implicit params in instance contexts

[Ganesh Sittampalam]

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]