It doesn't typecheck, no, but it also doesn't check out in scope. It
complains in
[FunD 'mousePosition [| mousePositionf |] ...
that mousePositionf isn't in scope.
What I believe I need to do is use mkName "mousePositionf", but how do
I bind the record getter "mousePositionf" that is defined by the code
in the function named "additions" to mousePosition -- a.k.a. how do I
write that template? Is it a ConE? And how do I encode
a{ mousePositionf = b }
in template haskell without using the [| |] syntax, so that I can use mkName?
-- Jeff
On Tue, Jan 6, 2009 at 6:23 PM, Eelco Lempsink
On 6 jan 2009, at 18:08, Jeff Heard wrote:
Alright... I *think* I'm nearly there, but I can't figure out how to derive a class instance using record accessors and updaters... Can anyone help? There are [| XXXf |] instances at the end of the module and they all need replaced, but I can't figure out what to replace them with.
...
-- usage: $(deriveUIState ''MyTypeWithUIState) {- - Derive an instance of UIState from some type that has had UIState fields added to it. -} deriveUIState tp = do return [InstanceD [] (appUIState $ appType tp []) [FunD 'mousePosition [| mousePositionf |]
...
,FunD 'setMousePosition [| \b a -> a{ mousePositionf=b } |]
...
Quick guess: this doesn't typecheck?
FunD :: Name -> [Clause] -> Dec
while [| ... |] will return something of type ExpQ (which is the same as Q Exp).
You're indeed nearly there, but if you use the quotation brackets you need to write monadic code (for the Q monad) and use functions like clause and funD. The tutorials on the wiki (you've probably seen them, http://www.haskell.org/haskellwiki/Template_Haskell) or pretty good and you could also look at packages at hackage for inspiration/examples, e.g. http://hackage.haskell.org/packages/archive/haxr-th/3000.0.0/doc/html/src/Ne...
-- Regards,
Eelco Lempsink