Summary - Interface and error

Combine interface and error to write a validator.

Interfaces and errors summary

An interface = a list of methods; types satisfy it implicitly. Small interfaces (io.Reader, fmt.Stringer) are powerful abstractions. error is an interface too: Error() string. Errors are created with errors.New and fmt.Errorf("...: %w", err), returned as the last value, and inspected with errors.Is (value) and errors.As (type).

Type switch

To branch on the real type of an interface value, use switch v := x.(type):

func tanimla(x any) string {
    switch v := x.(type) {
    case int:
        return fmt.Sprintf("tam sayı %d", v)
    case string:
        return "metin " + v
    case error:
        return "hata: " + v.Error()
    default:
        return "bilinmeyen"
    }
}

Exercise Describe it

Write the tanimla(x any) string function above with a type switch. Print the results for 42, "go", errors.New("x") and 3.5 on separate lines.

Show example solution

package main

import (
    "errors"
    "fmt"
)

func tanimla(x any) string {
    switch v := x.(type) {
    case int:
        return fmt.Sprintf("tam sayı %d", v)
    case string:
        return "metin " + v
    case error:
        return "hata: " + v.Error()
    default:
        return "bilinmeyen"
    }
}

func main() {
    for _, x := range []any{42, "go", errors.New("x"), 3.5} {
        fmt.Println(tanimla(x))
    }
}

Test yourself

What can the empty interface <code>any</code> (<code>interface{}</code>) hold?
- Only structs
- Only nil
- Only numbers
- A value of any type
Show answer

A value of any type — Every type satisfies an interface with no methods; using the value requires a type assertion or a type switch.

Type switch

switch v := x.(____) {
case int:
    fmt.Println("int", v)
}

Show answer

switch v := x.(type) {
case int:
    fmt.Println("int", v)
}

Group summary

interface

Behavior contract
any

Any type
x.(type)

Branching by type
errors.Is / As

Error chain

Next lesson: Challenge 1 - Combined program

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