RE: GADT problems

[sent to list as well this time] On Mon, 2008-09-15 at 14:00 +0100, Simon Peyton-Jones wrote:

Ah -- you used an *existential* there! Yes, existentially-bound type variables are rigid. They stand for themselves, as it were.

That resolves the mystery -- but it existentials admittedly introduce a new complication

How should this be clarified?

For me, "existentially-bound variables are rigid" works well enough. They're a somewhat non-obvious case of 'coming from an annotation' though, and it does warrant mention. -- Philippa Cowderoy