On Fri, 6 Nov 2009, Stefan Holdermans wrote:
Henning,
I have read in the History of Haskell, that originally 'seq' should be a method of a type class, that can be automatically derived by a 'deriving' clause. It was also mentioned that this clean solution was dropped because of particular experiences with developing a parser. However the arguments appeared to me, like these were problems of debugging.
If I understood it correctly, the problem was more general than just debugging. Every introduction of seq in a function could result in the requirement to also adapt the type signatures of calling functions.
Sure, but why was this a problem? Because they had to re-arrange a lot, and had to change the signature each time. But once that re-arrangement settles, it would be nice to have the Seq type constraint, right?
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.
I'll have a look into it. Thanks for the hint!