haskell: > 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. >
I also wonder if we could give a String syntax to the formatting language, using -XOverloadedStrings and the IsString class. -- Don _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
