Merge pull request #50 from joelmccracken/patch-1

Create Maintaining-Applicative-Form-Behavior-In-Monadic-Forms.md

Cookbook.md

@@ -19,6 +19,7 @@ For non-Yesod code, see also [Snippets](https://github.com/yesodweb/yesod-cookbo
 * [Change the order of form fields](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/OrderFormFields.md)
 * [Require input for Maybe field](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/RequireInputMaybeField.md)
 * [Custom form layout](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/CustomFormLayout.md)
+* [Maintaining Applicative Form Behavior In Monadic Forms](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Maintaining-Applicative-Form-Behavior-In-Monadic-Forms.md)
 * [Password confirm field](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Password-Confirm-Field.md)
 * [Multi-field validation](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Cookbook-Multi-field-validation.md)
 * [Select field populated from database](https://github.com/yesodweb/yesod-cookbook/blob/master/cookbook/Select-field-populated-from-database.md)

cookbook/Maintaining-Applicative-Form-Behavior-In-Monadic-Forms.md

@@ -0,0 +1,68 @@
+# Maintaining Applicative Form Behavior In Monadic Forms
+
+Applicative forms have a number of useful features that you lose when 
+switching to Monadic forms, such as generating labels, displaying errors
+beside the input field, etc, any of which you would then have to re-implement
+yourself in the monadic context. 
+
+However, you can separate the overall form into individual applicative forms,
+one per field, and then embed them within a monadic form, you can maintain 
+the individual behavior.
+
+Here is an example: 
+
+```haskell
+joinForm :: Html -> MForm Handler (FormResult Join, Widget)
+joinForm extra = do
+  let
+    mustAgreeField :: Text -> Field Handler Bool
+
+    mustAgreeField errorMsg = checkBool id errorMsg checkBoxField
+    accept1Def = areq (mustAgreeField
+                         "You must specify that you accept Section 1")
+                    "Accept Section 1" Nothing
+    accept2Def = areq (mustAgreeField
+                         "You must specify that you accept Section 2")
+                    "Accept Section 2" Nothing
+    fullNameDef = areq
+                    (checkBool
+                      ((>0) . length)
+                      ("You must include your full, legal name" :: Text)
+                      textField)
+                    "Full Name"
+                    Nothing
+
+  (accept1, accept1Widget) <- renderDivs accept1Def mempty
+  (accept2, accept2Widget) <- renderDivs accept2Def mempty
+
+  (fullName, fullNameWidget) <- renderDivs fullNameDef mempty
+
+  let
+    widget =
+      [whamlet|
+              ^{extra}
+              <p.my-4> this is a sentence
+              ^{accept1Widget}
+              <p.my-4> a sep
+              ^{accept2Widget}
+              <p.my-4> another sep
+              ^{fullNameWidget}
+              |]
+
+    joinRes = Join <$> accept1 <*> accept2 <*> fullName
+
+  return (joinRes, widget)
+
+data Join = Join
+  { joinSignedSection1 :: Bool
+  , joinSignedSection2 :: Bool
+  , joinFullName :: Text
+  }
+  deriving Show
+
+```
+
+- each applicative field is converted to the monadic context via `renderDivs`
+- `mempty` is passed to `renderDivs` instead of the usual `extra`, and `extra` 
+  is embedded in the parent widget. Otherwise, CSRF will be included multiple 
+  times, and this will not work