The following Haskell code reads a Pandoc document from the DB and then serves it to the user after making a transformation, after the first run it is stored in memory via IORef State, inside a [(String, Pandoc)] so the document can be quickly looked up again, without hitting the DB or doing the transformation. I don’t feel like this code reflects that this is what is happening that well, there is a lot of handling Maybe‘s that I feel clouds the actual steps that make up this action.

module Api.ReadDocument (Route, action) where  import Control.Monad.Reader import Data.Aeson.Types import Data.Int import Data.IORef import Data.List import Database.Persist.Postgresql (ConnectionPool, fromSqlKey, runSqlPool) import Servant import Servant.Auth.Server import Text.Pandoc (Pandoc, def, readHtml, writeJSON) -- import Config import Queries import State import Types.DB import Types.Pandoc  type Route = "read" :> Capture "id" Int64 :> Get '[JSON] Pandoc  action :: IORef State -> ConnectionPool -> AuthResult Account -> Int64 -> Handler Pandoc action state pool (Authenticated _) id = viaCache state pool id action _ _ _ = throwAll err401  viaCache :: IORef State -> ConnectionPool -> Int64 -> Handler Pandoc viaCache state pool id = do   let key = show id   state' <- liftIO $   readIORef state   maybe (retrieve state pool id) (return . snd) $   find ((== key) . fst) (cached state')  retrieve :: IORef State -> ConnectionPool -> Int64 -> Handler Pandoc retrieve state pool id = do   doc <- runSqlPool (Queries.getDocumentFromId id) pool   maybe (throwError err404) (loadDocument state id) doc  loadDocument :: IORef State -> Int64 -> Document -> Handler Pandoc loadDocument state id doc = do   let key = show id       pd = transform . documentBody $   doc   _ <- liftIO $   updateState (CacheFile (key, pd)) state   return pd 

Previously this was one function, which contained nested case statements like so (pseudo code):

action state pool (Authenticated _) id = do   state' <- fromState state              -- get the state   case find id state of                  -- find the data     Just (_, doc) ->       return doc                         -- return cached document     Nothing -> do       doc <- getDocumentFromId id pool   -- get document from db       case doc of         Just d -> do           let doc = performOp $   d        -- perform computation on doc           _ <- saveToState state doc     -- save the document to in memory cache           return doc                     -- serve document         Nothing ->                       -- document not actually in DB           throwError err404 

This is obviously much more explicit but I didn’t like the nested case statements, and obviously there is a bit more to the code than what I have typed out here.