WithValue

Learn to carry values inside a context with WithValue.

WithValue

context.WithValue(parent, key, value) carries request-scoped data (request ID, user, trace info). The key should be a private unexported type so it doesn't collide with other packages:

type anahtar int
const userKey anahtar = 0

ctx = context.WithValue(ctx, userKey, "ada")
ad, ok := ctx.Value(userKey).(string)

Type-safe helpers

A common pattern is to hide the key inside the package and export only helper functions:

func WithUser(ctx context.Context, ad string) context.Context {
    return context.WithValue(ctx, userKey, ad)
}

func User(ctx context.Context) (string, bool) {
    ad, ok := ctx.Value(userKey).(string)
    return ad, ok
}

Callers then don't deal with type assertions and the key can't be changed from outside.

Exercise Carry the user

Define type ctxKey struct{}. Write the WithUser and User helpers. Add the user "ada", read it back and print the value and ok.

Show example solution

package main

import (
    "context"
    "fmt"
)

type ctxKey struct{}

func WithUser(ctx context.Context, ad string) context.Context {
    return context.WithValue(ctx, ctxKey{}, ad)
}

func User(ctx context.Context) (string, bool) {
    ad, ok := ctx.Value(ctxKey{}).(string)
    return ad, ok
}

func main() {
    ctx := WithUser(context.Background(), "ada")
    fmt.Println(User(ctx))
}

Test yourself

Which is <strong>not</strong> suitable for WithValue?
- A request ID
- The authenticated user
- Trace information
- A function's required parameters (e.g. a database connection)
Show answer

A function's required parameters (e.g. a database connection) — Required dependencies should be explicit parameters; hiding them in the context makes code unclear and type-unsafe.

Read the value

id, ok := ctx.____(istekKey).(string)

Show answer

id, ok := ctx.Value(istekKey).(string)

Usage

Request ID

WithValue fits
Database connection

Explicit parameter
Private key type

Prevents collisions
Helper function

Type-safe access

Next lesson: WithDeadline

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