- ghc-devs - Haskell.org

Re: gitlab upgrade - change in approval system
by Ryan Scott 18 May '19

18 May '19

To be clear, your workaround is precisely what I used to do, but that is no longer working after the GitLab upgrade. I can add myself as a reviewer, but only to a group besides the default approvers list. Because Marge only counts approvals from those who belong to the default approvers list, my approvals don't count anymore. Ryan S.

1 0

Re: gitlab upgrade - change in approval system
by Ryan Scott 18 May '19

18 May '19

I would definitely appreciate changing Marge's behavior back to the old one. As things stand currently, I am unable to approve anything that I'm not explicitly listed as a reviewer for. See here [1] for an example of where this has happened. Ryan S. ----- [1] https://gitlab.haskell.org/ghc/ghc/merge_requests/559#note_198714

2 1

Bug or feature?
by Iavor Diatchki 17 May '19

17 May '19

Hello, I've encountered some odd behavior related to quantification and type families, but I am not sure if it is a bug or by design. Here is an example which works fine: > {-# Language ExistentialQuantification, RankNTypes, TypeFamilies #-} > > data T a b = T a > > t :: T Bool a > t = T True > > data Q1 = forall a. Q1 (forall b. T a b) > > q1 :: Q1 > q1 = Q1 t Now, suppose we wanted to constraint the universally quantified type of the field. This also works fine: > data Q2 = forall a. Q2 (forall b. (b ~ Int) => T a b) > > q2 :: Q2 > q2 = Q2 t However, things break down if the restriction involves a type family: > type family F a > > data Q3 = forall a. Q3 (forall b. (F b ~ Int) => T a b) > > q3 :: Q3 > q3 = Q3 t This causes some complaints about *a*, resulting from checking ambiguity. If we `AllowAmbiguousTypes`, then the definition of `Q3` is accepted, but `q3` fails with a similar error, complaining that GHC cannot match `a` with `Bool` because it is "untouchable". This seems confusing---I would have thought that when we use the `Q3` constructor we are providing the value for `a` (e.g., it is `Bool`), so I am not sure why GHC is trying to match anything. Any ideas? -Iavor

1 0

gitlab upgrade - change in approval system
by Matthew Pickering 17 May '19

17 May '19

Hi, I just noticed that the gitlab upgrade has caused the approval system to change. Before all that was required was 1 approval from anyone. Now, at least one approval from the default group is required (me, Ben, Omer, Tobias, David) I couldn't see a way to turn back to the old behaviour from a brief look at the UI. Do you know if there is a way Ben? Cheers, Matt

1 0

How to contribute a patch
by Simon Peyton Jones 16 May '19

16 May '19

Ben The "how to contribute a patch" page https://gitlab.haskell.org/ghc/ghc/wikis/working-conventions/fixing-bugs doesn't say anything about the process of actually getting it committed, notably: * Picking approvers * Assigning to Marge * Monitoring progress if it doesn't land within 24 hrs; even knowing when to time out would help. There's a bit of info, rather oddly, on the GHC wiki root page https://gitlab.haskell.org/ghc/ghc/wikis/home The initial stuff about opening a MR is details, but there is not enough on these latter steps I think. I'm cc'ing Sandy He's asking "what next"<https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fgitlab.ha…> and I wanted to say "look at the instructions here" but failed to find a suitable "here." Thanks Simon

3 2

GHC API, GHCi, profiling, shared objects
by Matthew Pickering 16 May '19

16 May '19

Hi, If I write a program using the GHC API then the resulting executable is sensitive to the version of GHC which I used to build the program. For example, if I build a version of GHC which has profiling enabled (prof flavour) then building my application (static linking) with this version means that I can only load packages which were also built with profiling enabled. This happens in particular when you load a module which uses TemplateHaskell which causes all the dependencies to be loaded. If the so isn't built with profiling then you get errors such as ``` <command line>: can't load .so/.DLL for: libHSblaze-html-0.9.1.1-E0enfQtU1ZTDNQ8G8pIpJp.dylib (dlopen(libHSblaze-html-0.9.1.1-E0enfQtU1ZTDNQ8G8pIpJp.dylib, 5): image not found) ``` This seemed to somewhat make sense to me because compiling with/without profiling changes the heap representation but then I considered whether this also happens in GHCi and I concluded that it didn't. In particular, it seems to me that 1. GHCi is not built by a profiled version of GHC and is still a "GHC API application" 2. There is no problem loading profiled packages into GHCi despite this fact? 1. Does GHCi do something special to mitigate against these kinds of failures? 2. Is this a difference between staticly and dynamically linking the application executable (`ghc` in the case for ghci). ? Hopefully someone familiar with the implementation of the linker will be able to clear this up. Cheers, Matt

1 1

clarification re Note [Deeper level on the left]
by Nicolas Frisby 16 May '19

16 May '19

https://gitlab.haskell.org/ghc/ghc/blob/master/compiler/typecheck/TcUnify.h… contains ``` Note [Deeper level on the left]~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~The most important thing is that we want to put tyvars withthe deepest level on the left. The reason to do so differs forWanteds and Givens, but either way, deepest wins! Simple.* Wanteds. Putting the deepest variable on the left maximise the chances that it's a touchable meta-tyvar which can be solved.* Givens. Suppose we have something like forall a[2]. b[1] ~ a[2] => beta[1] ~ a[2] If we orient the Given a[2] on the left, we'll rewrite the Wanted to (beta[1] ~ b[1]), and that can float out of the implication. Otherwise it can't. By putting the deepest variable on the left we maximise our changes of eliminating skolem capture. See also TcSMonad Note [Let-bound skolems] for another reason to orient with the deepest skolem on the left. IMPORTANT NOTE: this test does a level-number comparison on skolems, so it's important that skolems have (accurate) level numbers.See #15009 for an further analysis of why "deepest on the left"is a good plan.``` I'm wondering about the claim "and that can float out of the implication". The Note does go on to mention #15009 --- and I understand how that Wanted floats *with* #15009 --- but as written, it sounds like the Wanted could float even without #15009. Am I missing something? Roughly: I'm asking because I need to decide if there's a reason for my typechecker plugin to bother with "eliminating skolem capture" when #15009 does *not* apply. Thanks. -Nick

2 1

Re: CI on forked projects: Darwin woes
by Carter Schonwald 15 May '19

15 May '19

Cool. I recommend irc and devs list plus urls / copies of error messages. Hard to debug timeout if we don’t have the literal url or error messages shared ! -Carter ________________________________ From: Kevin Buhr <buhr(a)asaurus.net> Sent: Sunday, May 12, 2019 11:01 AM To: Carter Schonwald Cc: Iavor Diatchki Subject: Re: CI on forked projects: Darwin woes Thanks! I'll send a note if it starts happening again. On 5/12/19 7:23 AM, Carter Schonwald wrote: > [ . . . ] > Next time you hit a failure could you share with the devs list and or > #ghc irc ? -- Kevin Buhr <buhr(a)asaurus.net>

2 2

[hadrian] happy 1.19.10
by Shayne Fletcher 15 May '19

15 May '19

Hi Vlad, Are there imminent plans to update hadrian/stack.yaml with something like, ``` # Specifies the GHC version and set of packages available (e.g., lts-3.5, nightly-2015-09-21, ghc-7.10.2) resolver: lts-13.21 ``` I think this is necessary to get the recent happy upgrade? -- Shayne Fletcher Language Engineer c: +1 917 699 7763 e: shayne.fletcher(a)daml.com Digital Asset Holdings, LLC 4 World Trade Center 150 Greenwich Street, 47th Floor <https://maps.google.com/?q=150+Greenwich+Street,+47th+Floor%C2%A0+%C2%A0+%C…> New York, NY 10007, USA <https://maps.google.com/?q=150+Greenwich+Street,+47th+Floor%C2%A0+%C2%A0+%C…> digitalasset.com <http://www.digitalasset.com/> -- This message, and any attachments, is for the intended recipient(s) only, may contain information that is privileged, confidential and/or proprietary and subject to important terms and conditions available at http://www.digitalasset.com/emaildisclaimer.html <http://www.digitalasset.com/emaildisclaimer.html>. If you are not the intended recipient, please delete this message.

2 2

Parser performance: 10% regression in 8.8
by Vladislav Zavialov 15 May '19

15 May '19

Hello ghc-devs, This February I did some changes to the parser that require higher rank types support in ‘happy’. Unfortunately, as I discovered, happy’s --coerce option is severely broken in the presence of higher rank types, so I had to disable it. My benchmarks have shown a 10% slowdown from disabling --coerce (https://gist.github.com/int-index/38af0c5dd801088dc1de59eca4e55df4 <https://gist.github.com/int-index/38af0c5dd801088dc1de59eca4e55df4>). Alongside my changes I submitted a pull request to happy which fixes the issue (https://github.com/simonmar/happy/pull/134 <https://github.com/simonmar/happy/pull/134>), in the hope that it would get merged, released, and I could re-enable --coerce in GHC ‘happy' configuration. Unfortunately, my patch has been ignored to this day (for 3 months now), and the performance regression reached 8.8-alpha. We need to act swiftly if we want to avoid a performance regression in the actual release. Here’s what needs to be done: 1. Merge https://github.com/simonmar/happy/pull/134 <https://github.com/simonmar/happy/pull/134> 2. Release a new ‘happy’ 3. (Optional) Specify in GHC’s build system that it builds only with the latest 'happy' release 4. Restore the --coerce option in GHC’s build system ‘happy’ configuration 5. Backport it to the ghc-8.8 branch I have no access to do 1 & 2, I believe Simon Marlow does. I’d appreciate if someone took care of 3, currently the build system does not install ‘happy’ and assumes a system-wide installation without checking its version. This means that users of all but the newly released version will encounter obscure error messages. We need a version check. Then I will do 4, as planned, and create a merge request for 5. All the best, - Vladislav

2 2