Give FormResult an Alternative instance

yesod-form/ChangeLog.md

@@ -1,3 +1,7 @@
+## 1.4.15
+
+* Added `Alternative` instance to `FormResult` to simplify handling pages with multiple forms.
+
 ## 1.4.14
 
 * Added `WForm` to reduce the verbosity using monadic forms.

yesod-form/Yesod/Form/Types.hs

@@ -30,7 +30,7 @@ import Text.Blaze (Markup, ToMarkup (toMarkup), ToValue (toValue))
 #define Html Markup
 #define ToHtml ToMarkup
 #define toHtml toMarkup
-import Control.Applicative ((<$>), Applicative (..))
+import Control.Applicative ((<$>), Alternative (..), Applicative (..))
 import Control.Monad (liftM)
 import Control.Monad.Trans.Class
 import Data.String (IsString (..))
@@ -45,6 +45,8 @@ import Data.Foldable
 --
 -- The 'Applicative' instance will concatenate the failure messages in two
 -- 'FormResult's.
+-- The 'Alternative' instance will choose 'FormFailure' before 'FormSuccess',
+-- and 'FormMissing' last of all.
 data FormResult a = FormMissing
                   | FormFailure [Text]
                   | FormSuccess a
@@ -80,6 +82,16 @@ instance Data.Traversable.Traversable FormResult where
       FormFailure errs -> pure (FormFailure errs)
       FormMissing -> pure FormMissing
 
+-- | @since 1.4.15
+instance Alternative FormResult where
+    empty = FormMissing
+
+    FormFailure e    <|> _             = FormFailure e
+    _                <|> FormFailure e = FormFailure e
+    FormSuccess s    <|> FormSuccess _ = FormSuccess s
+    FormMissing      <|> result        = result
+    result           <|> FormMissing   = result
+
 -- | The encoding type required by a form. The 'ToHtml' instance produces values
 -- that can be inserted directly into HTML.
 data Enctype = UrlEncoded | Multipart

yesod-form/yesod-form.cabal

@@ -1,5 +1,5 @@
 name:            yesod-form
-version:         1.4.14
+version:         1.4.15
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>