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[英]How to write a multi-arity function (variable number of arguments) in Haskell
[英]Template Haskell: How to extract the number of arguments of a function?
我有一個函數,它給出了一個函數Name
,它增加了它,產生了另一個應用於其他東西的函數(細節不太相關):
mkSimple :: Name -> Int -> Q [Dec]
mkSimple adapteeName argsNum = do
adapterName <- newName ("sfml" ++ (capitalize . nameBase $ adapteeName))
adapteeFn <- varE adapteeName
let args = mkArgs argsNum
let wrapper = mkApply adapteeFn (map VarE args)
-- generates something like SFML $ liftIO $ ((f a) b) c)
fnBody <- [| SFML $ liftIO $ $(return wrapper) |]
return [FunD adapterName [Clause (map VarP args) (NormalB fnBody) []]]
where
mkArgs :: Int -> [Name]
mkArgs n = map (mkName . (:[])) . take n $ ['a' .. 'z']
-- Given f and its args (e.g. x y z) builds ((f x) y) z)
mkApply :: Exp -> [Exp] -> Exp
mkApply fn [] = fn
mkApply fn (x:xs) = foldr (\ e acc -> AppE acc e) (AppE fn x) xs
這是有效的,但從外部傳遞適配器功能所具有的args數量是很繁瑣的。 有一些TH函數來提取args的數量? 我懷疑它可以通過reify實現,但我不知道如何。
謝謝!
當然,你應該可以做到
do (VarI _ t _ _) <- reify adapteeName
-- t :: Type
-- e.g. AppT (AppT ArrowT (VarT a)) (VarT b)
let argsNum = countTheTopLevelArrowTs t
...
where
countTheTopLevelArrowTs (AppT (AppT ArrowT _) ts) = 1 + countTheTopLevelArrowTs
countTheTopLevelArrowTs _ = 0
以上只是我的想法,可能不太正確。
jberryman的答案略有改進,處理類型約束,如(Ord a) -> a -> a
是:
arity :: Type -> Integer
arity = \case
ForallT _ _ rest -> arity rest
AppT (AppT ArrowT _) rest -> arity rest +1
_ -> 0
用法:
do (VarI _ t _ _) <- reify adapteeName
let argsNum = arity t
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