Ah -- you used an *existential* there! Yes, existentially-bound type variables are rigid. They stand for themselves, as it were.
How should this be clarified?
I'd leave it. I wanted a simple set of rules stating "_if_ you provide the following type signatures your code _will_ compile", which is what you currently provide, albeit I interpreted 'result' slightly differently. If people want to learn where these type signatures can be omitted (because a type is already rigid) people can follow the papers, or learn by trial and error. Thanks Neil ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================