How to perform multiple Http requests (Tasks) in bulk in Elm lang

Question

I want to load user profile before rendering something into the page but the whole user profile is composed of different parts that are loaded by multiple HTTP requests.

So far I'm loading user profile in sequence (one by one)

type alias CompanyInfo = 
  { name: String
  , address: ...
  , phone: String
  , ...
  }

type alias UserProfile = 
  { userName: String
  , companyInfo: CompanyInfo
  , ... 
  }

Cmd.batch
  [ loadUserName userId LoadUserNameFail LoadUserNameSuccess
  , loadCompanyInfo userId LoadCompanyInfoFail LoadCompanyInfoSuccess
  ...
  ]

But that's not very effective. Is there a simple way how to perform a bunch of Http requests and return just one complete value?

Something like this

init = 
    (initialModel, loadUserProfile userId LoadUserProfileFail LoadUserProfileSuccess)

....

Answer 1

You can achieve this using Task.map2:

Edit: Updated to Elm 0.18

Task.attempt LoadUserProfile <|
    Task.map2 (\userName companyInfo -> { userName = userName, companyInfo = companyInfo })
        (Http.get userNameGetUrl userDecoder |> Http.toTask)
        (Http.get companyInfoGetUrl companyInfoDecoder |> Http.toTask)

You can then get rid of the individual LoadUserName... and LoadCompanyInfo... Msgs. In Elm 0.18, the need for separate Fail and Succeed Msgs is addressed by Task.attempt expecting a Result Error Msg type, so that LoadUserProfile is defined like this:

type Msg
    = ...
    | LoadUserProfile (Result Http.Error UserProfile)

map2 will only succeed once both tasks succeed. It will fail if any of the tasks fail.