- Glasgow-haskell-users

Annotations: status and documentation
by Greg Weber 06 Nov '14

06 Nov '14

I would like to be able to retrieve user-supplied annotations in Template Haskell. The 7.8 user manual doesn't mention this ability. However, I see this Trac page: https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/Annotations Where it appears that this feature may exist in 7.8. I will gladly tinker with annotations and update the user manual if someone will help me understand what the situation is. On a related note, it looks like this patch pointed to from the Wiki page for annotations got ignored. Should i submit it to phabricator? https://ghc.haskell.org/trac/ghc/ticket/8460 Thanks, Greg Weber

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Re: GHC Trac
by John Wiegley 03 Nov '14

03 Nov '14

>>>>> Simon Peyton Jones <simonpj(a)microsoft.com> writes: > Is it just me, or is the GHC Trac soul-destroyingly slow at the moment? IT > takes minutes to load one page! You may be interested in the Mylyn plugin for the Eclipse IDE, which allows for downloading and offline browsing of issues in Trac. That could speed up your "hop to next bug" time, at the cost of installing Eclipse solely for that purpose. John

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Recursion on TypeNats
by Barney Hilken 29 Oct '14

29 Oct '14

If you define your own type level naturals by promoting data Nat = Z | S Nat you can define data families recursively, for example data family Power :: Nat -> * -> * data instance Power Z a = PowerZ data instance Power (S n) a = PowerS a (Power n a) But if you use the built-in type level Nat, I can find no way to do the same thing. You can define a closed type family type family Power (n :: Nat) a where Power 0 a = () Power n a = (a, Power (n-1) a) but this isn't the same thing (and requires UndecidableInstances). Have I missed something? The user guide page is pretty sparse, and not up to date anyway. If not, are there plans to add a "Successor" constructor to Nat? I would have thought this was the main point of using Nat rather than Int. Barney.

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Irreducible predicates error in Template Haskell
by Sreenidhi Nair 29 Oct '14

29 Oct '14

Hello, we were trying to reify a typeclass, which had a ConstraintKind and we hit upon this error: "Can't represent irreducible predicates in Template Haskell:". It seems that there is already a ghc bug [ https://ghc.haskell.org/trac/ghc/ticket/7021 ] filed and its status is set as fixed, but there is a comment at the bottom in which the reviewer recommends against merging immediately. Does anybody know when it would get merged in? -- Yours truly, Sreenidhi Nair

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Hiding module *exports*
by Tom Murphy 27 Oct '14

27 Oct '14

(Not to be confused with the "hiding import behavior" discussion also going on) -- Currently, I'm able to write "module Foo where" to export everything defined in Foo. If, though, I add to the module some definitions which I don't want to export... data Lockbox = MkLockbox Int internalFunction = ... ...I then have to explicitly enumerate everything that I *do* want the module to export, and add to that list each time I add to the module. I propose that instead, we're able to simply say what we mean: module Foo hiding (Lockbox(MkLockbox), internalFunction) where I think its semantics are immediately clear to the reader. There's a little bit of bikeshedding that needs to happen (e.g. is "hiding (Foo(..))" sufficient to hide the type Foo and not just its constructors), but are people +1 on this? I've frequently wanted this behavior. Tom

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ghci balkiness on OS X?
by Evan Laforge 25 Oct '14

25 Oct '14

On my OS X, if I load a couple hundred modules into ghci as bytecode, text entry gets balky and laggy. So e.g. if I hold down a key, the letters will stream in but have frequent hiccups. This seems to adversely affect haskeline as well, such that, using vi mode, sometimes ^[ to move to command mode will be lost, or it will spontaneously go to insert mode, or ^[ followed by 'h' will be just beep and remain in insert mode. If I load most of the modules compiled, this doesn't happen, or at least is not nearly so bad. It doesn't seem to be due to background work by ghci or anything, because it doesn't matter how long I wait after loading the modules. My previous random guess was that increased memory use made the GC work more, and my pauses were due to major collections. But when I try on linux, ghci remains responsive no matter how many modules I load, so maybe it's an OS X only problem. Do any other OS X users see this effect?

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Hiding import behaviour
by Malcolm Gooding 24 Oct '14

24 Oct '14

With the prelude changes that people have been discussing recently I've been wondering is there any reason why importing an identifier explicitly and unqualified doesn't automatically hide any implicit imports of the same identifier? Specifically I'm wondering about cases where you've imported an identifier explicitly from only one module, like this: module Foo (x, ...) where { ... } module Bar (x, ...) where { ... } import Bar import Foo (x) Even if you needed a pragma to enable it I can't think of any sensible reason why that shouldn't be equivalent to: import Bar hiding (x) import Foo (x) I don't know much of GHC's internals, but it seems like a pretty minimal change. Typing rules remain the same; explicit imports just shadow implicits. So importing multiple identifiers both implicitly or both explicitly would remain ambiguous.

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Type checker plugins
by Adam Gundry 23 Oct '14

23 Oct '14

Hi folks, Those of you at HIW last week might have been subjected to my lightning talk on plugins for the GHC type checker, which should allow us to properly implement nifty features like units of measure or type-level numbers without recompiling GHC. I've written up a wiki page summarising the idea: https://ghc.haskell.org/trac/ghc/wiki/Plugins/TypeChecker Feedback is very welcome, particularly if (a) you have an interesting use for this feature or (b) you think this is a terrible idea! Thanks, Adam -- Adam Gundry, Haskell Consultant Well-Typed LLP, http://www.well-typed.com/

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Re: GHC 7.4.2 on Ubuntu Trusty
by Yitzchak Gale 22 Oct '14

22 Oct '14

I wrote: >> How do I get a working GHC 7.4.2 on Trusty? Thanks to all for the suggestions. Daniel Trstenjak wrote, off-list: > I can only recommend: https://launchpad.net/~hvr/+archive/ubuntu/ghc Thanks Daniel! I hadn't looked at Herbert's ppa for a while. Despite the comments, which say it is only updated for precise but should also work for trusty, the ppa actually *is* updated for trusty now. This is trivially simple to use and worked great, so that's what I ended up using. But still - something really really needs to be done about GHC portability. This thread makes it clear what a mess we have inherited from the days when GHC was primarily a research compiler. Let's face it - GHC is now also a serious production compiler, and this urgently needs to be cleaned up. Thanks, Yitz

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GHC 7.4.2 on Ubuntu Trusty
by Yitzchak Gale 22 Oct '14

22 Oct '14

In order support some older software that we released, we need to get a working GHC 7.4.2 on Ubuntu Trusty. We currently have GHC 7.8.3. The binary tarball for GHC 7.4.2 does not install on Trusty due to multiple incompatibilities. For example, GHC requires GMP 3, but Trusty only provides GMP >= 4. Etc. I tried building GHC 7.4.2. from source on Trusty. But the process won't boot from our currently installed GHC 7.8.3. The oldest GHC binary I can get is GHC 7.6.3, which happens to be still available from the Ubuntu distribution itself (neither the binary tarball nor compiling from source work for GHC 7.6.3 on Trusty either). But booting from GHC 7.6.3 won't work either. How do I get a working GHC 7.4.2 on Trusty? Thanks, Yitz

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