support optional fields

frontend/app/Form/Internal.hs

@@ -2,9 +2,11 @@ module Form.Internal
   ( Form (..),
     mapValues,
     runForm,
+    optional,
   )
 where
 
+import Data.Text qualified as T
 import Miso
 
 data Form i o = Form
@@ -57,3 +59,16 @@ runForm form i =
   form_ [onSubmit (either (\_ -> Left i) (Right) (form.fill i))] $
     (fmap Left <$> form.view i)
       <> [button_ [type_ "submit"] [text "submit"]]
+
+class IsEmpty i where
+  isEmpty :: i -> Bool
+
+instance IsEmpty T.Text where
+  isEmpty = T.null . T.strip
+
+optional :: (IsEmpty i) => Form i o -> Form i (Maybe o)
+optional form =
+  Form
+    { view = \i -> form.view i,
+      fill = \i -> if isEmpty i then Right Nothing else Just <$> form.fill i
+    }