General validation helpers
==========================

.. code:: haskell

   --
   validateLength :: (Foldable t, Monoid e, MonadIO m) => Text -> (Int, Int) -> Getting (t a) s (t a) -> s -> m e

   -- NOTE: The type signature is probably incomplete. Please refer to the usage
   -- sample to figure out what the actual type signature needs to be.
   validateFormat :: (MonadIO m, Monoid e) => m RE -> Lens' s a -> s -> m e

   -- Strips the field of all leading and trailing whitespace and then ensures
   -- that is not a blank string. TODO: Should the whitespace-stripped string be
   -- stored in the DB? How do we ensure that?
   validatePresence :: (Monoid e, MonadIO m) => Text -> Getting Text s Text -> s -> m e

   -- Ensures that a field is either Nothing OR a blank string (ignoring all
   -- leading and trailing whitespace). TODO: How do we ensure that a blank-string
   -- is actually treated as a Nothing when storing into the DB? Also, is there a
   -- use-case for having a non-Maybe (i.e. NOT NULL) field, which is validated to
   -- be a blank string?
   validateAbsence :: (Monoid e, MonadIO m) => Text -> Getting (Maybe Text) s (Maybe Text) -> s -> m e

   -- This will end up making a DB call, because of which, more class -
   -- constraints will get added. Like `Default Constant a1 (Column a1)`. Also,
   -- please NOTE - you have to be careful while querying the DB for rows with the
   -- same fields to NOT match the record which is being validated. This can be
   -- ensured by passing another condition to `filterN` -
   -- (id, pgNotEq, record ^.id)
   validateUnique1 :: (Monoid e, HasDatabase m) => Text -> (Getting a1 s a1) -> s -> m e
   validateUnique2 :: (Monoid e, HasDatabase m) => Text -> (Getting a1 s a1, Getting a2 s a2) -> s -> m e
   validateUnique3 :: (Monoid e, HasDatabase m) => Text -> (Getting a1 s a1, Getting a2 s a2, Getting a3 s a3) -> s -> m e
   -- and so on... til validateUnique5

   --
   validateIn :: (Monoid e, MonadIO m) => Text -> [a] -> Getting [a] s [a] -> s -> m e

Strict model validations
========================

.. code:: haskell

   module Models.User
     (
       module Models.User
     , module Models.User.Types
     , module Autogenerated.Models.User
     ) where

   instance DbModel User where
     strictValidations :: (MonadIO m) => User -> m [Error]
     strictValidations user =
       (validateUnique "Email must be unique" email)
       <> (validateLength "Name must be between 5 and 100 chars" (5, 100) name)
       <> (validateFormat "Doesn't seem like a valid email." (compiledRegex "(.*)@(.*)\.(.*)") email)
       <> (validatePresence "Name should be present" name) -- strips the field of whitespace
       <> (validateIn "Should be one of black or gray" ["black", "gray"] colourCode)
       <> (if (present $ user ^. firstName)
           then (validatePresence "Last name should be present if first name is given" lastName)
           else [])