Hello, On Tue, 27 Dec 2005 16:39:34 +0000, Chris Kuklewicz wrote:
Happy Holidays,
I was wondering if it this small snippet of code could be altered to require fewer OPTIONS -fallow-... switches.
Here is a partial solution using only -fglasgow-exts:
module MyShow where
class MyShow t where swim :: String -> t
instance MyShow String where swim = id
instance (Show a, MyShow t) => MyShow (a -> t) where swim s x = swim (s ++ show x)
test foo = foo " and " 7 " are "
main = do putStrLn $ swim "Hello" " " "World #" [17,18,19] "!" putStrLn $ test (swim "I also think " [4,5,6]) "cool" "."
The problem of this solution is that, because we are using show in the definition of swim for (a -> t), the strings are beeing printed with extra "\"". *MyShow> main Hello" ""World #"[17,18,19]"!" I also think [4,5,6]" and "7" are ""cool""." You can, ofcourse, add a new instance for (String -> t) but that will cost you -fallow-incoherent-instances and -fallow-overlapping-instances.
instance MyShow t => MyShow (String -> t) where swim s x = swim (s ++ x)
Could this be improved by TypeEq machinery? Is there a Haskell 98 way to make myShow work this way?
The problem of the previous solution is that you do not want, in the general case, use the default instance of Show for Strings. An alternative is to just introduce a newtype S (isomorphic to String) that allows you to handle Strings differently:
module MyShow where
class MyShow t where swim :: S -> t
newtype S = S {unS :: String}
instance MyShow S where swim = id
instance Show S where show = unS
instance (Show a, MyShow t) => MyShow (a -> t) where swim s x = swim (S $ unS s ++ show x)
putSLn = putStrLn . unS
test foo = foo (S " and ") 7 (S " are ")
main = do putSLn $ swim (S "Hello") (S " ") (S "World #") [17,18,19] (S "!") putSLn $ test (swim (S "I also think ") [4,5,6]) (S "cool") (S ".")
By using the newtype S instead of Haskell String, you obtain an Haskell 98 solution. I am not sure if this is good enough for you but it seems a good compromise. Hope it helps! Cheers, Bruno