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@chshersh
Last active Jul 5, 2022
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CPS transformed code
-- Code for the following blog post:
-- https://kodimensional.dev/cps
{-# LANGUAGE LambdaCase #-}
module CPS where
data AppError
= UserSessionIsInvalid
| DbError InternalDbError
| UserAlreadyHasEmail
| UserHasDifferentEmail
| EmailIsTaken
data InternalDbError
data UserSession
data UserId
data Email = Email deriving Eq
data ID
validateUserSession :: UserSession -> IO (Maybe UserId)
validateUserSession = error "TODO: Not implemented"
getEmailByUserId :: UserId -> IO (Maybe Email)
getEmailByUserId = error "TODO: Not implemented"
getUserIdByEmail :: Email -> IO (Maybe UserId)
getUserIdByEmail = error "TODO: Not implemented"
insertUserEmail :: UserId -> Email -> IO (Either InternalDbError ID)
insertUserEmail = error "TODO: Not implemented"
withUserSession
:: UserSession
-> (UserId -> IO (Either AppError a))
-> IO (Either AppError a)
withUserSession userSession next = validateUserSession userSession >>= \case
Nothing -> pure $ Left UserSessionIsInvalid
Just userId -> next userId
withCheckedUserEmail
:: UserId
-> Email
-> IO (Either AppError a)
-> IO (Either AppError a)
withCheckedUserEmail userId email next = getEmailByUserId userId >>= \case
Just otherEmail
| email == otherEmail -> pure $ Left UserAlreadyHasEmail
| otherwise -> pure $ Left UserHasDifferentEmail
Nothing -> next
withCheckedOtherUserEmail
:: Email
-> IO (Either AppError a)
-> IO (Either AppError a)
withCheckedOtherUserEmail email next = getUserIdByEmail email >>= \case
Just otherUserId -> pure $ Left EmailIsTaken
Nothing -> next
withEmailInsert
:: UserId
-> Email
-> (ID -> IO (Either AppError a))
-> IO (Either AppError a)
withEmailInsert userId email next = insertUserEmail userId email >>= \case
Left dbErr -> pure $ Left $ DbError dbErr
Right id' -> next id'
associateEmail
:: UserSession
-> Email
-> IO (Either AppError ID)
associateEmail userSession email =
withUserSession userSession $ \userId ->
withCheckedUserEmail userId email $
withCheckedOtherUserEmail email $
withEmailInsert userId email $ \id' ->
pure $ Right id'
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