Template Haskell: How to extract the number of arguments of a function?

Question

I have a function which given a `Name` of a function it augments it, yielding another function applied to some other stuff (details not very relevant): ``` 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 ``` This works, but it's tedious to pass externally the number of args the adaptee function has. There exists some TH function to extract the number of args? I s…

Accepted Answer

Sure, you should be able to do

```
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
```

The above is just from my head and may not be quite right.

Template Haskell: How to extract the number of arguments of a function?

Solution

Problem