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
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