I tend to agree with this (I was just waiting for someone to express this viewpoint, so I would have less typing to do). I agree that explaining this difference to a nonuser is often nontrivial, but after a few months and some deliberation, it's started to make sense to me :). That said, if adding 'exists' to the data constructor is a precursor to having existential types that aren't datatypes, then I'm completely in favor of it :). - Hal -- Hal Daume III | hdaume@isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume On Thu, 17 Apr 2003, George Russell wrote:
1) The problem is basically whether you regard the type constructor as a way of constructing a type or of taking it apart. So using the current notation
data A = forall x . B x => C x
forall seems sensible if you think of a constructor as a way of constructing a type (for all x that's a B, C gives you an A), but not if want to take it apart, where your attitude is (there exists an x that's a B that's inside this C).
I really can't see any reason for preferring the viewpoint of the constructor to the destructor, or vice-versa. So my vote would be for keeping things as they are.
2) I currently existential datatypes 17 times or thereabout. Replacing "forall" by "exists" through all these would be annoying, but not take very long. Also there seem to be two modules which use the variable name "exists", but while having to replace that would be annoying, again it wouldn't take very long. Altogether perhaps 20 minutes, if that. So while the suggested change-over would be a pain, it wouldn't be a major pain.
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