-XStrict: Why some binders are not made strict?

[Simon Peyton Jones]

| 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 at haskell.org] On Behalf Of Roman
| > > | Cheplyaka
| > > | Sent: 11 December 2015 12:57
| > > | To: Johan Tibell <johan.tibell at gmail.com>
| > > | Cc: ghc-devs at haskell.org
| > > | Subject: Re: -XStrict: Why some binders are not made strict?
| > > |
| > > | On 12/11/2015 02:21 PM, Johan Tibell wrote:
| > > | > If we force strictness all the way down it's not really call-by-value
| > > | > either, because the caller doesn't know what to evaluate (I think).
| > > |
| > > | Not sure what you mean here.
| > > |
| > > | > In addition, making pattern matching strict in this way makes it hard
| to
| > > | > mix and match strict and lazy data types (e.g. Maybe), because using
| a
| > > | > lazy data type from another module will make it appear strict in your
| > > | > code (hurting modularity).
| > > |
| > > | I don't think this is a case about modularity. A lazy Maybe value
| > > | defined in a lazy module remains lazy; and you can pass it to lazy
| > > | functions without forcing it. Only when you pattern match on it *in the
| > > | strict module*, the evaluation happens.
| > > |
| > > | 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?
| > > |
| > > | Roman
| > >
| >
| >
| > _______________________________________________
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| > ghc-devs at haskell.org
| >
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