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 <simonpj@microsoft.com> wrote:
| 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