Re: Proposal to solve Haskell's MPTC dilemma

27 May 2010

      On Thu, May 27, 2010 at 5:43 PM, David Menendez  wrote:
...
On Thu, May 27, 2010 at 10:39 AM, Carlos Camarao
 wrote:
...
Isaac Dupree:
...
Your proposal appears to allow /incoherent/ instance selection.
This means that an expression can be well-typed in one module, and
well-typed in another module, but have different semantics in the
two modules.  For example (drawing from above discussion) :
module C where
class F a b where f :: a -> b
class O a where o :: a
module P where
import C
instance F Bool Bool where f = not
instance O Bool where o = True
k :: Bool
k = f o
module Q where
import C
instance F Int Bool where f = even
instance O Int where o = 0
k :: Bool
k = f o
module Main where
import P
import Q
-- (here, all four instances are in scope)
main = do { print P.k ; print Q.k }
-- should result, according to your proposal, in
-- False
-- True
-- , am I correct?
If qualified importation of k from both P and from Q was specified, we
would have two *distinct* terms, P.k and Q.k.
I think Isaac's point is that P.k and Q.k have the same definition (f
o). If they don't produce the same value, then referential
transparency is lost.
--
Dave Menendez 
<http://www.eyrie.org/~zednenem/ http://www.eyrie.org/%7Ezednenem/>
The definitions of P.k and Q.k are textually the same but the contexts are
different. "f" and "o" denote distinct values in P and Q. Thus, P.k and Q.k
don't have the same definition.

Thanks for the clarification. I thought the point was about coherence.

Cheers,

Carlos