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