// 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

// 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

// 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 { ... }

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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, `
Common pitfall	Don't genericize single-use code or when interfaces suffice

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Related Skills

• Interfaces vs generics: See go-interfaces when deciding whether an interface already models the shared behavior without generics
• Type declarations: See go-declarations when defining new types, type aliases, or choosing between type definitions and aliases
• Documenting generic APIs: See go-documentation when writing doc comments and runnable examples for generic functions
• Naming type parameters: See go-naming when choosing names for type parameters or constraint interfaces