go-kit middlewares / decorators ... confused :S

[Alex Leonhardt]

Hi,

I'm new-ish to go-kit (and to Go for that matter) and actually not even a proper developer, but I've been learning Go and started reading through the stringsvc example here: https://gokit.io/examples/stringsvc.html ... 

I'm a bit confused by this here:

func loggingMiddleware(logger log.Logger) Middleware {
	return func(next endpoint.Endpoint) endpoint.Endpoint {
		return func(ctx context.Context, request interface{}) (interface{}, error) {
			logger.Log("msg", "calling endpoint")
			defer logger.Log("msg", "called endpoint")
			return next(ctx, request)
		}
	}
}

and then the use of

count = loggingMiddleware(log.With(logger, "method", "count"))(count)

I do understand the calling of loggingMiddleware (as such) but what confused me is what the additional 

(count)

at the end of the function call is doing ?

It's probably much more a generic-ish Go / programming question than specific to go-kit, but it's used here and I'd like to understand it :) 

Thanks!

Alex

[Alex Leonhardt]

I think I got it ... 

it's because 

loggingMiddleware(log.With(logger, "method", "count"))

returns a function which requires a endpoint.Endpoint as argument ... 

which is 

(count)

from

count = makeCountEndpoint(svc)




I hope I got that right ? It's still really a bit confusing... In my head, this is what's happening...  

var count endpoint.Endpoint
count = makeCountEndpoint(svc) // creates endpoint.Endpoint for stringService{}
loggingCount = loggingMiddleware(log.With(logger, "method", "count")) // returns func(next endpoint.Endpoint) endpoint.Endpoint
count = loggingCount(count) // returns the "count" endpoint passed in as "next" wrapped into loggingMiddleware

Cheers,

Alex

[Gulácsi Tamás]

Yes, it is.

go-kit uses such constructs (function returning a function which encloses over some specific object) a lot. AFAIK this is called "closure" as it "closes over" the object.

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[Alex Leonhardt]

Cool thanks, I do (mostly) understand closures but I haven't used it a lot so it's taking some time to get my head "wrapped" around it ;) (pun intended) hehe :D

Cheers!