[Haskell-cafe] Re: [Haskell] The initial view on typed sprintf and sscanf

[Don Stewart]

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