Tue, 17 Jul 2001 12:08:51 +0200 (MEST), Tobias Haeberlein <T.Haeberlein@gmx.de> pisze:
show (Succ ( ... (Succ Zero)..)) = n (where n is the number of Succ's) and
show (Succ ( ... (Succ x)..)) = show x (when x != Zero)
This is not possible in Haskell 98 - you can't overload an operation on "any type except this one". There are no "negative assertions" or "exceptions". It might be possible in Hugs run with -98 option (define "instance Count a" and "instance Count a => Count (Bind a)" and "instance Count a => Show (Bind a)", where class Count has a function of type a->Int), but I would not be surprised if it didn't work correctly in all cases, e.g. if the answer was too small sometimes. Overlapped instances is not a well behaved concept. -- __("< Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK