On 08-Feb-2001, Dylan Thurston
On Thu, Feb 08, 2001 at 09:41:56PM +1100, Fergus Henderson wrote:
One point that needs to be resolved is the interaction with default methods. Consider
class foo a where f :: ... f = ... f2 :: ... f2 = ...
class (foo a) => bar a where b :: ...
instance bar T where -- no definitions for f or f2 b = 42
Should this define an instance for `foo T'? (I think not.)
Whyever not?
Because too much Haskell code uses classes where the methods are defined in terms of each other: class Foo a where -- you should define either f or f2 f :: ... f = ... f2 ... f2 :: ... f2 = ... f ...
Because there is no textual mention of class Foo in the instance for Bar?
Right, and because allowing the compiler to automatically generate instances for class Foo without the programmer having considered whether those instances are OK is too dangerous.
Think about the case of a superclass with no methods; wouldn't you want to allow automatic instances in this case?
Yes.
I think Marcin has a better idea:
| So maybe there should be a way to specify that default definitions
| are cyclic and some of them must be defined?
--
Fergus Henderson