Re: [Haskell-cafe] Beginner's TH question

20 Dec 2008


      Hello Jeff,

Saturday, December 20, 2008, 6:59:42 PM, you wrote:

my experience tells that you should insert whole language sentences,
like

$(add_fields [d| data T = T { myfield :: Field, ... } ] )

where original declaration passed as a parameter
...
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
       |]
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-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin@gmail.com

Re: [Haskell-cafe] Beginner's TH question

Bulat Ziganshin