Two sequential HTTP requests - frp

Question

Sorry for newbie's question (and for my english) :)
I tries to write the following function:
the function downloads a content from URL1 (it's received as argument)
the function parses this content and extract URL2
the function downloads a content from URL2
the content from URL2 is a result of this function
if an error was occured, this function should return Nothing
I know how to execute the HTTP requests. I have a function to parse the request from URL1. But I don't know how:
to execute new request with extracted URL2
to ignore second request if URL2 isn't extracted (or error in URL1 is occured)

Answer 1

I principle you want something like this:
import Maybe
import Http
type Url = String
getContentFromUrl : Maybe Url -> Maybe String
getContentFromUrl url = --your implementation
extractUrlFromContent : Maybe String -> Maybe Url
extractUrlFromContent content = --your implementation
content = getContentFromUrl (Just "http://example.com")
|> extractUrlFromContent
|> getContentFromUrl

Answer 2

Sending an Http means talking to the outside world, which involves Signals in Elm. So the final result from URL2 will come packed in a Signal. As long as you're ok with that, you can use maybe to return the content of in a Maybe in a Signal. For example:
import Maybe
import Http
-- helper functions
isSuccess : Http.Response a -> Bool
isSuccess response = case response of
Http.Success _ -> True
_ -> False
responseToMaybe : Http.Response a -> Maybe.Maybe a
responseToMaybe response = case response of
Http.Success a -> Just a
_ -> Nothing
parseContentAndExtractUrl : String -> String
parseContentAndExtractUrl = identity -- this still requires your implementation
-- URL1
startUrl : String
startUrl = "www.example.com" -- replace this with your URL1
result1 : Signal (Http.Response String)
result1 = Http.sendGet <| constant startUrl
-- URL2
secondUrl : Signal String
secondUrl = result1
|> keepIf isSuccess (Http.Success "")
|> lift (\(Http.Success s) -> s)
|> lift parseContentAndExtractUrl
result2 : Signal (Maybe String)
result2 = secondUrl
|> Http.sendGet
|> lift responseToMaybe
Note that there are plans to make all of this easier to work with: https://groups.google.com/d/topic/elm-discuss/BI0D2b-9Fig/discussion