ex3

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Exercise 3

Question

1. Assume you have a simple logging function that looks like this:

func Log(ctx context.Context, level Level, message string) {
    var inLevel Level
    // TODO get a logging level out of the context and assign it to inLevel
	if level == Debug && inLevel == Debug {
        fmt.Println(message)
    }
    if level == Info && (inLevel == Debug || inLevel == Info) {
        fmt.Println(message)
    }
}

Define a type called Level whose underlying type is string. Define two constants of this type, Debug and Info, set to "debug" and "info", respectively.

Create functions to store the log level in the context and to extract it.

Create a middleware function to get the logging level from a query parameter called log_level. The valid values for log_level are debug and info.

Finally, fill in the TODO in Log to properly extract the log level from the context. If the log level is not assigned or is not a valid value, nothing should be printed.

Solution

We start by defining Level and its constants:

type Level string

const (
	Debug Level = "debug"
	Info  Level = "info"
)

Next, we need to make our log level context management functions. We first define an unexported type for the key's type, and an unexported constant for the key:

type logKey int

const (
	_ logKey = iota
	key
)

After that, we use the two types we've defined to write our context value management functions:

func ContextWithLevel(ctx context.Context, level Level) context.Context {
	return context.WithValue(ctx, key, level)
}

func LevelFromContext(ctx context.Context) (Level, bool) {
	level, ok := ctx.Value(key).(Level)
	return level, ok
}

Now we can use the ContextWithLevel function and the Level type to write our middleware:

func Middleware(h http.Handler) http.Handler {
	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		level := r.URL.Query().Get("log_level")
		ctx := ContextWithLevel(r.Context(), Level(level))
		r = r.WithContext(ctx)
		h.ServeHTTP(w, r)
	})
}

Finally, we can fill in the missing lines in the Log function using LevelFromContext:

	inLevel, ok := LevelFromContext(ctx)
	if !ok {
		return
	}