Griesemer Precise Go

Robert Griesemer Style Guide

Overview

Robert Griesemer co-created Go and was the primary designer of Go's generics. He previously worked on the V8 JavaScript engine and the Java HotSpot VM. His focus: precise semantics, clean syntax, and type system clarity.

Core Philosophy

"The language should help you think clearly."

"Every feature adds complexity. Is the complexity worth it?"

Griesemer values precision and clarity. Go's spec is remarkably small and clear because every construct has well-defined semantics.

Design Principles

1. Precise Semantics: Every language construct has exactly one meaning.

2. Orthogonal Features: Features should combine predictably.

3. No Surprises: Behavior should be obvious from reading the code.

4. Spec-Driven: If it's not in the spec, it's not guaranteed.

When Writing Code

Always

• Understand the exact semantics of operations
• Use types to express constraints
• Make nil behavior explicit and safe
• Write code that matches Go's spec, not implementation details
• Use generics when type safety improves clarity
• Define clear type constraints

Never

• Rely on unspecified behavior
• Assume implementation details (memory layout, etc.)
• Create ambiguous APIs
• Use empty interface when a constraint works
• Ignore the distinction between value and pointer receivers

Prefer

• Explicit type constraints over any
• Named types for domain concepts
• Method receivers that match semantics (value vs pointer)
• Clear zero values

Code Patterns

Generics Done Right (Go 1.18+)

// BAD: Empty interface loses type safety
func Max(a, b interface{}) interface{} {
    // runtime type assertion needed
    switch v := a.(type) {
    case int:
        if v > b.(int) { return v }
        return b
    // ... repeat for every type
    }
    panic("unsupported type")
}

// GOOD: Type constraints preserve safety
type Ordered interface {
    ~int | ~int8 | ~int16 | ~int32 | ~int64 |
    ~uint | ~uint8 | ~uint16 | ~uint32 | ~uint64 |
    ~float32 | ~float64 | ~string
}

func Max[T Ordered](a, b T) T {
    if a > b {
        return a
    }
    return b
}

// Usage: type-safe, no assertions
result := Max(3, 5)        // int
result := Max(3.14, 2.71)  // float64

Precise Type Constraints

// Constraint: any type with these methods
type Stringer interface {
    String() string
}

// Constraint: underlying type is int
type Integer interface {
    ~int | ~int64
}

// Constraint: must be pointer to struct with Name field
type Named[T any] interface {
    *T
    GetName() string
}

// Combining constraints
type OrderedStringer interface {
    Ordered
    Stringer
}

// Generic data structure with constraint
type Set[T comparable] struct {
    items map[T]struct{}
}

func (s *Set[T]) Add(item T) {
    if s.items == nil {
        s.items = make(map[T]struct{})
    }
    s.items[item] = struct{}{}
}

func (s *Set[T]) Contains(item T) bool {
    _, ok := s.items[item]
    return ok
}

Value vs Pointer Semantics

// Value receiver: method doesn't modify, type is small
type Point struct {
    X, Y float64
}

func (p Point) Distance(q Point) float64 {
    dx := p.X - q.X
    dy := p.Y - q.Y
    return math.Sqrt(dx*dx + dy*dy)
}

// Pointer receiver: method modifies, or type is large
func (p *Point) Scale(factor float64) {
    p.X *= factor
    p.Y *= factor
}

// IMPORTANT: Be consistent within a type
// If ANY method needs pointer receiver, use pointer for ALL
type Buffer struct {
    data []byte
}

func (b *Buffer) Write(p []byte) (int, error) {
    b.data = append(b.data, p...)
    return len(p), nil
}

func (b *Buffer) String() string {  // Also pointer, for consistency
    return string(b.data)
}

Safe Nil Handling

// Make nil receiver safe
type Stack[T any] struct {
    items []T
}

func (s *Stack[T]) Push(item T) {
    if s == nil {
        panic("nil stack")  // Or return error
    }
    s.items = append(s.items, item)
}

func (s *Stack[T]) Len() int {
    if s == nil {
        return 0  // Safe: nil stack has zero length
    }
    return len(s.items)
}

// Named types for clarity
type UserID int64
type OrderID int64

// Now these can't be accidentally swapped
func GetOrder(uid UserID, oid OrderID) (*Order, error) {
    // ...
}

Precise Interface Design

// Small, precise interfaces
type Reader interface {
    Read(p []byte) (n int, err error)
}

type Writer interface {
    Write(p []byte) (n int, err error)
}

type Closer interface {
    Close() error
}

// Compose precisely
type ReadCloser interface {
    Reader
    Closer
}

type WriteCloser interface {
    Writer
    Closer
}

type ReadWriteCloser interface {
    Reader
    Writer
    Closer
}

// Generic interface with type parameter
type Container[T any] interface {
    Add(T)
    Remove(T) bool
    Contains(T) bool
    Len() int
}

Mental Model

Griesemer designs by asking:

1. What does the spec say? Not the implementation—the specification.
2. Is this unambiguous? Can two people read this differently?
3. What are the edge cases? nil, zero values, overflow?
4. Is the type constraint minimal? Don't over-constrain.

Spec-Level Thinking

Write code that works regardless of implementation details.