Writing Flows

When you set out to write the logic for your app, what you are doing is actually laying out how the user will move through various steps towards attaining a goal. This going through steps towards a goal is what we call a flow. To write flows in Presto is very simple. Abstractions like 'Remote' and 'UI' will help you organize your code better and makes code more readable.

This app has a primary goal of Paying a Bill.

App Goal

billPayFlow :: Flow BillPayFailure StatusScreenAction
billPayFlow = do
  _            <- UI.splashScreen
  operators    <- Remote.fetchOperators
  operator     <- UI.chooseOperator operators
  mobileNumber <- UI.askMobileNumber
  amount       <- UI.askAmount
  result       <- Remote.payBill mobileNumber amount operator
  UI.billPayStatus mobileNumber amount result

UI Flows

Choose Operator

chooseOperator :: Array Operator -> Flow BillPayFailure Operator
chooseOperator operators = do
  action <- runUI' (ChooseOperatorScreen operators)
  case action of 
    OperatorSelected operator-> pure operator
    ChooseOperatorScreenAbort -> liftLeft UserAbort

Ask MobileNumber

askMobileNumber :: Flow BillPayFailure MobileNumber
askMobileNumber = do
  action <- runUI' AskMobileNumberScreen
  case action of
    SubmitMobileNumber mobileNumber -> pure mobileNumber
    AskMobileNumberScreenAbort -> liftLeft UserAbort

Ask Amount

askAmount :: Flow BillPayFailure Amount
askAmount = do
  action <- runUI' AskAmountScreen
  case action of
    SubmitAmount amount -> pure amount
    AskAmountScreenAbort -> liftLeft UserAbort

Show Status

billPayStatus :: MobileNumber -> Amount -> BillPayStatus -> Flow BillPayFailure StatusScreenAction
billPayStatus mobileNumber amount payStatus = do
  action <- runUI' (StatusScreen mobileNumber amount payStatus)
  case action of
    SuccessResult -> pure SuccessResult
    StatusScreenAbort -> liftLeft UserAbort

SplashScreen

billPayStatus :: MobileNumber -> Amount -> BillPayStatus -> Flow BillPayFailure StatusScreenAction
billPayStatus mobileNumber amount payStatus = do
  action <- runUI' (StatusScreen mobileNumber amount payStatus)
  case action of
    SuccessResult -> pure SuccessResult
    StatusScreenAbort -> liftLeft UserAbort

Api Flows

Fetch Operators Api

fetchOperators :: Flow BillPayFailure (Array Operator)
fetchOperators = do
    FetchOperatorsResponse { operators } <- callApi FetchOperatorsRequest
    pure operators

Bill Pay Api

payBill :: Operator -> MobileNumber -> Amount -> Flow BillPayFailure BillPayStatus
payBill operator mobileNumber amount = 
    BillPayResponse { status } <- callApi $ BillPayRequest { operator : operator, mobileNumber : mobileNumber, amount : amount }
    pure status