Context values in Go

A context can carry request-scoped values, keyed by an unexported type.

type contextKey struct{ name string }

var requestIDKey = contextKey{"requestID"}

func withRequestID(ctx context.Context, id string) context.Context {
    return context.WithValue(ctx, requestIDKey, id)
}

func requestID(ctx context.Context) (string, bool) {
    id, ok := ctx.Value(requestIDKey).(string)
    return id, ok
}

func demo() (string, bool) {
    ctx := withRequestID(context.Background(), "abc123")
    return requestID(ctx)
}

How it works

1. A private key type stops two packages colliding.
2. Values are for request metadata, not for passing arguments.
3. The lookup returns any, so it needs a type assertion.

Keywords and builtins used here

• bool
• func
• return
• string
• struct
• type
• var

The run, in numbers

Lines

17

Characters to type

421

Tokens

102

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 58 seconds.

Type this snippet

Step 3 of 3 in Context, step 13 of 25 in Concurrency.

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