Error when ($) is used, but no error without

[Brian Hulley]

Brian Hulley wrote:
> In contrast, (forall s. ST s a) is not a true existential, since we
> are here just simulating an existential by making use of the ST
> constructor to store the info about what 's' was used so that it can
> be recovered by pattern matching.

Sorry the above paragraph is wrong ;-)
(But see my previous post for how to simulate existentials by a data type 
(to hide the forall) and use them with $ if this is what you wanted to do)

Regards, Brian.