Helper to allow validateField to work on Maybe values (#1879)

* Add validateMaybe

* Update docs

* Revert flake.lock changes

* Revert flake.lock

Guide/validation.markdown

@@ -94,6 +94,18 @@ Works with ints:
 
 You can find [the full list of built-in validators in the API Documentation](https://ihp.digitallyinduced.com/api-docs/IHP-ValidationSupport-ValidateField.html).
 
+### Validate `Maybe` Fields.
+
+You can use all the existing validators with `Maybe` fields. The validator will only be applied when the field is not `Nothing`.
+
+```haskell
+buildPost :: Post -> Post
+buildPost post = post
+    |> validateField #title nonEmpty
+    -- Assuming sourceUrl is optional.
+    |> validateField #sourceUrl (validateMaybe nonEmpty)
+```
+
 ### Fill Validation
 
 When using [`fill`](https://ihp.digitallyinduced.com/api-docs/IHP-Controller-Param.html#v:fill), like `|> fill @'["title", "body"]`, any error parsing the input is also added as a validation error.

IHP/ValidationSupport/ValidateField.hs

@@ -99,6 +99,30 @@ validateFieldIO fieldProxy customValidation model = do
     pure (attachValidatorResult fieldProxy result model)
 {-# INLINE validateFieldIO #-}
 
+-- | Validate a Maybe field.
+--
+-- Validate a Maybe field using a given validator function.
+-- >>> validateMaybe nonEmpty (Just "foo")
+-- Success
+--
+-- >>> validateMaybe nonEmpty (Just "")
+-- Failure "This field cannot be empty"
+--
+-- If the value is 'Nothing', the validation will succeed.
+-- >>> validateMaybe nonEmpty Nothing
+-- Success
+--
+-- This function is useful when you want to validate a field that is optional.
+-- >>> buildPost :: Post -> Post
+-- >>> buildPost post = post
+-- >>>     |> validateField #title nonEmpty
+-- >>>     -- Assuming sourceUrl is optional.
+-- >>>     |> validateField #sourceUrl (validateMaybe nonEmpty)
+validateMaybe :: (val -> ValidatorResult) -> Maybe val -> ValidatorResult
+validateMaybe _ Nothing = Success
+validateMaybe validator (Just value) = validator value
+{-# INLINE validateMaybe #-}
+
 -- | Overrides the error message of a given validator function.
 --
 -- >>> (nonEmpty |> withCustomErrorMessage "Custom error message") ""