I still don't think you can do it just from the default method's type. A
typical case is the following:
class C a where
op :: a -> Int
default op :: (Generic a, GC (Rep a)) => a -> Int
When giving an instance C [a], you might well find out that you need C a
=>, but this is not something
you can see in the type of the default method; it follows only after the
expansion of Rep [a] and resolving
the GC constraint a number of times.
Best regards,
Pedro
On Fri, Jun 17, 2016 at 12:43 PM, Simon Peyton Jones
| My question is then: why does DeriveAnyClass take the bizarre approach | of co-opting the DeriveFunctor algorithm? Andres, you originally | proposed this in #7346 [2], but I don't quite understand why you | wanted to do it this way. Couldn't we infer the context simply from | the contexts of the default method type signatures?
That last suggestion makes perfect sense to me. After all, we are going to generate an instance looking like
instance .. => C (T a) where op1 = <default-op1> op2 = <default-op2>
so all we need in ".." is enough context to satisfy the needs of <default-op1> etc.
Well, you need to take account of the class op type sig too:
class C a where op :: Eq a => a -> a default op :: (Eq a, Show a) => a -> a
We effectively define default_op :: (Eq a, Show a) => a -> a
Now with DeriveAnyClass for lists, we effectively get
instance ... => C [a] where op = default_op
What is ..? Well, we need (Eq [a], Show [a]); but we are given Eq [a] (because that's op's instantiated type. So Show a is all we need in the end.
Simon _______________________________________________ ghc-devs mailing list ghc-devs@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs