it seems we can almost do this now without adding any new API calls, just have 'thawArray' and 'freezeArray' perform the check, and behave like 'unsafeThawArray' or 'unsafeFreezeArray' when they are the only reference. The compiler may even be able to statically replace some calls to thawArray or freezeArray with the unsafe versions with an extension of the rules syntax {-# RULES forall a | unique a . freezeArray a = unsafeFreezeArray a #-} this syntax actually came up in a previous discussion about wanting to fire rules only when the argument is a constant literal (though, you don't care about the particular constant value it is) I imagine infering the uniqueness of values shouldn't be that hard as a form of it is already done for avoiding thunk-updates. John -- John Meacham - ⑆repetae.net⑆john⑈