[Haskell-cafe] Beginner's TH question

[Jeff Heard]

Two things...  can I add fields to records using Template Haskell, like:

data T = T { $fields, myfield :: Field, ... }

I assume the answer there is no, and then what's wrong with this?  I get:

    Illegal instance declaration for `UIState t'
        (All instance types must be of the form (T a1 ... an)
         where a1 ... an are type *variables*,
         and each type variable appears at most once in the instance head.
         Use -XFlexibleInstances if you want to disable this.)
    In the instance declaration for `UIState t'
    In the expression:
        [d|
            instance UIState t where
                { setSizeY v a = setSizeY v . uist $ a
                  setSizeX v a = setSizeX v . uist $ a
                  setDrawing v a = setDrawing v . uist $ a
                  setKey v a = setKey v . uist $ a
                  .... } |]
    In the definition of `deriveUIState':
        deriveUIState uist t
                        = [d|
                              instance UIState t where
                                  { setSizeY v a = setSizeY v . uist $ a
                                    setSizeX v a = setSizeX v . uist $ a
                                    setDrawing v a = setDrawing v . uist $ a
                                    .... } |]

in this module:

-# LANGUAGE TemplateHaskell #-}
module Graphics.Rendering.Thingie.TH where

import Language.Haskell.TH
import Graphics.Rendering.Thingie.UIState
import qualified Graphics.Rendering.Thingie.BasicUIState as S


deriveUIState uist t =
      [d| instance UIState t where
            mousePosition a = S.mousePosition . uist $ a
            mouseLeftButtonDown a = S.mouseLeftButtonDown . uist $ a
            mouseRightButtonDown a = S.mouseRightButtonDown . uist $ a
            mouseMiddleButtonDown a = S.mouseMiddleButtonDown . uist $ a
            mouseLeftButtonClicked a = S.mouseLeftButtonClicked . uist $ a
            mouseRightButtonClicked a = S.mouseRightButtonClicked . uist $ a
            mouseMiddleButtonClicked a = S.mouseMiddleButtonClicked . uist $ a
            mouseWheel a = S.mouseWheel . uist $ a
            keyCtrl a = S.keyCtrl . uist $ a
            keyShift a = S.keyShift . uist $ a
            keyAlt a = S.keyAlt . uist $ a
            key a = S.key . uist $ a
            drawing a = S.drawing . uist $ a
            sizeX a = S.sizeX . uist $ a
            sizeY a = S.sizeY . uist $ a
            setMousePosition v a = setMousePosition v . uist $ a
            setMouseLeftButtonDown v a = setMouseLeftButtonDown v . uist $ a
            setMouseRightButtonDown v a = setMouseRightButtonDown v . uist $ a
            setMouseMiddleButtonDown v a = setMouseMiddleButtonDown v . uist $ a
            setMouseLeftButtonClicked v a = setMouseLeftButtonClicked
v . uist $ a
            setMouseRightButtonClicked v a =
setMouseRightButtonClicked v . uist $ a
            setMouseMiddleButtonClicked v a =
setMouseMiddleButtonClicked v . uist $ a
            setMouseWheel v a = setMouseWheel v . uist $ a
            setKeyCtrl v a = setKeyCtrl v . uist $ a
            setKeyShift v a = setKeyShift v . uist $ a
            setKeyAlt v a = setKeyAlt v . uist $ a
            setKey v a = setKey v . uist $ a
            setDrawing v a = setDrawing v . uist $ a
            setSizeX v a = setSizeX v . uist $ a
            setSizeY v a = setSizeY v . uist $ a
       |]