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

Hello, label has the type: label :: (Monad m, ToMarkup children) => children -> Form m input error Html () () Meaning the argument to 'label' can be anything which can be embedded as markup. Unfortunately, when you use OverloadedStrings and write: label "username: " It doesn't know if it should treat "username: " as a String value or an Html value -- as both have IsString instances. You can fix it by typing: label ("username: " :: String) So that it knows. That is pretty annoying, and so we should probably have a function like: labelString :: (Monad m) => String -> Form m input error Html () () which is not ambiguous, and you can do: labelString "username: " I believe I have added that to the reform-hsp library, but not reform-blaze. In your second attempt, you imported the functions from .Common. But inputtext takes an extra parameter there: inputText :: (Monadhttp://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Control-Mo... m, FormErrorhttp://hackage.haskell.org/packages/archive/reform/0.1.2/doc/html/Text-Refor... error, ToValuehttp://hackage.haskell.org/packages/archive/blaze-markup/0.5.1.4/doc/html/Te... text) => (input -> Eitherhttp://hackage.haskell.org/packages/archive/base/4.6.0.1/doc/html/Data-Eithe... error text) -> text -> Formhttp://hackage.haskell.org/packages/archive/reform/0.1.2/doc/html/Text-Refor... m input error Htmlhttp://hackage.haskell.org/packages/archive/blaze-html/0.6.0.0/doc/html/Text... () text The .Common module is really only for common code that is shared between .String, .Text, and whatever may come in the future. It should have a comment at the top explaining its purpose. Sorry about that. - jeremy On Sat, Jun 15, 2013 at 4:19 AM, Adrian May wrote:

Hi All,

I'm trying to get this example working:

http://patch-tag.com/r/stepcut/reform/snapshot/current/content/pretty/exampl...

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.