Invariants about UnivCo?

Simon Peyton Jones simonpj at
Thu Sep 21 09:59:49 UTC 2017

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.


From: ghc-devs [mailto:ghc-devs-bounces at] On Behalf Of Nicolas Frisby
Sent: 19 September 2017 16:51
To: ghc-devs at
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.<>

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.


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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