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

20 Dec 2008

      Seems like GHC had already told you what's wrong. Instance  
declarations like "instance UIState t" are illegal without  
FlexibleInstances language feature enabled. Also, I don't quite  
understand, what you're trying to achieve; argument "t" and the letter  
"t" in the TH body are two different beasts, so your "derive..." would  
be of no use.

May be, you want something like this:

{-# LANGUAGE TemplateHaskell #-}
module TH where
import Language.Haskell.TH
import Language.Haskell.TH.Syntax
class C a where c :: a -> a
deriveC t =
      do decs <- [d| c x = x |]
        tp <- t
        return [InstanceD [] (AppT (ConT ''C) tp) decs]

{-# LANGUAGE TemplateHaskell #-}
module THTest where
import TH
$(deriveC [t| Int |])

*THTest> c (1 :: Int)
1

On 20 Dec 2008, at 18:59, Jeff Heard wrote:
...
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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Re: [Haskell-cafe] Beginner's TH question

Miguel Mitrofanov