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

[ChrisK]

Matthew Brecknell wrote:
> 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.


How about this:

> fprintf :: Handle -> F (IO ()) b -> 
> fprintf h fmt = write fmt id  where
>   write :: F a b -> (IO () -> a) -> b
>   write (FLit str) k = k (hPutStr h str)
>   write FInt k = \i -> k (hPutStr h (show i))
>   write FChr k = \c -> k (hPutChar h c)
>   write (FPP (PrinterParser pr _)) k = \x -> k (hPutStr h (pr x))
>   write (a :^ b)  k = write a (\sa -> write b (\sb -> k (sa >> sb)))

*PrintScan> fprintf stdout fmt5 15 1.3 '!'
abc15cde1.3!*PrintScan>

The first thing I did last night was change String to
type ShowS = String -> String :

> intps :: F a b -> (ShowS -> a) -> b
> intps (FLit str) k = k (str++)
> intps FInt       k = \x -> k (shows x)
> intps FChr       k = \x -> k (x:)
> intps (FPP (PrinterParser pr _))  k = \x -> k (pr x ++)
> intps (a :^ b)   k = intps a (\sa -> intps b (\sb -> k (sa . sb)))

> sprintfs :: F ShowS b -> b
> sprintfs fmt = intps fmt id

Ideally PrinterParser would display using "ShowS" as well:

> data PrinterParser a
>   = PrinterParser (a -> ShowS) (String -> Maybe (a, String))

Or one could use instance witnesses via GADTs to wrap up Show:

> data F a b where
>     FSR  :: (Show b,Read b) => F a (b -> a)

But I think changing PrinterParser would result in simpler code.

Cheers,
   Chris