2013-04-26 04:31, wren ng thornton skrev:
On 4/25/13 9:49 PM, Dan Doel wrote:
I don't really think they're worth saving in general, though. I haven't
missed them, at least.
The thing I've missed them for (and what I believe they were originally
designed for) is adding constraints to derived instances. That is, if I
data Bar a => Foo a = ... deriving Baz
Then this is equivalent to:
data Foo a = ...
instance Bar a => Baz (Foo a) where ...
where the second ellipsis is filled in by the compiler. Now that these
constraints have been removed from the language, I've had to either (a)
allow instances of derived classes which do not enforce sanity
constraints, or (b) implement the instances by hand even though they're
I think standalone deriving solves this:
deriving instance Bar a => Baz (Foo a)
Haskell-Cafe mailing list