One day I was playing around with types and I came across this type:
data S m a = Nil | Cons a (m (S m a))
The idea being one could use generic functions with whatever monad m (of course m doesn't need to be a monad but my original idea was to be able to make mutable lists with some sort of monad m.) Anyway in attempting to define a generic show instance for the above datatype I finally came upon:
instance (Show a, Show (m (S m a))) => Show (S m a) where show Nil = "Nil" show (Cons x y) = "Cons " ++ show x ++ " " ++ show y
which boggles my mind. But hugs +98 and ghci -fallow-undecidable-instances both allow it to compile but when i try
show s
on
s = Cons 3 [Cons 4 [], Cons 5 [Cons 2 [],Cons 3 []]]
(btw yes we are "nesting" an arbitrary lists here! however structurally it really isnt much different from any tree datatype) we get ERROR - *** The type checker has reached the cutoff limit while trying to *** determine whether: *** Show (S [] Integer) *** can be deduced from: *** () *** This may indicate that the problem is undecidable. However, *** you may still try to increase the cutoff limit using the -c *** option and then try again. (The current setting is -c998) funny thing is apparently if you set -c to an odd number (on hugs) it gives *** The type checker has reached the cutoff limit while trying to *** determine whether: *** Show Integer *** can be deduced from: *** () why would it try to deduce Show integer? Anyway, is my instance declaration still a bit mucked up? Also, could there be a way to give a definition of show for S [] a? Heres my sample definition just in case anybody is curious.
myshow Nil = "Nil" myshow (Cons x y) = "Cons "++ show x ++ " [" ++ blowup y ++ "]" where blowup (x:y:ls) = myshow x ++ "," ++ blowup (y:ls) blowup (x:[]) = myshow x blowup [] = ""
Thanks, Jay Cox yet another non-academic person on this list.