implicit params in instance contexts

[Krzysztof Gogolewski]

GHC ticket:
http://ghc.haskell.org/trac/ghc/ticket/7624


2013/7/16 Ganesh Sittampalam <ganesh at earth.li>

> Hi,
>
> It seems that from GHC 7.4, the prohibition on implicit parameter
> constraints in instance declarations has been relaxed. The program below
> gives the error "Illegal constraint ?fooRev::Bool" in GHC 7.2.1 but
> loads fine in GHC 7.4.2 and GHC 7.6.2.
>
> I can't spot anything about this in the release notes, and the
> documentation
> (
> http://www.haskell.org/ghc/docs/7.6.3/html/users_guide/other-type-extensions.html#idp49069584
> )
> still says "You can't have an implicit parameter in the context of a
> class or instance declaration."
>
> So I wonder if this happened by accident, perhaps as part of the
> ConstraintKinds work or similar?
>
> I've wanted this feature a few times so if it's going to stay I might
> start using it. However it is a bit dangerous, so if it was added by
> accident it might warrant some discussion before deciding to keep it.
> For example as the value "set2" below shows, it can be used to violate
> datatype invariants.
>
> Cheers,
>
> Ganesh
>
>
> {-# LANGUAGE ImplicitParams #-}
> module Ord where
>
> import Data.Set ( Set )
> import qualified Data.Set as Set
>
> newtype Foo = Foo Int
>     deriving (Eq, Show)
>
> instance (?fooRev :: Bool) => Ord Foo where
>     Foo a `compare` Foo b =
>         if ?fooRev then b `compare` a else a `compare` b
>
> set1 = let ?fooRev = False in Set.fromList [Foo 1, Foo 3]
>
> set2 = let ?fooRev = True in Set.insert (Foo 2) set1
> -- Ord> set2
> -- fromList [Foo 2,Foo 1,Foo 3]
>
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