Generics Basics

Map Key Helper

The comparable constraint allows a generic helper to use map keys.

comparable Map keys must be comparable, so a generic map helper can require `K comparable`. The trace uses a concrete string-keyed map and shows the generic shape in a comment.

Map Key Helper

key
map_key_helper.go
Replay: real traced execution (multi-file project)
package main

import "fmt"

// Generic shape: Lookup[K comparable, V any](items map[K]V, key K) (V, bool)
func LookupScore(items map[string]int, key string) (int, bool) {
	value, ok := items[key]
	return value, ok
}

func main() {
	var key = "beta"
	scores := map[string]int{"alpha": 10, "beta": 20}
	value, ok := LookupScore(scores, key)

	fmt.Println("key=", key)
	fmt.Println("found=", ok)
	fmt.Println("value=", value)
}

package main

import "fmt"

// Generic shape: Lookup[K comparable, V any](items map[K]V, key K) (V, bool)
func LookupScore(items map[string]int, key string) (int, bool) {
	value, ok := items[key]
	return value, ok
}

func main() {
	var key = "alpha"
	scores := map[string]int{"alpha": 10, "beta": 20}
	value, ok := LookupScore(scores, key)

	fmt.Println("key=", key)
	fmt.Println("found=", ok)
	fmt.Println("value=", value)
}

package main

import "fmt"

// Generic shape: Lookup[K comparable, V any](items map[K]V, key K) (V, bool)
func LookupScore(items map[string]int, key string) (int, bool) {
	value, ok := items[key]
	return value, ok
}

func main() {
	var key = "delta"
	scores := map[string]int{"alpha": 10, "beta": 20}
	value, ok := LookupScore(scores, key)

	fmt.Println("key=", key)
	fmt.Println("found=", ok)
	fmt.Println("value=", value)
}

  1. key ← "beta", scores ← map[string]int{"alpha":10, "beta":20}

    11func main() {12  var key→ "beta" = "beta" //@key="alpha", "delta"13  scores→ map[string]int{"alpha":10, "beta":20} := map[string]int{"alpha": 10, "beta": 20}14  value, ok := LookupScore(scoresmap[string]int{"alpha":10, "beta":20}, key"beta")

  2. value ← 20, ok ← true

    5// Generic shape: Lookup[K comparable, V any](items map[K]V, key K) (V, bool)6func LookupScore(itemsmap[string]int{"alpha":10, "beta":20} map[string]int, key"beta" string) (int, bool) {7  value→ 20, ok→ true := items[key]208  return value20, oktrue9}

  3. value ← 20, ok ← true

    13  scores := map[string]int{"alpha": 10, "beta": 20}14  value→ 20, ok→ true := LookupScore(scoresmap[string]int{"alpha":10, "beta":20}, key"beta")1516  fmt.Println("key=", key"beta")17  fmt.Println("found=", oktrue)18  fmt.Println("value=", value20)19}
    outputkey= beta
found= true
value= 20

  1. key ← "alpha", scores ← map[string]int{"alpha":10, "beta":20}

    11func main() {12  var key→ "alpha" = "alpha"13  scores→ map[string]int{"alpha":10, "beta":20} := map[string]int{"alpha": 10, "beta": 20}14  value, ok := LookupScore(scoresmap[string]int{"alpha":10, "beta":20}, key"alpha")

  2. value ← 10, ok ← true

    5// Generic shape: Lookup[K comparable, V any](items map[K]V, key K) (V, bool)6func LookupScore(itemsmap[string]int{"alpha":10, "beta":20} map[string]int, key"alpha" string) (int, bool) {7  value→ 10, ok→ true := items[key]108  return value10, oktrue9}

  3. value ← 10, ok ← true

    13  scores := map[string]int{"alpha": 10, "beta": 20}14  value→ 10, ok→ true := LookupScore(scoresmap[string]int{"alpha":10, "beta":20}, key"alpha")1516  fmt.Println("key=", key"alpha")17  fmt.Println("found=", oktrue)18  fmt.Println("value=", value10)19}
    outputkey= alpha
found= true
value= 10

  1. key ← "delta", scores ← map[string]int{"alpha":10, "beta":20}

    11func main() {12  var key→ "delta" = "delta"13  scores→ map[string]int{"alpha":10, "beta":20} := map[string]int{"alpha": 10, "beta": 20}14  value, ok := LookupScore(scoresmap[string]int{"alpha":10, "beta":20}, key"delta")

  2. value ← 0, ok ← false

    5// Generic shape: Lookup[K comparable, V any](items map[K]V, key K) (V, bool)6func LookupScore(itemsmap[string]int{"alpha":10, "beta":20} map[string]int, key"delta" string) (int, bool) {7  value→ 0, ok→ false := items[key]08  return value0, okfalse9}

  3. value ← 0, ok ← false

    13  scores := map[string]int{"alpha": 10, "beta": 20}14  value→ 0, ok→ false := LookupScore(scoresmap[string]int{"alpha":10, "beta":20}, key"delta")1516  fmt.Println("key=", key"delta")17  fmt.Println("found=", okfalse)18  fmt.Println("value=", value0)19}
    outputkey= delta
found= false
value= 0