模板Haskell显示实例不工作
有谁知道为什么这个代码：
module Language.P4.UtilTest where
import Language.P4.Util (mkShow)
data Dummy = Bogus Char
| Nonsense Int
$(mkShow ''Dummy)
正在产生这个错误：
Davids-Air-2:P4 dbanas$ stack ghc -- UtilTest.hs -ddump-splices
[1 of 1] Compiling Language.P4.UtilTest ( UtilTest.hs, UtilTest.o )
UtilTest.hs:24:3-16: Splicing declarations
mkShow ''Dummy
======>
instance Show Dummy where
show (Bogus x) = show x
show (Nonsense x) = show x
UtilTest.hs:24:3: error:
Conflicting definitions for ‘show’
Bound at: UtilTest.hs:24:3-16
UtilTest.hs:24:3-16
|
24 | $(mkShow ''Dummy)
| ^^^^^^^^^^^^^^
？
TH拼接扩展对我来说看起来是正确的。 如果我注释掉第二个构造函数（ Nonsense Int ），则代码无错地编译。 另外，如果我手动输入显示的TH拼接扩展（当然注释掉$(mkShow ''Dummy)行）代码，则编译时不会出错。
mkShow :: Name -> Q [Dec]
mkShow typName = do
t@(TyConI (DataD _ _ _ _ constructors _)) <- reify typName -- Get all the information on the type.
let func_name = mkName "show"
let var_name = mkName "x"
let func_decs = map ( c@(NormalC nm _) -> FunD func_name
[ Clause [ConP (simpleName nm) [VarP var_name]]
(NormalB (AppE (VarE func_name) (VarE var_name)))
[]
]
)
constructors
return [InstanceD Nothing [] (AppT (ConT (mkName "Show")) (ConT (simpleName typName))) func_decs]
simpleName :: Name -> Name
simpleName nm =
let s = nameBase nm
in case dropWhile (/=':') s of
[] -> mkName s
_:[] -> mkName s
_:t -> mkName t
上面@ user2407038的评论给出了答案。 这是mkShow（）的更正后的代码：
mkShow :: Name -> Q [Dec]
mkShow typName = do
t@(TyConI (DataD _ _ _ _ constructors _)) <- reify typName -- Get all the information on the type.
let func_name = mkName "show"
let var_name = mkName "x"
let clause_decs = map ( c@(NormalC nm _) ->
Clause [ConP (simpleName nm) [VarP var_name]]
(NormalB (AppE (VarE func_name) (VarE var_name)))
[]
)
constructors
return [InstanceD Nothing [] (AppT (ConT (mkName "Show")) (ConT (simpleName typName))) [FunD func_name clause_decs]]