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course/15-generics/exercises/3-constraints/code.go
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111
course/15-generics/exercises/3-constraints/code.go
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package main
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import (
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"fmt"
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"time"
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)
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func chargeForLineItem[T lineItem](newItem T, oldItems []T, balance float64) ([]T, float64, error) {
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// ?
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}
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// don't edit below this line
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type lineItem interface {
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GetCost() float64
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GetName() string
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}
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type subscription struct {
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userEmail string
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startDate time.Time
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interval string
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}
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func (s subscription) GetName() string {
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return fmt.Sprintf("%s subscription", s.interval)
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}
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func (s subscription) GetCost() float64 {
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if s.interval == "monthly" {
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return 25.00
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}
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if s.interval == "yearly" {
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return 250.00
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}
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return 0.0
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}
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type oneTimeUsagePlan struct {
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userEmail string
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numEmailsAllowed int
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}
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func (otup oneTimeUsagePlan) GetName() string {
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return fmt.Sprintf("one time usage plan with %v emails", otup.numEmailsAllowed)
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}
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func (otup oneTimeUsagePlan) GetCost() float64 {
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const costPerEmail = 0.03
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return float64(otup.numEmailsAllowed) * costPerEmail
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}
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func main() {
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test(subscription{
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userEmail: "john@example.com",
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startDate: time.Now().UTC(),
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interval: "yearly",
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},
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[]subscription{},
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1000.00,
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)
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test(subscription{
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userEmail: "jane@example.com",
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startDate: time.Now().UTC(),
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interval: "monthly",
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},
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[]subscription{
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{
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userEmail: "jane@example.com",
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startDate: time.Now().UTC().Add(-time.Hour * 24 * 7),
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interval: "monthly",
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},
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{
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userEmail: "jane@example.com",
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startDate: time.Now().UTC().Add(-time.Hour * 24 * 7 * 52 * 2),
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interval: "yearly",
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},
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},
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686.20,
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)
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test(oneTimeUsagePlan{
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userEmail: "dillon@example.com",
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numEmailsAllowed: 5000,
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},
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[]oneTimeUsagePlan{},
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756.20,
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)
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test(oneTimeUsagePlan{
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userEmail: "dalton@example.com",
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numEmailsAllowed: 100000,
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},
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[]oneTimeUsagePlan{
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{
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userEmail: "dalton@example.com",
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numEmailsAllowed: 34200,
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},
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},
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32.20,
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)
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}
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func test[T lineItem](newItem T, oldItems []T, balance float64) {
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fmt.Println(" --- ")
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fmt.Printf("Charging customer for a '%s', current balance is %v...\n", newItem.GetName(), balance)
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newItems, newBalance, err := chargeForLineItem(newItem, oldItems, balance)
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if err != nil {
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fmt.Printf("Got error: %v\n", err)
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return
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}
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fmt.Printf("New balance is: %v. Total number of line items is now %v\n", newBalance, len(newItems))
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}
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117
course/15-generics/exercises/3-constraints/complete.go
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117
course/15-generics/exercises/3-constraints/complete.go
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package main
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import (
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"errors"
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"fmt"
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"time"
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)
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func chargeForLineItem[T lineItem](newItem T, oldItems []T, balance float64) ([]T, float64, error) {
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newBalance := balance - newItem.GetCost()
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if newBalance < 0 {
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return nil, 0, errors.New("insufficient funds")
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}
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oldItems = append(oldItems, newItem)
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return oldItems, newBalance, nil
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}
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// don't edit below this line
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type lineItem interface {
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GetCost() float64
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GetName() string
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}
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type subscription struct {
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userEmail string
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startDate time.Time
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interval string
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}
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func (s subscription) GetName() string {
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return fmt.Sprintf("%s subscription", s.interval)
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}
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func (s subscription) GetCost() float64 {
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if s.interval == "monthly" {
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return 25.00
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}
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if s.interval == "yearly" {
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return 250.00
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}
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return 0.0
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}
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type oneTimeUsagePlan struct {
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userEmail string
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numEmailsAllowed int
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}
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func (otup oneTimeUsagePlan) GetName() string {
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return fmt.Sprintf("one time usage plan with %v emails", otup.numEmailsAllowed)
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}
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func (otup oneTimeUsagePlan) GetCost() float64 {
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const costPerEmail = 0.03
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return float64(otup.numEmailsAllowed) * costPerEmail
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}
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func main() {
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test(subscription{
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userEmail: "john@example.com",
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startDate: time.Now().UTC(),
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interval: "yearly",
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},
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[]subscription{},
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1000.00,
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)
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test(subscription{
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userEmail: "jane@example.com",
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startDate: time.Now().UTC(),
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interval: "monthly",
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},
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[]subscription{
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{
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userEmail: "jane@example.com",
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startDate: time.Now().UTC().Add(-time.Hour * 24 * 7),
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interval: "monthly",
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},
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{
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userEmail: "jane@example.com",
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startDate: time.Now().UTC().Add(-time.Hour * 24 * 7 * 52 * 2),
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interval: "yearly",
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},
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},
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686.20,
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)
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test(oneTimeUsagePlan{
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userEmail: "dillon@example.com",
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numEmailsAllowed: 5000,
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},
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[]oneTimeUsagePlan{},
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756.20,
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)
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test(oneTimeUsagePlan{
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userEmail: "dalton@example.com",
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numEmailsAllowed: 100000,
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},
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[]oneTimeUsagePlan{
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{
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userEmail: "dalton@example.com",
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numEmailsAllowed: 34200,
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},
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},
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32.20,
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)
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}
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func test[T lineItem](newItem T, oldItems []T, balance float64) {
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fmt.Println(" --- ")
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fmt.Printf("Charging customer for a '%s', current balance is %v...\n", newItem.GetName(), balance)
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newItems, newBalance, err := chargeForLineItem(newItem, oldItems, balance)
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if err != nil {
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fmt.Printf("Got error: %v\n", err)
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return
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}
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fmt.Printf("New balance is: %v. Total number of line items is now %v\n", newBalance, len(newItems))
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}
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12
course/15-generics/exercises/3-constraints/expected.txt
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12
course/15-generics/exercises/3-constraints/expected.txt
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---
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Charging customer for a 'yearly subscription', current balance is 1000...
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New balance is: 750. Total number of line items is now 1
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---
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Charging customer for a 'monthly subscription', current balance is 686.2...
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New balance is: 661.2. Total number of line items is now 3
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---
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Charging customer for a 'one time usage plan with 5000 emails', current balance is 756.2...
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New balance is: 606.2. Total number of line items is now 1
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---
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Charging customer for a 'one time usage plan with 100000 emails', current balance is 32.2...
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Got error: insufficient funds
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44
course/15-generics/exercises/3-constraints/readme.md
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44
course/15-generics/exercises/3-constraints/readme.md
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# Constraints
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Sometimes you need the logic in your generic function to know *something* about the types it operates on. The example we used in the first exercise didn't need to know *anything* about the types in the slice, so we used the built-in `any` constraint:
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```go
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func splitAnySlice[T any](s []T) ([]T, []T) {
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mid := len(s)/2
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return s[:mid], s[mid:]
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}
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```
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Constraints are just interfaces that allow us to write generics that only operate within the constraint of a given interface type. In the example above, the `any` constraint is the same as the empty interface because it means the type in question can be *anything*.
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## Creating a custom constraint
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Let's take a look at the example of a `concat` function. It takes a slice of values and concatenates the values into a string. This should work with *any type that can represent itself as a string*, even if it's not a string under the hood. For example, a `user` struct can have a `.String()` that returns a string with the user's name and age.
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```go
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type stringer interface {
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String() string
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}
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func concat[T stringer](vals []T) string {
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result := ""
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for _, val := range vals {
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// this is where the .String() method
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// is used. That's why we need a more specific
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// constraint instead of the any constraint
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result += val.String()
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}
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return result
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}
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```
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## Assignment
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We have different kinds of "line items" that we charge our customer's credit cards for. Line items can be things like "subscriptions" or "one-time payments" for email usage.
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Complete the `chargeForLineItem` function. First, it should check if the user has a balance with enough funds to be able to pay for the cost of the `newItem`. If they don't then return an "insufficient funds" error.
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If they *do* have enough funds:
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* Add the line item to the user's history by appending the `newItem` to the slice of `oldItems`. This new slice is your first return value.
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* Calculate the user's new balance by subtracting the cost of the new item from their balance. This is your second return value.
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