On 16/10/2010, at 12:00, Max Bolingbroke wrote:
Hi Cafe,
I've run across a problem with my use of existential data types, whereby programs using them are forced to become too strict, and I'm looking for possible solutions to the problem.
I'm going to explain what I mean by using a literate Haskell program. First, the preliminaries:
{-# LANGUAGE ExistentialQuantification #-} import Control.Arrow (second) import Unsafe.Coerce
Let's start with a simple example of an existential data type:
data Stream a = forall s. Stream s (s -> Maybe (a, s))
[...] In fact, to define a correct cons it would be sufficient to have some function (eta :: Stream a -> Stream a) such that (eta s) has the same semantics as s, except that eta s /= _|_ for any s.
That's easy. eta :: Stream a -> Stream a eta s = Stream s next where next (Stream s next') = case next' s of Just (x,s') -> Just (x,Stream s' next') Nothing -> Nothing Making GHC optimise stream code involving eta properly is hard :-) Roman