What is the best approach to handle this case: `lo...

# arrow

[mmaillot]

What is the best approach to handle this case:
loadURL().flatMap { url -> callWS(url) }.flatMap { saveResulte(url: ???, response: it) }

How can I get the url to save it in the last flatMap ? Should I use a pair in the second flatmap .flatMap { url -> callWS(url).map { Pair(it, url) } } ?

[Jannis]

Using fx blocks instead of flatMap chains: (in what monad are you? IO?

fx {
  val url = loadURL().bind()
  val resp = callWS(url).bind()
  saveResult(url, resp).bind()
}

or similar depending on your code ^^

[Jannis]

Making a pair also works, but makes weird code as you need to do all the plumbing

[mmaillot]

I use only Try at this moment

[mmaillot]

Currently, I use the monad Try to catch java exception (when I call java lib).

[Jannis]

You should probably use IO because Try will probably be removed soon (we deprecated it a while ago). Anyway this works the same for any monad, but not all have an ext method called fx so for Try you can do Try.monad().fx.monad {...} to get the same thing

[Scott Christopher]

One of the laws behind flatMap also allows you to safely change it to the following:

loadURL().flatMap { url -> callWS(url).flatMap { saveResulte(url, it) } }

This allows you to close over the url variable by calling flatMap against the result of callWS rather than the result of the first flatMap.

[Jannis]

that also works, but I find using fx blocks leads to more readable code. It's a style thing and I am so used to fx that I did not even think of your solution 😄
With arrow-meta those will also have the same runtime representation, but thats a few months out ^^

[Scott Christopher]

Yep, I'll take preference for the fx version too

[mmaillot]

I'll try the fx version, thanks for your help!