Working on HasFormlet type class

Instances.hs

{-# LANGUAGE TypeSynonymInstances #-}
module Text.Digestive.Blaze.Instances where

import Data.Maybe
import Text.Digestive.Blaze.Html5 (inputText, inputCheckBox, inputTextRead)
import Text.Digestive.Forms (FormInput)
import Text.Digestive.Types (Form)

class HasFormlet a where
    formlet :: (Monad m, Functor m, FormInput i f) => e -> Maybe a -> Form m i e v a

instance HasFormlet String where
    formlet _ d = inputText d

instance HasFormlet Bool where
    formlet _ d = inputCheckBox $ fromMaybe False d

instance HasFormlet Int where
    formlet = inputTextRead

instance HasFormlet Float where
    formlet = inputTextRead

instance HasFormlet Double where
    formlet = inputTextRead

{-
This code gets five errors like the following:

Foo.hs:13:18:
    Couldn't match expected type `v'
           against inferred type `Text.Digestive.Forms.Html.FormHtml
                                    Text.Blaze.Internal.Html'
      `v' is a rigid type variable bound by
          the type signature for `formlet' at Foo.hs:10:81
      Expected type: Form m i e v String
      Inferred type: Form
                       m i e Text.Digestive.Blaze.Html5.BlazeFormHtml String
    In the expression: inputText d
    In the definition of `formlet': formlet _ d = inputText d
-}