yesod-form: Add Monad AForm instance for transformers >=0.6

yesod-form/ChangeLog.md

@@ -1,5 +1,9 @@
 # ChangeLog for yesod-form
 
+## 1.7.4
+
+* Added a `Monad AForm` instance only when `transformers` >= 0.6 [#1795](https://github.com/yesodweb/yesod/pull/1795)
+
 ## 1.7.3
 
 * Fixed `radioField` according to Bootstrap 3 docs. [#1783](https://github.com/yesodweb/yesod/pull/1783)

yesod-form/Yesod/Form/Types.hs

@@ -166,6 +166,18 @@ instance Monad m => Applicative (AForm m) where
         (a, b, ints', c) <- f mr env ints
         (x, y, ints'', z) <- g mr env ints'
         return (a <*> x, b . y, ints'', c `mappend` z)
+
+#if MIN_VERSION_transformers(0,6,0)
+instance Monad m => Monad (AForm m) where
+    (AForm f) >>= k = AForm $ \mr env ints -> do
+        (a, b, ints', c) <- f mr env ints
+        case a of
+          FormSuccess r -> do 
+            (x, y, ints'', z) <- unAForm (k r) mr env ints'
+            return (x, b . y, ints'', c `mappend` z)
+          FormFailure err -> pure (FormFailure err, b, ints', c)
+          FormMissing -> pure (FormMissing, b, ints', c)
+#endif
 instance (Monad m, Monoid a) => Monoid (AForm m a) where
     mempty = pure mempty
     mappend a b = mappend <$> a <*> b

yesod-form/yesod-form.cabal

@@ -1,6 +1,6 @@
 cabal-version:   >= 1.10
 name:            yesod-form
-version:         1.7.3
+version:         1.7.4
 license:         MIT
 license-file:    LICENSE
 author:          Michael Snoyman <michael@snoyman.com>