[Haskell-cafe] Increasing Haskell modularity

[Jason McCarty]

Here's another fun question. The following module is legal in GC, and
does what you would expect.

\begin{code}
{-# LANGUAGE FlexibleInstances, OverlappingInstances #-}
module Parametric where

class IsInt a where
  isInt :: a -> Bool

instance IsInt a where isInt _ = False
instance IsInt Int where isInt _ = True

myList1 :: IsInt a => a -> [a]
myList1 x = if isInt x then [x] else [x, x]
\end{code}

Would the following module be legal under the proposal? If so,
parametricity is gone, because you can define typecase without any
constraints.

\begin{code}
{-# LANGUAGE ScopedTypeVariables, FlexibleInstances, OverlappingInstances, LocalInstances #-}
module NotParametric where
import Parametric

myList2 :: a -> [a]
myList2 (x :: a) =
  let instance IsInt a where isInt _ = False in
  let instance IsInt Int where isInt _ = True in
  myList1 x
  -- the IsInt constraint should be discharged by the local instances
\end{code}

The only way I see of avoiding this is if local instances can't
discharge a constraint; that is, a local instance can only shadow an
existing instance. It can't create an instance for a type that didn't
already have one. Or maybe there's just a bad interaction with
flexible/overlapping instances, but the extension seems less useful
without these.

-- 
Jason McCarty <jmccarty at sent.com>