Ian Lynagh wrote:
Here I will just quote what Malcolm said in his original message:
The definition of seq is seq _|_ b = _|_ seq a b = b, if a/= _|_
In the circular expression let q = FinCons 3 q in q it is clear that the second component of the FinCons constructor is not _|_ (it has at least a FinCons constructor), and therefore it does not matter what its full unfolding is.
Well, in a sense, it's exactly the defining property of strict constructors that they are not automatically different from _|_. The translation
q = FinCons 3 q === (by Haskell 98 report 4.2.1/Strictness Flags/Translation q = (FinCons $ 3) $! q
is rather subtle: the first FinCons is a strict constructor whereas the second is "the real constructor". In other words, the translation loops as we could (should?) apply FinCons => \x y -> FinCons x $! y => \x y -> (\x' y' -> FinCons x' $! y') x $! y => ... ad infinitum.
and in his recent e-mail to me:
Yes, I still think this is a reasonable interpretation of the Report. I would phrase it as "After evaluating the constructor expression to WHNF, any strict fields contained in it are also be guaranteed to be in WHNF."
Referring to WHNF would break the report's preference of not committing to a particular evaluation strategy. That's already a good reason to stick with FinCons 3 _|_ = _|_. Besides, having let q = FinCons 3 q in q not being _|_ crucially depends on memoization. Even with the characterization by WHNF, let q x = FinCons 3 (q x) in q () is _|_. Regards, apfelmus