September 2016 - Glasgow-haskell-users

Allow extra commas in module declarations or lists?
by Michael Burge 17 Dec '16

17 Dec '16

Some languages like Perl allow you to include additional commas in your lists, so that you can freely reorder them without worrying about adding or removing commas from the last or first item: my @array = ( 1, 2, 3, ); Perl even allows multiple redundant commas everywhere after the first element, which is less useful but has come up on occasion: my @array = (1,,,2,,,,,,,,,3,,,,,); I propose allowing an optional single extra comma at the end in module declarations, record declarations, record constructors, and list constructors: module Example ( module Example, SomeConstructor(..), ) where data SomeConstructor = SomeConstructor { foo :: Int, bar :: Int, } baz :: SomeConstructor -> SomeConstructor baz x = x { foo = 5, bar = 10, } qux :: [ Int ] qux = [ 1, 2, 3, ] What do you think?

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RE: Getting rid of -XImpredicativeTypes
by Simon Peyton Jones 26 Sep '16

26 Sep '16

1) ImpredicativeTypes enables types like `Maybe (forall a. a)`. Do those just disappear, or are they also enabled anyway? (I would guess the former.) Yes, they’d disappear. 2) There was a sketch drawn up around a year ago (I think) aiming to actually fix ImpredicativeTypes. I don't recall who was working on it, but I think when I mentioned it in the context of something else, you didn't seem to be aware of it. I guess it's safe to say that nothing ever came of it, at least inasmuch as no one ever showed you their proposal for a properly functioning ImpredicativeTypes? It’s just a swamp. I have tried multiple times to fix ImpredicativeTypes, and failed every time. Which is not to say that someone shouldn’t try again, with new thinking. Simon From: Dan Doel [mailto:dan.doel@gmail.com] Sent: 26 September 2016 00:54 To: Simon Peyton Jones <simonpj(a)microsoft.com> Cc: ghc-users(a)haskell.org; ghc-devs(a)haskell.org Subject: Re: Getting rid of -XImpredicativeTypes I don't use the extension, because it's more pleasant to use newtypes with polymorphic contents. But here are some questions: 1) ImpredicativeTypes enables types like `Maybe (forall a. a)`. Do those just disappear, or are they also enabled anyway? (I would guess the former.) 2) There was a sketch drawn up around a year ago (I think) aiming to actually fix ImpredicativeTypes. I don't recall who was working on it, but I think when I mentioned it in the context of something else, you didn't seem to be aware of it. I guess it's safe to say that nothing ever came of it, at least inasmuch as no one ever showed you their proposal for a properly functioning ImpredicativeTypes? Anyhow, if it can't be fixed, I think not having the extension is superior to its current state. And really, I think even if fixing it were on the roadmap, it'd be better to get rid of it until it were actually fixed. -- Dan On Sun, Sep 25, 2016 at 2:05 PM, Simon Peyton Jones via ghc-devs <ghc-devs(a)haskell.org<mailto:ghc-devs@haskell.org>> wrote: Friends GHC has a flag -XImpredicativeTypes that makes a half-hearted attempt to support impredicative polymorphism. But it is vestigial…. if it works, it’s really a fluke. We don’t really have a systematic story here at all. I propose, therefore, to remove it entirely. That is, if you use -XImpredicativeTypes, you’ll get a warning that it does nothing (ie. complete no-op) and you should remove it. Before I pull the trigger, does anyone think they are using it in a mission-critical way? Now that we have Visible Type Application there is a workaround: if you want to call a polymorphic function at a polymorphic type, you can explicitly apply it to that type. For example: {-# LANGUAGE ImpredicativeTypes, TypeApplications, RankNTypes #-} module Vta where f x = id @(forall a. a->a) id @Int x You can also leave out the @Int part of course. Currently we have to use -XImpredicativeTypes to allow the @(forall a. a->a). Is that sensible? Or should we allow it regardless? I rather think the latter… if you have Visible Type Application (i.e. -XTypeApplications) then applying to a polytype is nothing special. So I propose to lift that restriction. I should go through the GHC Proposals Process for this, but I’m on a plane, so I’m going to at least start with an email. Simon _______________________________________________ ghc-devs mailing list ghc-devs(a)haskell.org<mailto:ghc-devs@haskell.org> http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs<https://na01.safelinks.protection.outlook.com/?url=http%3A%2F%2Fmail.haskel…>

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FW: Getting rid of -XImpredicativeTypes
by Simon Peyton Jones 26 Sep '16

26 Sep '16

Friends GHC has a flag -XImpredicativeTypes that makes a half-hearted attempt to support impredicative polymorphism. But it is vestigial…. if it works, it’s really a fluke. We don’t really have a systematic story here at all. I propose, therefore, to remove it entirely. That is, if you use -XImpredicativeTypes, you’ll get a warning that it does nothing (ie. complete no-op) and you should remove it. Before I pull the trigger, does anyone think they are using it in a mission-critical way? Now that we have Visible Type Application there is a workaround: if you want to call a polymorphic function at a polymorphic type, you can explicitly apply it to that type. For example: {-# LANGUAGE ImpredicativeTypes, TypeApplications, RankNTypes #-} module Vta where f x = id @(forall a. a->a) id @Int x You can also leave out the @Int part of course. Currently we have to use -XImpredicativeTypes to allow the @(forall a. a->a). Is that sensible? Or should we allow it regardless? I rather think the latter… if you have Visible Type Application (i.e. -XTypeApplications) then applying to a polytype is nothing special. So I propose to lift that restriction. I should go through the GHC Proposals Process for this, but I’m on a plane, so I’m going to at least start with an email. Simon

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Type families in kind signatures with TypeInType
by David Menendez 23 Sep '16

23 Sep '16

Should the code below work in GHC 8.0.1? {-# LANGUAGE TypeInType, TypeFamilies #-} import Data.Kind (Type) type family K t :: Type type family T t :: K t -> Type data List type instance K List = Type type instance T List = [] Right now, I get an error like this one: min.hs:12:24: error: • Expected kind ‘K List -> Type’, but ‘[]’ has kind ‘* -> *’ • In the type ‘[]’ In the type instance declaration for ‘T’ which is puzzling, since K List -> Type and * -> * should be the same. Obviously, TypeInType is experimental and incomplete. I’m just wondering if this sort of thing is expected to work, or if I’m doing something not yet supported or never to be supported In particular, the kind signature for T is forall t -> K t -> Type, which looks like DependentHaskell. -- Dave Menendez <dave(a)zednenem.com> <http://www.eyrie.org/~zednenem/>

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RE: File Type of source code
by Clive Lee 12 Sep '16

12 Sep '16

I am a beginner in a mess over file type I wrote a little Haskell source code 6 months ago and returning I find the source files have type Haskell Source File" When I try to store new source code it fails to load either "not found" or "does not contain module or source code" The files all have type Text Document even when I try extension ".hs" The "Save as" in Word or NotePad does not give me the option of Haskell Source File. I've even tried to start with my original files (OK load) but they save as Text Documents and then won't load I'm embarrassed I can't get over first step - I find the user guide a little complicated for a raw beginner and I must miss something in tutorials. Any help appreciated Best regards Clive

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Call for Nominations: GHC Steering Committee
by Ben Gamari 05 Sep '16

05 Sep '16

Hello everyone, As you likely know, over the last few months we have been discussing options for reforming the process for proposing language and compiler changes to GHC. After much discussion, we have a process [1] which, while not perfect, is acceptable to a majority of our contributor base and will be an improvement over the status quo. While we invite suggestions for future improvements, we are at a point where we can move ahead with implementation. Consequently, we are seeking nominations for the initial GHC steering committee. This body is responsible for overseeing the progression of proposals through the process, working with authors on refining their ideas, and evaluating proposals for acceptance. The committee will consist of five to eight members of diverse backgrounds. We would like to offer the following as a criteria for membership. Note that a candidate is not expected to satisfy all or even most of these criteria, but a good candidate should satisfy at least one: * A history of contributions to the design of new language features * Experience developing Haskell libraries and applications * A demonstrated track record of contributing code to GHC * A pedagogical background, with experience in either teaching or authoring educational materials * Experience in compiler development * Knowledge of functional programming language theory I'd like to emphasize that committee membership is as much a duty as it is a privilege. Membership is not intended to be a platform to be used by members to drive their own ideas; rather it is a way of serving the Haskell community by helping other community members refine and advance their proposals. This, of course, requires an investment of time and effort, which you should be willing and able to consistently put forth. If you'd like to be considered for committee membership then please write a statement describing why you feel you are well-qualified to serve, in terms of the criteria above and any others that you would like to offer. Please send your statements to ben(a)well-typed.com by September 30th. The initial committee selection will be made by the Simons soon thereafter. Thanks to everyone for their feedback and cooperation so far! Cheers, - Ben [1] http://github.com/ghc-proposals/ghc-proposals

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GHC proposal for OverloadedRecordFields (plus some amendments to OverloadedLabels)
by Adam Gundry 05 Sep '16

05 Sep '16

Hi GHC users, I've recently posted a slightly revised proposal for the next step in the epic OverloadedRecordFields saga. If you're interested, please read it and give your feedback at: https://github.com/ghc-proposals/ghc-proposals/pull/6 Many thanks, Adam P.S. Apologies for jumping the gun on the GHC proposals process being formally sanctioned. ;-) -- Adam Gundry, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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Exposing target language in Haskell with GHC API
by Matthew Farkas-Dyck 02 Sep '16

02 Sep '16

A colleague and i are writing, as an unofficial side project, a Haskell→Bluespec compiler, using GHC as our Haskell front-end. The source language of the part we are writing is GHC Core. We need to somehow expose some Bluespec terms and types to the Haskell source program. We had a few ideas: 1. Some "NO_MANGLE" pragma which would tell GHC to not mangle the emitted name, e.g. `x = {-# NO_MANGLE #-} x` to expose `x` 2. `foreign import prim`, not quite sure how yet 3. "CORE" pragmas, e.g. `x = {-# CORE "foo" #-} x` to expose `x` 4. "ANN" pragmas, e.g. `{-# ANN x "no_mangle" #-} x = x` to expose `x` 1 and 2 would mean modifying GHC which we'd rather not do. For 3, we're not sure how to find the "CORE"-pragmatic annotations in a `Core` AST. 4 seems it would work but be a little cumbersome, as the annotation is not on the `Core` AST. Anyone know a good way to do this?

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