oleg [1]:
We demonstrate typed sprintf and typed sscanf sharing the same formatting specification.
[1]http://www.haskell.org/pipermail/haskell/2008-August/020605.html Reading Oleg's post, I noticed that it is quite straightforward to generalise printing to arbitrary output types.
class Monoid p => Printf p where printChar :: Char -> p --etc
intp :: Printf p => F a b -> (p -> a) -> b intp FChr k = \c -> k (printChar c) intp (a :^ b) k = intp a (\sa -> intp b (\sb -> k (mappend sa sb))) --etc
printf :: Printf p => F p b -> b printf fmt = intp fmt id
The Printf instances for String, ShowS, ByteStrings, and IO () are all trivial. Printing directly to a file (without building an intermediate string) is slightly more interesting:
instance Monad m => Monoid (ReaderT r m ()) where mempty = return () mappend = (>>)
instance Printf (ReaderT Handle IO ()) where printChar c = ask >>= \h -> liftIO (hPutChar h c) --etc
(<<) :: Handle -> ReaderT Handle IO () -> IO () (<<) = flip runReaderT
test_fprintf h = h << printf (lit "Hello " ^ lit "World" ^ chr) '!'
Unfortunately, I don't seem to be able to make the expected fprintf function, because printf's format-dependent parameter list makes it impossible to find a place to pass the handle. Hence the C++-like (<<) ugliness.