See http://stackoverflow.com/questions/29389064/yesods-th-generates-incorrect-code/29389188

Data.hs

{-# LANGUAGE QuasiQuotes     #-}
{-# LANGUAGE TemplateHaskell #-}
{-# LANGUAGE TypeFamilies    #-}
{-# LANGUAGE FlexibleContexts    #-}
module Yesod.DataSource.Data where

import Yesod
import Data.Text (Text)

-- Subsites have foundations just like master sites.
data DataSource = DataSource

class (RenderMessage master FormMessage, Yesod master) => YesodDataSource master where
    dummyThing :: HandlerT master IO Bool
    dummyThing = return True

-- We have a familiar analogue from mkYesod, with just one extra parameter.
-- We'll discuss that later.
mkYesodSubData "DataSource" [parseRoutes|
/ SubHomeR GET
/datasource DataSourceInputR POST GET
|]

data DataSourceInput = DataSourceInput
    { dataSourceName        :: Text
    , dataSourceStart      :: Int
    , dataSourceEnd         :: Int
    }
  deriving Show

datasource.cabal

-- Initial datasource.cabal generated by cabal init.  For further 
-- documentation, see http://haskell.org/cabal/users-guide/

name:                datasource
version:             0.1.0.0
-- synopsis:            
-- description:         
-- license:             
license-file:        LICENSE
author:              Beerend Lauwers
-- maintainer:          
-- copyright:           
-- category:            
build-type:          Simple
-- extra-source-files:  
cabal-version:       >=1.10

library
  Exposed-modules:     
    Yesod.DataSource
    Yesod.DataSource.Data
  -- other-modules:       
  -- other-extensions:    
  build-depends:       
    base >=4.7 && <4.8,
    text,
    yesod,
    classy-prelude,
    autogen,
    haskell-src-exts,
    directory
  -- hs-source-dirs:      
  default-language:    Haskell2010

DataSource.hs

{-# LANGUAGE FlexibleInstances     #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE OverloadedStrings     #-}
{-# LANGUAGE QuasiQuotes           #-}
{-# LANGUAGE TemplateHaskell       #-}
{-# LANGUAGE RankNTypes            #-}

module Yesod.DataSource (module Yesod.DataSource.Data, Yesod.DataSource) where

import Yesod.DataSource.Data
import Yesod
import ClassyPrelude

instance YesodDataSource master => YesodSubDispatch DataSource (HandlerT master IO) where
    yesodSubDispatch = $(mkYesodSubDispatch resourcesHelloSub)
    
type DataSourceHandler a = forall master. YesodDataSource master 
                         => HandlerT DataSource (HandlerT master IO) a

getDataSourceInputR :: DataSourceHandler Html
getDataSourceInputR = do 
  toMaster <- getRouteToParent
  (widget, enctype) <- lift $ generateFormPost simpleSourceForm
  lift $ defaultLayout
            [whamlet|
                <p>
                    The widget generated contains only the contents
                    of the form, not the form tag itself. So...
                <form method=post action=@{toMaster DataSourceInputR} enctype=#{enctype}>
                    ^{widget}
                    <p>It also doesn't include the submit button.
                    <button>Submit
            |]

-- And we'll spell out the handler type signature.
getSubHomeR :: DataSourceHandler Html
getSubHomeR = do
 toMaster <- getRouteToParent
 lift $ defaultLayout [whamlet|<a href=@{toMaster SubHomeR}> |]
 
 
simpleSourceForm = renderDivs $ DataSourceInput
  <$> areq textField "Name" Nothing
  <*> areq intField "Start" Nothing
  <*> areq intField "End" Nothing

  
postDataSourceInputR :: DataSourceHandler Html
postDataSourceInputR = do
    toMaster <- getRouteToParent
    ((result, widget), enctype) <- lift $ runFormPost simpleSourceForm
    case result of
        FormSuccess datasource -> lift $ defaultLayout [whamlet|<p>#{show datasource}|]
        _ -> lift $ defaultLayout
            [whamlet|<a href=@{toMaster SubHomeR}> |]