Prairie.Traverse

This module shows how to traverse over a Record, allowing you to perform side-effects for each field and return a transformed value.

Since: 0.0.3.0

Documentation

traverseRecord :: forall rec f. (Record rec, Applicative f) => (forall ty. ty -> Field rec ty -> f ty) -> rec -> f rec

Apply an effectful function over each field of a Record, producing a new Record in the process.

This example use increments a Users age and requests a new name.

happyBirthday :: User -> IO User
happyBirthday =
    traverseRecord
         (\val field ->
             case field of
                 UserName -> do
                     putStrLn $ "Current name is: " <> val
                     putStrLn "Please input a new name: "
                     getLine
                 UserAge -> do
                     putStrLn $ "Current age is " <> show val
                     pure (val + 1)
         )

If you only want to target a single field, you can use a wildcard match and pure. This example also uses LambdaCase.

nameAtLeastOneCharacter :: User -> Maybe User
nameAtLeastOneCharacter =
     traverseRecord
         (val ->
             case
                 UserName -> do
                     guard (length val >= 1)
                     pure val
                 _ ->
                     pure val
         )

Since: 0.0.3.0