On Thu, May 27, 2010 at 10:39 AM, Carlos Camarao
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