| Given the upcoming 8.0 feature freeze I think the correct approach for
| 8.0 is to document the current implementation (I'll try to do that this
| week).
Yes, that's right. Thanks!
Simon
|
| It would probably be good if interested parties would document their
| input in a ticket.
|
| Cheers,
| --Adam
|
|
| On Sat, 12 Dec 2015, at 12:55 AM, Roman Cheplyaka wrote:
| > Thanks Simon, this is an interesting and compelling interpretation. But
| > I'm wondering whether it is enough to specify the dynamic semantics
| > unambiguously.
| >
| > Two examples:
| >
| > 1.
| >
| > let _ = undefined in ()
| >
| > Intuitively, since we are talking about /strictness/, this should
| > evaluate to bottom. However, it seems that your rule also admits () as
| > an answer; it is equivalent to () under lazy evaluation *and* it does
| > not create any thunks.
| >
| > 2.
| >
| > f g = g undefined
| >
| > When compiled lazily, this code doesn't construct any thunks:
| >
| > f = \r srt:SRT:[02v :-> undefined] [g_suM] g_suM undefined;
| >
| > So, under your rule, this is an admissible code for -XStrict, too. But
| > we can hardly call it strict.
| >
| > On 12/12/2015 12:38 AM, Simon Peyton Jones wrote:
| > > | As I said, I prefer this semantics mainly because it's easier to
| > > | explain: all variables (and underscores) bound in a strict module refer
| > > | to WHNF values. Do you have a similarly simple explanation for the
| > > | semantics you're suggesting?
| > >
| > > Here's one, which is roughly what the current implementation does (modulo
| bugs):
| > >
| > > * Code compiled under -XStrict constructs no thunks.
| > >
| > > So consider
| > >
| > > module M1 where data T = C Int Int
| > > module M2 where f n = C (n+1) (n-1)
| > > module M3 where g x = let C y z = f x in ...
| > >
| > > Look at M3. Usually we'd get a thunk for (f 4), but not with -XStrict.
| But even with -XStrict in M3, y,z might be bound to thunks.
| > >
| > > If you compile M2 with -XStrict, function f won't build thunks for (n+1),
| (n-1) but will evaluate them instead.
| > >
| > > If you compile M1 with StrictData, then C is made strict, so again M2
| will build no thunks even if M2 was compiled without -XStrict.
| > >
| > > I quite like this design. It's not clear to me that anything useful is
| gained by forcing y and z in M3 before evaluating the body "...".
| > >
| > >
| > > So Roman's design makes sense, but so does the implemented design (modulo
| any bugs). The trouble is that the implemented design is not well described.
| > >
| > > Simon
| > >
| > > | -----Original Message-----
| > > | From: ghc-devs [mailto:ghc-devs-bounces@haskell.org] On Behalf Of Roman
| > > | Cheplyaka
| > > | Sent: 11 December 2015 12:57
| > > | To: Johan Tibell