On Mon, 8 Oct 2001 15:03:15 -0700 Ashley Yakeley
At 2001-10-08 09:27, Diego Dainese wrote:
what are the reasons behind the rule stating that a type must not be declared as an instance of a particular class, more than once in the program?
It's so that the members of the class are unambiguous.
-- class C t where foo :: t -> Integer
instance C Bool where foo _ = 3;
instance C Bool where foo _ = 5;
ambiguous = foo True; --
OK, this is reasonable; but why are instance declarations always automatically exported and imported across modules boundary? This goes against information hiding. Consider this situation:
module M(T, f, g) where data T = ... ...
module N where import M
instance Eq T where ...
now, suppose that in a second revision of the module M, an instance of Eq is made for T; even if this instance is needed for internal use only, it outlaws the instance defined in the module N. I think this problem could be a real show-stopper for big programs... Why aren't instance declarations handled by the module system like every other symbol, so that a module can decide when to import an instance declaration and when to export it? Are there technical difficulties with this approach?
GHC has a flag that will turn the rule off.
I cannot find it! -- Diego To reply remove the 2 `x' from the address.