Generics Basics

Type Constraints

A constraint limits which types a generic function can accept.

constraint A constraint describes the operations that are valid for a type parameter. The trace uses a concrete integer helper and shows the constrained generic shape in a comment.

Type Constraints

count
type_constraints.go
Replay: real traced execution (multi-file project)
package main

import "fmt"

// Generic shape: Twice[T ~int | ~float64](value T) T
func TwiceInt(value int) int {
	return value + value
}

func main() {
	var count = 4
	doubled := TwiceInt(count)

	fmt.Println("count=", count)
	fmt.Println("doubled=", doubled)
	fmt.Println("type=", fmt.Sprintf("%T", doubled))
}

package main

import "fmt"

// Generic shape: Twice[T ~int | ~float64](value T) T
func TwiceInt(value int) int {
	return value + value
}

func main() {
	var count = 2
	doubled := TwiceInt(count)

	fmt.Println("count=", count)
	fmt.Println("doubled=", doubled)
	fmt.Println("type=", fmt.Sprintf("%T", doubled))
}

package main

import "fmt"

// Generic shape: Twice[T ~int | ~float64](value T) T
func TwiceInt(value int) int {
	return value + value
}

func main() {
	var count = 7
	doubled := TwiceInt(count)

	fmt.Println("count=", count)
	fmt.Println("doubled=", doubled)
	fmt.Println("type=", fmt.Sprintf("%T", doubled))
}

  1. count ← 4

    10func main() {11  var count→ 4 = 4 //@count=2, 712  doubled := TwiceInt(count4)

  2. func TwiceInt(value int) int

    5// Generic shape: Twice[T ~int | ~float64](value T) T6func TwiceInt(value4 int) int {7  return value4 + value8}

  3. doubled ← 8

    11  var count = 4 //@count=2, 712  doubled→ 8 := TwiceInt(count4)1314  fmt.Println("count=", count4)15  fmt.Println("doubled=", doubled8)16  fmt.Println("type=", fmt.Sprintf("%T", doubled8))17}
    outputcount= 4
doubled= 8
type= int

  1. count ← 2

    10func main() {11  var count→ 2 = 212  doubled := TwiceInt(count2)

  2. func TwiceInt(value int) int

    5// Generic shape: Twice[T ~int | ~float64](value T) T6func TwiceInt(value2 int) int {7  return value2 + value8}

  3. doubled ← 4

    11  var count = 212  doubled→ 4 := TwiceInt(count2)1314  fmt.Println("count=", count2)15  fmt.Println("doubled=", doubled4)16  fmt.Println("type=", fmt.Sprintf("%T", doubled4))17}
    outputcount= 2
doubled= 4
type= int

  1. count ← 7

    10func main() {11  var count→ 7 = 712  doubled := TwiceInt(count7)

  2. func TwiceInt(value int) int

    5// Generic shape: Twice[T ~int | ~float64](value T) T6func TwiceInt(value7 int) int {7  return value7 + value8}

  3. doubled ← 14

    11  var count = 712  doubled→ 14 := TwiceInt(count7)1314  fmt.Println("count=", count7)15  fmt.Println("doubled=", doubled14)16  fmt.Println("type=", fmt.Sprintf("%T", doubled14))17}
    outputcount= 7
doubled= 14
type= int