seq as type class method

[Henning Thielemann]

On Fri, 6 Nov 2009, Stefan Holdermans wrote:

> What wasn't understood by then is that making a seq a type-class method is 
> not enough to recover parametricity, which was the goal. This is explained in 
> a recent paper by Daniel Seidel and Janis Voigtlaender:
>
> Daniel Seidel and Janis Voigtlaender. Taming selective strictness.
> In Stefan Fischer, Erik Maehle, and Ruediger Reischuk, editors,
> INFORMATIK 2009 – Im Focus das Leben, Beitraege der 39.
> Jahrestagung der Gesellschaft fuer Informatik e.V. (GI), 28.
> September – 2. Oktober, in Luebeck, volume 154 of Lecture Notes
> in Informatics, pages 2916–2930. GI, 2009.
>
> http://www.iai.uni-bonn.de/~jv/atps09.pdf

I'm still trying to understand the initial example. As far as I can see, 
in all of these examples an Eval constraint would prevent wrong 
application of implications/rules, but it might prevent application of a 
rule when it could be correctly applied. Is my conclusion valid?