Thu, 8 Feb 2001 21:41:56 +1100, Fergus Henderson
Should this define an instance for `foo T'? (I think not.)
How about if the instance declaration is changed to
instance bar T where f = 41 -- no definition for f2 b = 42
? (In that case, I think it should.)
I don't like the idea of treating the case "no explicit definitions were given because all have default definitions which are OK" differently than "some explicit definitions were given". When there is a superclass, it must have an instance defined, so if we permit such thing at all, I would let it implicitly define all superclass instances not defined explicitly, or something like that. At least when all methods have default definitions. Yes, I know that they can be mutually recursive and thus all will be bottoms... So maybe there should be a way to specify that default definitions are cyclic and some of them must be defined? It is usually written in comments anyway, because it is not immediately visible in the definitions. If not formally in the language (now any method definition can be omitted even if it has no default!), then perhaps the compiler could detect most cases when methods are defined in terms of one another and give a warning. Generally the compiler could warn if the programmer has written bottom in an unusual way. For example f x = g some_expression g x = f some_expression is almost certainly a programmer error. -- __("< Marcin Kowalczyk * qrczak@knm.org.pl http://qrczak.ids.net.pl/ \__/ ^^ SYGNATURA ZASTÊPCZA QRCZAK