The key points of printf
is the ability to either return a String or a function. Copied from http://www.haskell.org/ghc/docs/6.12.2/html/libraries/base-4.2.0.1/src/Text-Printf.html,
printf :: (PrintfType r) => String -> r
printf fmts = spr fmts []
class PrintfType t where
spr :: String -> [UPrintf] -> t
instance (IsChar c) => PrintfType [c] where
spr fmts args = map fromChar (uprintf fmts (reverse args))
instance (PrintfArg a, PrintfType r) => PrintfType (a -> r) where
spr fmts args = \a -> spr fmts (toUPrintf a : args)
and the basic structure we can extract out is
variadicFunction :: VariadicReturnClass r => RequiredArgs -> r
variadicFunction reqArgs = variadicImpl reqArgs mempty
class VariadicReturnClass r where
variadicImpl :: RequiredArgs -> AccumulatingType -> r
instance VariadicReturnClass ActualReturnType where
variadicImpl reqArgs acc = constructActualResult reqArgs acc
instance (ArgClass a, VariadicReturnClass r) => VariadicReturnClass (a -> r) where
variadicImpl reqArgs acc = \a -> variadicImpl reqArgs (specialize a `mappend` acc)
For instance:
class SumRes r where
sumOf :: Integer -> r
instance SumRes Integer where
sumOf = id
instance (Integral a, SumRes r) => SumRes (a -> r) where
sumOf x = sumOf . (x +) . toInteger
then we could use
*Main> sumOf 1 :: Integer
1
*Main> sumOf 1 4 7 10 :: Integer
22
*Main> sumOf 1 4 7 10 0 0 :: Integer
22
*Main> sumOf 1 4 7 10 2 5 8 22 :: Integer
59