Invariants about UnivCo?

Sun Oct 8 00:19:01 UTC 2017

I can happily report some progress: I'm seeing no more Core lint errors!

1) Thank you both Richard and Simon for your pointers --
-fprint-typechecker-elaboration in particular was a revelation.

2) Simon, I intend to match the spirit of the favor you requested, but not
to the letter. My goal with this project is to write a typechecker plugin
for achieving row types _without_ editing GHC's source code. I'm keeping an
annotated bibliography of things I've studied (papers, guide/wiki/blog,
source Notes, etc). (It's nice to put a bunch of related notes in the same
text file!) I'm also logging my epiphanies, which I do intend to write-up
in some kind of document (probably on the dev wiki). I'm planning a section
for suggesting which Notes should be adjusted/expanded, but I don't
anticipate feeling comfortable enough to actually edit the Notes myself.
This is unfortunately just a hobby project. My intent is to offer you,
Richard, and other experts the details of what wasn't clear to me.

3) I confirmed that the lack of cobox uniques in the dump output was indeed
due to `ppr_co' deferring to `ppr @IfaceType'; it does that (at least) for
every coercion with a head of `TyConAppCo'. With a tiny kludgy patch I was
able to persist those uniques just for debugging purposes.

4) My top-level error is an "out of scope cobox" Lint error, but (once I
patched the dumper) the output of -fprint-typechecker-elaboration showed
sufficient bindings for all of the cobox occurrences, even the one that the
Lint error was flagging! Stymied, I finally did a -DDEBUG build of the
ghc-8.2.2-rc1 tag and used that. It ultimately lead to me finding my
mistakes. (New wisdom: always use a DEBUG build when authoring a plugin.
(... Duh.))

4a) ASSERT failures showed that I was invoking `substTy' without correctly
initializing the `InScopeSet'. I also was ignorant that I should be using
`extendTvSubstAndInScope' instead of just `extendTvSubst'. I don't think
this was relevant to my particular Lint error, but I fixed it if only to
see further ASSERT failures.

4b) Fixing my `InScopeSet's ASSERT failure revealed another:
`extendIdSubst' was being called with a CoVar! That's something that my
plugin code absolutely does not do, so at that point I knew that some
higher-level operation I was doing was knocking the rest of GHC's pipeline
off the rails. (In particular, I traced this ASSERT callstack to
extendIdSubst called from simpleOptExpr called from
mkInlineUnfoldingWithArity called from DsBinds. I stopped there.)

5) The first suspect turned out to be the culprit: I was using my plugin's
by-fiat coercions in the most naive possible way, always simply `EvCast ev
(fiatCoercion ty0 ty1)`. In particular, I was even doing that to create new
Given unlifted equality witnesses from existing Given unlifted equality
witnesses when simplifying Given constraints (e.g. for example reducing a
plugin-specific type family application on one side of an unlifted equality
type ~#).

In summary, I see no more ASSERT failures or Lint errors having now changed
my plugin to prefer `EvCoercion (TransCo U (TransCo co U))` to `EvCast
(EvCoercion co) U`. The actual diff excerpt is here:
https://github.com/nfrisby/coxswain/issues/3#issuecomment-334972227

I have not figured out exactly why that change matters, but it does seem a
reasonable preference to require. In particular, Note [Coercion evidence
terms] in TcEvidence.hs explicitly says that `EvCast (EvCoercion co1) co2`
is a valid form of evidence for ~#. So perhaps that Note deserves
elaboration --- I'm guessing the missing part may be specific to Givens?

-Nick

On Thu, Sep 21, 2017 at 2:59 AM Simon Peyton Jones <simonpj at microsoft.com>
wrote:

> Some thoughts
>
>
>
>    - Read Note [Coercion holes] in TyCoRep.
>
>
>
>    - As you’ll see, generally we don’t create value-bindings for
>    (unboxed) coercions of type t1 ~# t2.  (yes for boxed ones t1 ~ t2).
>    Reasons in the Note.  Exception: for superclasses of Givens we do create
>    (co :: a ~# b) = sc_sel1 d
>
> where d is some dictionary with a superclass of type (a ~# b).
>
>
>
> Side note: the use of “cobox” is wildly unhelpful.  These Ids are
> specifically *unboxed*!  I’m going to change it to just “co”.
>
>
>
>    - You appear to have bindings like[G]  cobox_a67J = CO Sym
>    cobox_a654.  That is suspicious.  Who is creating them?  It may not
>    actually be wrong but it’s suspicious.  The time it’d be outright wrong is
>    if you dropped the ev-binds on the floor.
>
>
>
> Ha!  runTcSEqualites makes up an ev_binds_var, and solves the equalities –
> but it should be the case that no value bindings end up in the
> ev_binds_var.  (reason: we are solving equalities in a type signature, so
> there is no place to put the evidence bindigns)   I suggest you add a
> DEBUG-only assertion to check this.
>
>
>
>    - Do -ddump-tc -fprint-typechecker-elaboration; that should show you
>    the evidence binds.
>
>
>
> Can I ask you a favour?  Separately from your branch, can you start a
> branch of small patches to GHC that include
>
>    - Extra assertions, such as that above
>    - *Notes* that explain things you wish you’d known earlier, with
>    references to those Notes from the places you were studying when you that
>    information would have been useful
>
>
>
> Richard and I know too much! – your learning curve is very valuable and I
> don’t want to lose it.
>
>
>
> Keeping this separate from your branch is useful : you can commit (via
> Phab) these updates right away, so they aren’t predicated on adding row
> types to GHC.
>
>
>
> Simon
>
>
>
> *From:* ghc-devs [mailto:ghc-devs-bounces at haskell.org] *On Behalf Of *Nicolas
> Frisby
> *Sent:* 19 September 2017 16:51
> *To:* ghc-devs at haskell.org
> *Subject:* Invariants about UnivCo?
>
>
>
> [I summarize with some direct questions at the bottom of this email.]
>
>
>
> I spent time last night trying to eliminate -dcore-lint errors from my
> record and variant library using the coxswain row types plugin. I made some
> progress, but I'm currently stuck, as discussed on this github Issue.
>
>
>
> https://github.com/nfrisby/coxswain/issues/3#issuecomment-330577609
> <https://na01.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgithub.com%2Fnfrisby%2Fcoxswain%2Fissues%2F3%23issuecomment-330577609&data=02%7C01%7Csimonpj%40microsoft.com%7Cde0675bbb584495a2f8008d4ff764c72%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C636414330952932223&sdata=lPLcpIlb%2BhivQdCUoVOPUgYDHeEDaMX660NQS%2BQyyBw%3D&reserved=0>
>
>
>
> Here's the relevant bit:
>
>
>
> The latest unresolved -dcore-lint error is an out-of-scope cobox co var.
> I'm certainly not creating it *directly* (there are no
> U(plugin:coxswain,... in the Core Lint warning), but I have to wonder if
> my somewhat loose use of UnivCo is violating some assumptions somewhere
> that's causing GHC to drop the co var binding or overlook this occurrence
> of it on a renaming/subst pass. I checked UnivCo for source comments
> looking for anything it should *not* be used for, but I didn't find an
> obvious explanation along those lines.
>
>
>
> I haven't yet been able to effectively distill the test case.
>
>
>
> I'm doing this all at -O0.
>
>
>
> With `-ddump-tc-trace`, I can see the offending cobox (cobox_a67M) is
> present in an "implication evbinds" listing after a "solveImplication end
> }" delimiter, but that's the last obvious binding of it.
>
>
>
>                          [G] cobox_a67J = CO Sym cobox_a654,
>
>                          [G] cobox_a67M
>
>                            = cobox_a67J `cast` U(plugin:coxswain,...)
>
>
>
> cobox_a654 is introduced by a GADT pattern match.
>
>
>
> I'm also not seeing obvious occurrences of cobox_a67M, but I think the
> reason is that I'm seeing several (Sym cobox) with no uniques printed (even
> with `-dppr-debug`). Those are probably the cobox in question, but I can't
> confirm.
>
>
>
> Questions:
>
>
>
> 1) Is there a robust way to ensure that covar's uniques are always
> printed? (Is the pprIface reuse  with a free cobox part of the issue here?)
>
>
>
> 2) Is my plugin asking for this kind of trouble by using UnivCo to cast
> coboxes?
>
>
>
> 3) If I spent the effort to create non-UnivCo coercions where possible,
> would that likely help? This is currently an "eventually" task, but I
> haven't seen an urgency for it yet. I could bump its priority if it might
> help. E.G. I'm using UnivCo to cast entire givens when all I'm doing is
> reducing a type family application somewhere "deep" within the given's
> predtype. I could, with considerable effort, instead wrap a single,
> localized UnivCo within a bunch of non-UnivCo "lifting" coercion
> constructors. Would that likely help?
>
>
>
> 3) Is there a usual suspect for this kind of situation where a cobox
> binding is seemingly dropped (by the typechecker) even though there's an
> occurrence of it?
>
>
>
> Thank you for your time. -Nick
>
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