Challenge 3 — Solution

Task: Define a contextKey type and a requestIDKey constant. Use
context.WithValue to attach a request ID string to a context, then
write a function logRequest(ctx context.Context) that reads it back
with ctx.Value and prints it.

package main

import (
	"context"
	"fmt"
)

type contextKey string

const requestIDKey contextKey = "requestID"

func logRequest(ctx context.Context) {
	requestID := ctx.Value(requestIDKey)
	fmt.Println("Handling request:", requestID)
}

func main() {
	ctx := context.WithValue(context.Background(), requestIDKey, "req-12345")

	logRequest(ctx)
}


Expected output:

Handling request: req-12345

Notes:
- contextKey is a small custom string type, used instead of a plain
  string key, specifically to avoid collisions with other packages
  that might use the same literal string as a different key —
  defining a distinct type for context keys is the standard, safe
  convention.
- ctx.Value(requestIDKey) returns a value of type any, since
  WithValue/Value work for any kind of stored data — here it happens
  to print correctly as a string without any extra conversion,
  since Println accepts any value directly.
- If requestIDKey were never set on ctx, ctx.Value would simply
  return nil rather than an error — there's no built-in equivalent
  of the comma-ok idiom (Fundamentals Chapter 7) for context values.