Template Haskell determinism

Michael Sloan mgsloan at gmail.com
Wed Jun 29 18:46:48 UTC 2016 

Also, revisiting this issue, I don't think it is worth solving, just
worth documenting.

Why?  Because TH already lets you do lots of incorrect things.  TH
already allows you to shoot yourself in the foot all over the place,
and that's ok.  I'd much rather it be a dangerous power tool than a
weak safe tool.

When writing TH, you are writing something that's part of the
compiler, and the code can often resemble the sort of involved in the
middle bits of a compiler.  For the cases where stable Ord on Name
matters even for local names, you're definitely writing something
that's a fairly fancy, nearly compiler-like transformation.  Concerns
like the stability of Names should be handled by the TH user.

-Michael

On Wed, Jun 29, 2016 at 10:41 AM, Michael Sloan <mgsloan at gmail.com> wrote:
> No, NameU and NameL both lack package key / package id.
>
> -Michael
>
> On Wed, Jun 29, 2016 at 7:34 AM, Edward Z. Yang <ezyang at mit.edu> wrote:
>> No, nameBase is not the right thing to use here; you also need the
>> unit ID (in GHC 8.0 parlance; package key in GHC 7.10; package id
>> in GHC 7.8 and before).  If you have that information, then
>> GHC establishes an invariant that if two names compare stably equal,
>> then the uniques associated with them are the same.
>>
>> Edward
>>
>> Excerpts from Michael Sloan's message of 2016-06-10 17:16:44 -0400:
>>> Hey, sorry for not getting back to this sooner!
>>>
>>> Perhaps I should have added the following to my list of goals in contention:
>>>
>>> (3) (==) shouldn't yield True for Names that have different unique ids.
>>>
>>> We can only have stable comparisons if goal (3) isn't met, and two
>>> different unique Names would be considered to be equivalent based on the
>>> nameBase.  This is because Ord is a total order, not a partial order.  As
>>> described in my prior email, PartialOrd could be added, but it'd be
>>> inconvenient to use with existing Ord based containers.
>>>
>>> -Michael
>>>
>>> On Sun, Jun 5, 2016 at 10:15 AM, Edward Z. Yang <ezyang at mit.edu> wrote:
>>>
>>> > I must admit, I am a bit confused by this discussion.
>>> >
>>> > It is true that every Name is associated with a Unique.  But you don't
>>> > need the Unique to equality/ordering tests; the names also contain
>>> > enough (stable) information for stable comparisons of that sort.  So
>>> > why don't we expose that instead of the Unique?
>>> >
>>> > Edward
>>> >
>>> > Excerpts from Michael Sloan's message of 2016-06-04 18:44:03 -0700:
>>> > > On Thu, Jun 2, 2016 at 4:12 AM, Simon Peyton Jones <
>>> > simonpj at microsoft.com>
>>> > > wrote:
>>> > >
>>> > > > If names get different ordering keys when reified from different
>>> > modules
>>> > > > (seems like they'd have to, particularly given ghc's "-j"), then we
>>> > end up
>>> > > > with an unpleasant circumstance where these do not compare as equal
>>> > > >
>>> > > >
>>> > > >
>>> > > > The I believe that global, top level names (NameG) are not subject to
>>> > this
>>> > > > ordering stuff, so I don’t think this problem can occur.
>>> > > >
>>> > >
>>> > > True, top level names are NameG.  The reified Info for a top level Dec
>>> > may
>>> > > include NameU, though.  For example, the type variables in 'Maybe' are
>>> > > NameU:
>>> > >
>>> > > $(do TyConI (DataD _ _ [KindedTV (Name _ nf) _] _ _ _) <- reify ''Maybe
>>> > >      lift (show nf))
>>> > >
>>> > > The resulting expression is something like "NameU 822083586"
>>> > >
>>> > > > This is a breaking change and it doesn't fix the problem that
>>> > NameFlavour
>>> > > > is
>>> > > >
>>> > > > not abstract and leaks the Uniques. It would break at least:
>>> > > >
>>> > > >
>>> > > >
>>> > > > But why is NameU exposed to clients?   GHC needs to know, but clients
>>> > > > don’t.  What use are these packages making of it?
>>> > > >
>>> > >
>>> > > It's being leaked in the public inteface via Ord.  The Eq instance is
>>> > fine,
>>> > > because these are Uniques, so the results should be consistent.
>>> > >
>>> > > There are two goals in contention here:
>>> > >
>>> > > 1) Having some ordering on Names so that they can be used in Map or Set
>>> > > 2) Having law-abiding Eq / Ord instances.  We'd need a 'PartialOrd' to
>>> > > really handle these well.  In that case, the ordering would be based on
>>> > > everything but the NameU int, but 'Eq' would still follow it
>>> > >
>>> > > A few ideas for different approaches to resolving this:
>>> > >
>>> > > 1) Document it.  Less appealing than fixing it in the API, but still
>>> > would
>>> > > be good.
>>> > >
>>> > > 2) Remove the 'Ord' instance, and force the user to pick 'NamePartialOrd'
>>> > > newtype (partial ord on the non-unique info), or 'UnstableNameOrd'
>>> > newtype
>>> > > (current behavior).  A trickyness of this approach is that you'd need
>>> > > containers that can handle (PartialOrd k, Eq k) keys.  In lots of cases
>>> > > people are using the 'Ord' instance with 'Name's that are not 'NameU', so
>>> > > this would break a lot of code that was already deterministic.
>>> > >
>>> > > 3) Some approaches like this ordering key, but I'm not sure how it will
>>> > > help when comparing NameUs from different modules?
>>> > >
>>> > > > S
>>> > > >
>>> > > >
>>> > > >
>>> > > >
>>> > > >
>>> > > > *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of
>>> > *Michael
>>> > > > Sloan
>>> > > > *Sent:* 02 June 2016 02:07
>>> > > > *To:* Bartosz Nitka <niteria at gmail.com>
>>> > > > *Cc:* ghc-devs Devs <ghc-devs at haskell.org>
>>> > > > *Subject:* Re: Template Haskell determinism
>>> > > >
>>> > > >
>>> > > >
>>> > > > +1 to solving this.  Not sure about the approach, but assuming the
>>> > > > following concerns are addressed, I'm (+1) on it too:
>>> > > >
>>> > > >
>>> > > >
>>> > > > This solution is clever!  However, I think there is some difficulty to
>>> > > > determining this ordering key.  Namely, what happens when I construct
>>> > the
>>> > > > (Set Name) using results from multiple reifies?
>>> > > >
>>> > > >
>>> > > >
>>> > > > One solution is to have the ordering key be a consecutive supply that's
>>> > > > initialized on a per-module basis.  There is still an issue there,
>>> > though,
>>> > > > which is that you might store one of these names in a global IORef
>>> > that's
>>> > > > used by a later TH splice.  Or, similarly, serialize the names to a
>>> > file
>>> > > > and later load them.  At least in those cases you need to use 'runIO'
>>> > to
>>> > > > break determinism.
>>> > > >
>>> > > >
>>> > > >
>>> > > > If names get different ordering keys when reified from different
>>> > modules
>>> > > > (seems like they'd have to, particularly given ghc's "-j"), then we
>>> > end up
>>> > > > with an unpleasant circumstance where these do not compare as equal.
>>> > How
>>> > > > about having the Eq instance ignore the ordering key?  I think that
>>> > mostly
>>> > > > resolves this concern.  This implies that the Ord instance should also
>>> > > > yield EQ and ignore the ordering key, when the unique key matches.
>>> > > >
>>> > > >
>>> > > >
>>> > > > One issue with this is that switching the order of reify could
>>> > > > unexpectedly vary the behavior.
>>> > > >
>>> > > >
>>> > > >
>>> > > > Does the map in TcGblEnv imply that a reify from a later module will
>>> > get
>>> > > > the same ordering key?  So does this mean that the keys used in a given
>>> > > > reify depend on which things have already been reified?  In that case,
>>> > then
>>> > > > this is also an issue with your solution.  Now, it's not a big problem
>>> > at
>>> > > > all, just surprising to the user.
>>> > > >
>>> > > >
>>> > > >
>>> > > >
>>> > > >
>>> > > > If the internal API for Name does change, may as well address
>>> > > > https://ghc.haskell.org/trac/ghc/ticket/10311 too.  I agree with SPJ's
>>> > > > suggested solution of having both the traditional package identifier
>>> > and
>>> > > > package keys in 'Name'.
>>> > > >
>>> > > >
>>> > > >
>>> > > > -Michael
>>> > > >
>>> > > >
>>> > > >
>>> > > > On Tue, May 31, 2016 at 6:54 AM, Bartosz Nitka <niteria at gmail.com>
>>> > wrote:
>>> > > >
>>> > > > Template Haskell with its ability to do arbitrary IO is
>>> > non-deterministic
>>> > > > by
>>> > > >
>>> > > > design. You could for example embed the current date in a file. There
>>> > is
>>> > > >
>>> > > > however one kind of non-deterministic behavior that you can trigger
>>> > > >
>>> > > > accidentally. It has to do with how Names are reified. If you take a
>>> > look
>>> > > > at
>>> > > >
>>> > > > the definition of reifyName you can see that it puts the assigned
>>> > Unique
>>> > > > in a
>>> > > >
>>> > > > NameU:
>>> > > >
>>> > > >
>>> > > >
>>> > > >   reifyName :: NamedThing n => n -> TH.Name
>>> > > >
>>> > > >   reifyName thing
>>> > > >
>>> > > >     | isExternalName name = mk_varg pkg_str mod_str occ_str
>>> > > >
>>> > > >     | otherwise           = TH.mkNameU occ_str (getKey (getUnique
>>> > name))
>>> > > >
>>> > > >     ...
>>> > > >
>>> > > > NameFlavour which NameU is a constructor of has a default Ord instance,
>>> > > > meaning
>>> > > >
>>> > > > that it ends up comparing the Uniques. The relative ordering of
>>> > Uniques is
>>> > > > not
>>> > > >
>>> > > > guaranteed to be stable across recompilations [1], so this can lead to
>>> > > >
>>> > > > ABI-incompatible binaries.
>>> > > >
>>> > > >
>>> > > >
>>> > > > This isn't an abstract problem and it actually happens in practice. The
>>> > > >
>>> > > > microlens package keeps Names in a Set and later turns that set into a
>>> > > > list.
>>> > > >
>>> > > > The results have different orders of TyVars resulting in different ABI
>>> > > > hashes
>>> > > >
>>> > > > and can potentially be optimized differently.
>>> > > >
>>> > > >
>>> > > >
>>> > > > I believe it's worth to handle this case in a deterministic way and I
>>> > have
>>> > > > a
>>> > > >
>>> > > > solution in mind. The idea is to extend NameU (and potentially NameL)
>>> > with
>>> > > > an
>>> > > >
>>> > > > ordering key. To be more concrete:
>>> > > >
>>> > > >
>>> > > >
>>> > > > -   | NameU !Int
>>> > > >
>>> > > > +   | NameU !Int !Int
>>> > > >
>>> > > >
>>> > > >
>>> > > > This way the Ord instance can use a stable key and the problem reduces
>>> > to
>>> > > >
>>> > > > ensuring the keys are stable. To generate stable keys we can use the
>>> > fact
>>> > > > that
>>> > > >
>>> > > > reify traverses the expressions in the same order every time and
>>> > > > sequentially
>>> > > >
>>> > > > allocate new keys based on traversal order. The way I have it
>>> > implemented
>>> > > > now
>>> > > >
>>> > > > is to add a new field in TcGblEnv which maps Uniques to allocated keys:
>>> > > >
>>> > > >
>>> > > >
>>> > > > +        tcg_th_names :: TcRef (UniqFM Int, Int),
>>> > > >
>>> > > >
>>> > > >
>>> > > > Then the reifyName and qNewName do the necessary bookkeeping and
>>> > translate
>>> > > > the
>>> > > >
>>> > > > Uniques on the fly.
>>> > > >
>>> > > >
>>> > > >
>>> > > > This is a breaking change and it doesn't fix the problem that
>>> > NameFlavour
>>> > > > is
>>> > > >
>>> > > > not abstract and leaks the Uniques. It would break at least:
>>> > > >
>>> > > >
>>> > > >
>>> > > > - singletons
>>> > > >
>>> > > > - th-lift
>>> > > >
>>> > > > - haskell-src-meta
>>> > > >
>>> > > > - shakespeare
>>> > > >
>>> > > > - distributed-closure
>>> > > >
>>> > > >
>>> > > >
>>> > > > I'd like to get feedback if this is an acceptable solution and if the
>>> > > > problem
>>> > > >
>>> > > > is worth solving.
>>> > > >
>>> > > >
>>> > > >
>>> > > > Cheers,
>>> > > >
>>> > > > Bartosz
>>> > > >
>>> > > >
>>> > > >
>>> > > > [1]
>>> > > >
>>> > https://ghc.haskell.org/trac/ghc/wiki/DeterministicBuilds#NondeterministicUniques
>>> > > >
>>> > > >
>>> > > > _______________________________________________
>>> > > > ghc-devs mailing list
>>> > > > ghc-devs at haskell.org
>>> > > > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs
>>> > > > <
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>>> > >
>>> > > >
>>> > > >
>>> > > >
>>> >

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