On Tue, Jul 19, 2011 at 11:14 PM, yi huang wrote:

2011/7/20 Eugene Kirpichov

...

reallyUnsafePointerEq#, and it really is as unsafe as it sounds :)

Why is it so unsafe? i can't find any documentation on it. I think always compare pointer first is a good optimization.

Any number of conditions could make it fail, but most especially GC. It is almost always better to explicitly model a property when you want it. Uniqueness based on construction, for example, can be modeled with a monad and protected by an ADT.

module Unique ( HasUnique(..), Unique, newUnique, openUnique) where class HasUnique m uid where newUniqueID :: m uid

data Unique uid a = U !uid a

newUnique :: (HasUnique m uid, Monad m) => a -> m (Unique uid a) newUnique a = newUniqueID >>= \ uid -> return (U uid a)

openUnique :: (Unique m a) -> a openUnique (U _ x) = x

instance (Eq uid) => Eq (Unique uid a) instance (Ord uid) => Ord (Unique uid a)