Re: [Haskell-cafe] Restrictions on associated types for classes

[Conor McBride]

On 17 Dec 2009, at 15:31, Simon Peyton-Jones wrote:

Hmm.  If you have
  class (Diff (D f)) => Diff f where

then if I have
        f :: Diff f => ...
        f = e

then the constraints available for discharging constraints arising from e are

        Diff f
        Diff (D f)
        Diff (D (D f))
        Diff (D (D (D f)))
        ...

That's a lot of constraints.

But isn't it a bit like having an instance

  Diff f => Diff (D f)

?

Conor
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