我正在写一个代码生成器,其输出取决于存储在其类实例中的数据类型字段描述。但是,我找不到如何用TH生成的参数运行函数。如何规避GHC阶段限制?
{-# LANGUAGE TemplateHaskell, ScopedTypeVariables #-}
module Generator where
import Language.Haskell.TH
import Language.Haskell.TH.Syntax
data Description = Description String [Description] deriving Show
class HasDescription a where
getDescription :: a -> Description
instance HasDescription Int where
getDescription _ = Description "Int" []
instance (HasDescription a, HasDescription b) => HasDescription (a, b) where
getDescription (_ :: (a, b)) = Description "Tuple2" [getDescription (undefined :: a), getDescription (undefined :: b)]
-- | creates instance of HasDescription for the passed datatype changing descriptions of its fields
mkHasDescription :: Name -> Q [Dec]
mkHasDescription dName = do
reify dName >>= runIO . print
TyConI (DataD cxt name tyVarBndr [NormalC cName types] derives) <- reify dName
-- Attempt to get description of data to modify it.
let mkSubDesc t = let Description desc ds = getDescription (undefined :: $(return t)) in [| Description $(lift $ desC++ "Modified") $(lift ds) |]
let body = [| Description $(lift $ nameBase dName) $(listE $ map (mkSubDesc . snd) types) |]
getDescription' <- funD 'getDescription [clause [wildP] (normalB body) []]
return [ InstanceD [] (AppT (ConT ''HasDescription) (ConT dName)) [getDescription'] ]
当另一个模块尝试使用发电机
{-# LANGUAGE TemplateHaskell, ScopedTypeVariables #-}
import Generator
data MyData = MyData Int Int
mkHasDescription ''MyData
{- the code I want to generate
instance HasDescription MyData where
getDescription _ = Description "MyData" [Description "IntModified" [], Description "IntModified" []]
-}
似乎有一个错误
Generator.hs:23:85:
GHC stage restriction: `t'
is used in a top-level splice or annotation,
and must be imported, not defined locally
In the first argument of `return', namely `t'
In the expression: return t
In an expression type signature: $(return t)
编辑:
当问我想,这个问题似乎只是因为我只是没有掌握TH中至关重要的一些东西,可以通过将某些功能移到TH中来解决其他模块。
如果无法生成预先计算的数据,例如问题,我想了解更多关于TH的理论限制。
我发现......令人惊讶的是,那是行不通的。也许你还需要打开QuasiQuotes? – 2012-04-02 16:26:25