[Haskell-beginners] Reform and blaze: trouble with example code

[Adrian May]

Hi All,

I'm trying to get this example working:

http://patch-tag.com/r/stepcut/reform/snapshot/current/content/pretty/examples/BlazeMain.hs

It emitted what I took to be bitrot about ToHtml having apparently been
generallised to ToMarkup and similar stuff, so I banged it into this form:

    {-# LANGUAGE FlexibleContexts, FlexibleInstances, TypeFamilies,
OverloadedStrings #-}
    module Main where

    import Control.Applicative.Indexed
    import Control.Monad
    import qualified Data.ByteString.Char8 as C
*    import Text.Blaze *
    import qualified Text.Blaze.Html5 as H
    import qualified Text.Blaze.Html5.Attributes as A
    import Text.Blaze.Renderer.Utf8 (renderHtml)
    import Text.Reform
*    --import Text.Reform.Blaze.Common*
*    --import Text.Reform.Blaze.Text*
*    import Text.Reform.Blaze.String*
    import Text.Reform.Happstack
    import Happstack.Server
    import SharedForm

    instance *ToMarkup* (DemoFormError [Input]) where
*toMarkup* InvalidEmail    = "Email address must contain a @."
*toMarkup* InvalidUsername = "Username must not be blank."
*toMarkup* (CommonError (InputMissing fid))        = H.toHtml $ "Internal
Error. Input missing: " ++ show fid
*toMarkup* (CommonError (NoStringFound input))     = H.toHtml $ "Internal
Error. Could not extract a String from: " ++ show input
*toMarkup* (CommonError (MultiStringsFound input)) = H.toHtml $ "Internal
Error. Found more than one String in: " ++ show input

    usernameForm :: (Monad m, FormInput input, *ToMarkup* (DemoFormError
input)) =>
String
      -> Form m input (DemoFormError input) *Markup* NotNull Username
    usernameForm initialValue =
( *label "username: " ++>* (Username <<$>> inputText initialValue `prove`
(notNullProof InvalidUsername)))
    {-
    usernameForm :: (Monad m, FormInput input, ToMarkup (DemoFormError
input)) =>
String
      -> Form m input (DemoFormError input) Markup NotNull Username
    usernameForm initialValue =
errorList ++> (*label "username: " ++> *(Username <<$>> inputText
initialValue `prove` (notNullProof InvalidUsername)))
    -}
    blazeResponse :: *Markup* -> Response
    blazeResponse html = toResponseBS (C.pack "text/html;charset=UTF-8") $
renderHtml html

    blazeForm :: *Markup* -> *Markup*
    blazeForm html =
H.form ! A.action "/"
      ! A.method "POST"
      ! A.enctype "multipart/form-data" $
do html
    H.input ! A.type_ "submit"

    formHandler :: (*ToMarkup* error, Show a) => Form (ServerPartT IO)
[Input] error *Markup* proof a -> ServerPart Response
    formHandler form =
    msum [ do method GET
      html <- viewForm "user" form
      ok $ blazeResponse $ blazeForm html

, do method POST
      r <- eitherForm environment "user" form
      case r of
(Right a) -> ok $ toResponse $ show a
(Left view) ->
  ok $ blazeResponse $ blazeForm view

]

    main :: IO ()
    main =
do let form = usernameForm ""
  simpleHTTP nullConf $ do decodeBody (defaultBodyPolicy "/tmp" 0 10000
10000)
    formHandler form


where *italics* indicate the bits I changed, but now I'm stumped by the *
bold* bit barfing with:

Taser.hs:30:13:
    Ambiguous type variable `children0' in the constraints:
      (Data.String.IsString children0)
        arising from the literal `"username: "' at Taser.hs:30:13-24
      (ToMarkup children0)
        arising from a use of `label' at Taser.hs:30:7-11
    Probable fix: add a type signature that fixes these type variable(s)
    In the first argument of `label', namely `"username: "'
    In the first argument of `(++>)', namely `label "username: "'
    In the expression:
      (label "username: "
       ++>
         (Username
          <<$>>
            inputText initialValue `prove` (notNullProof InvalidUsername)))

If I take out label "username: " ++>, then it all works fine, except I
don't have a label. I also tried putting the label inside the Username
constructor with the same result.

I have the following versions installed:

* blaze-markup     (library)
    Versions installed: 0.5.1.5
* blaze-html       (library)
    Versions installed: 0.6.1.1
* reform           (library)
    Versions installed: 0.1.2
* reform-blaze     (library)
    Versions installed: 0.1.2
The Glorious Glasgow Haskell Compilation System, version 7.4.2

Perhaps the markup thing is what broke it, but I can't see ToHtml in any of
those modules.

Thanks in advance,
Adrian.
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