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
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