Yep, that's the current question: why does preferring `EvCoercion (TransCo UnivCo (TransCo co UnivCo))` to `EvCast (EvCoercion co) UnivCo` seem to matter? In my scenario, `co` is the evidence for a Given equality type. And the coercion I'm building is also a Given constraint's evidence -- I'm simplifying Givens.
The only hard indication I currently have of what "goes wrong" is the ASSERT failure described in the previous email. I'm planning to spend some time investigating. I would appreciate any cycles you spend on it! On Sun, Oct 8, 2017, 18:53 Richard Eisenberg <r...@cs.brynmawr.edu> wrote: > Thanks for this status report. If I'm to boil it down to the question you > seem to be asking: What does changing EvCast ... to EvCoercion ... fix the > problem? I'm not sure of the answer at this point, but I want to make sure > I understand the question before I go digging for an answer. It's always > possible a Note is wrong! > > Thanks for this! > > Richard > > On Oct 7, 2017, at 8:19 PM, Nicolas Frisby <nicolas.fri...@gmail.com> > wrote: > > 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 <simo...@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-boun...@haskell.org] *On Behalf Of *Nicolas >> Frisby >> *Sent:* 19 September 2017 16:51 >> *To:* ghc-devs@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 >> > _______________________________________________ > > > ghc-devs mailing list > ghc-devs@haskell.org > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs > >
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