Is possible to get a value out of a Handler Monad?

[Conee]

Hi,
Although I haven't fully understood yet, it seems that everything that is obtained from the database with the runDB and get404 functions is wrapped in a handler monad.
I need a value from the database which is of type Text, but I only managed to get the
Handler Text which is causing me a problem.
I'm probably complicated, but I'm a beginner, so I'm experimenting.

I have the following functions:

getCityFromMan :: Manifestation -> Handler Text   --I want here to be just Text type getCityFromMan man = do

       loc <- runDB $ get404 $ manifestationLocation man   --Manifestation has key to Location 

       ads <- runDB $ get404 $ locationAddress loc --Location has key to Address       

       let cityName = addressCity ads --Address has column city type Text

       return cityName    

applyFilters :: ManFilter -> Manifestation -> Bool

applyFilters f man = and

       [ go name filterSearch

        , go city filterCity ]

  where

        go :: (z -> Bool) -> (ManFilter -> Maybe z) -> Bool

        go x y =

             case y f of

               Nothing -> True

               Just z -> x z

         norm = T.filter validChar . T.map C.toLower . normalize NFKD

         validChar = not . C.isMark

         name x = norm x `T.isInfixOf` norm (manifestationName man)  -- working ok

         city' = getCityFromMan man    --want to get name of city in Text type but its type is HandlerFor Text

         city x = norm x == norm city'    --get true if filter value and city is the same

I get error:

Couldn't match type ‘HandlerFor App Text’ with ‘Text’

Expected type: Text        Actual type: Handler Text

How to deal with this and is it even possible to extract Text from Handler Text?

On several similar problems I have seen that it is not possible, then what is the solution??
Thanks a lot, any advice is welcome.

[Conee]

typing error in first function:

getCityFromMan :: Manifestation -> Handler Text    -- I want here to be just Text type

getCityFromMan man = do

       loc <- runDB $ get404 $ manifestationLocation man    --Manifestation has key to Location 

       ads <- runDB $ get404 $ locationAddress loc   --Location has key to Address       

       let cityName = addressCity ads   --Address has column city type Text

       return cityName   

[jsch...@gmail.com]

Conee schrieb am Donnerstag, 11. März 2021 um 15:41:56 UTC+1:

typing error in first function:

getCityFromMan :: Manifestation -> Handler Text    -- I want here to be just Text type

That's not possible: If you could return just Text, then it would be a pure function. A database operation can never be a pure function.

[Conee]

Thanks for your reply !

In the meantime, someone find me solution, although I don't fully understand the filterM and pure functions, since I'm a beginner, but this is working now.

applyFilters :: ManFilter -> Manifestation -> Handler Bool

applyFilters f man =

   city' <- getCityFromMan man

   let city x = norm x == norm city'

   pure $ and

       [ go name filterSearch

        , go city filterCity ]

  where

        go :: (z -> Bool) -> (ManFilter -> Maybe z) -> Bool

        go x y =

             case y f of

               Nothing -> True

               Just z -> x z

         norm = T.filter validChar . T.map C.toLower . normalize NFKD

         validChar = not . C.isMark

         name x = norm x `T.isInfixOf` norm (manifestationName man)  -- working ok

postManUserR :: Handler Html

postManUserR = do

   emans <- runDB getAllMan

   let mans = toValues emans

   filters <- runInputPost $ ManFilter

     <$>iopt textField "City"

     <*>iopt textField "Search"

   filteredMan <- filterM (applyFilters filters) mans

   defaultLayout $ do

     [whamlet|<h1>#{show $ filteredMan}|]