July 2015 - Haskell-Cafe

digestive-functors and listOf
by Neil 31 Jul '15

31 Jul '15

I've been able to use digestive-functors with simple forms successfully in a snap application; however, I'm getting stuck when trying to handle a list of hidden inputs. There is something I'm not quite getting on how to use 'listOf'. Can anyone point me in the right direction? Thanks. The rendered html from a previous page includes the following hidden fields: <div id='messageForm.recipients' class='inputList'> <input type='hidden' name='messageForm.recipients.indices' value='0' /> <input type='hidden' id='messageForm.recipients.-1.email' name='messageForm.recipients.-1.email' value /> <input type='hidden' id='messageForm.recipients.0.email' name='messageForm.recipients.0.email' value='emai1l(a)email.com' /> <input type='hidden' id='messageForm.recipients.1.email' name='messageForm.recipients.1.email' value='email2(a)email.com' /> </div> The data types are: data ConfirmMessage = ConfirmMessage { isoDate :: T.Text, subject :: T.Text, body :: T.Text, action :: T.Text, recipients :: [Recipient]} deriving (Show) data Recipient = Recipient { email :: T.Text } deriving (Show) The forms are: messageForm :: (Monad m) => Form T.Text m ConfirmMessagemessageForm = ConfirmMessage <$> "isoDate" .: stringRead "Missing ISOdate" Nothing <*> "subject" .: stringRead "Subject required" Nothing <*> "body" .: stringRead "Body required" Nothing <*> "action" .: stringRead "Missing action" Nothing <*> "recipients" .: listOf recipientForm' Nothing recipientForm' :: (Monad m) => Formlet T.Text m RecipientrecipientForm'= Recipient <$> "email" .: stringRead "Missing email" Nothing Error message: Couldn't match type `Text.Digestive.Form.Internal.FormTree m0 v0 m0 Recipient' with `Maybe Recipient -> Form T.Text m Recipient' Expected type: Formlet T.Text m Recipient Actual type: Text.Digestive.Form.Internal.FormTree m0 v0 m0 Recipient Relevant bindings include recipientForm' :: Formlet T.Text m Recipient (bound at src/Handler/TradeNotifyConfirm.hs:224:1) In the expression: Recipient <$> "email" .: stringRead "Missing email" Nothing In an equation for `recipientForm'': recipientForm' = Recipient <$> "email" .: stringRead "Missing email" Nothing Thanks in advance, Neil

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Taking over package erlang
by Poór Artúr 31 Jul '15

31 Jul '15

Hi, The package erlang (https://hackage.haskell.org/package/erlang) wasn't updated since 2008, and it became fairly outdated since then. The author was contacted half a year ago, but he didn't respond. So following the instructions on Hackage I state my intention to take over. I have made the package functional again, and I would like to make it available on Hackage. Artúr

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New GHC fails to specialize imported function
by William Yager 31 Jul '15

31 Jul '15

Hello all, I just installed the new Haskell platform for OS X. It seems to have caused a major performance regression for a program I'm working on, making it well over 25x slower. The culprit appears to be that the new GHC (7.10.2) fails to specialize an imported function. If I manually add a SPECIALIZE pragma, performance goes back to around what it used to be. The code in question is located at https://github.com/wyager/HNet . As it stands, the code takes about 30 seconds to run on my machine. With the previous haskell-platform, it only took about .9 seconds. If I modify Operations.hs and uncomment the SPECIALIZE pragma, runtime goes down to about 1.1 seconds (still slower for some reason, but not awful). To test, compile with -O2 (as the program makes heavy use of Data.Vector, which requires -fspec-constr for performance). Additionally, my experience doesn't seem to match https://downloads.haskell.org/~ghc/7.8.4/docs/html/users_guide/pragmas.html… in that if I add an INLINABLE pragma at the site of (••)'s definition and then try to SPECIALIZE it in a different file after importing, GHC tells me "Warning: SPECIALISE pragma on INLINE function probably won't fire: ‘••’". Cheers, Will

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Excessive whitespace with tasty all of a sudden
by Rogan Creswick 31 Jul '15

31 Jul '15

I recently have been refactoring chatter, and finally got the test suite semi-operational (after lots of extensive changes, both to the library and to the tests to add tests and to make them compatible with the new APIs). Somewhere during the changes, the whitespace output by tasty to format the test status lines nicely has grown out of hand. For example, tests that used to show the OK message on the same line, now look like this (see below). Has anyone else run across this? Does anyone have debugging suggestions? Thanks! Rogan NLP.POS.LiteralTagger Empty tagger always tags as UNK: OK +++ OK, passed 100 tests. Initial training Simple training test: OK Duplicate entries -- use the last value.: OK

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ANNOUNCE: GHC version 7.10.2
by Ben Gamari 30 Jul '15

30 Jul '15

===================================================================== The (Interactive) Glasgow Haskell Compiler -- version 7.10.2 ===================================================================== The GHC Team is pleased to announce a new bug-fix release of GHC, 7.10.2. There have been a number of significant bug fixes since the 7.10.1 (with over 70 defects fixed). These include fixes affecting type-checker correctness, runtime stability, and compiler performance. For this reason we highly recommend that users of 7.10.1 upgrade quickly. Note that due to changes made in this release to GHC's optimizer, previously fragile-but-working rewrite rules may fail to fire. One example of this was found in the widely-used `text` package late in the release cycle, which manifested in long compilation times and poor code generation for `Text` literals (see [Trac #10528]). Users are advised to use text >=1.2.1.2 which includes more robust rewrite rules. The full release notes including a complete listing of the changes in this release can be found here, https://downloads.haskell.org/~ghc/7.10.2/docs/html/users_guide/release-7-1… [Trac #10528]: https://ghc.haskell.org/trac/ghc/ticket/10528 How to get it ~~~~~~~~~~~~~ Both binary and source tarballs of GHC itself are available on the release download page, https://www.haskell.org/ghc/download_ghc_7_10_2 For a more smoother, better integrated experience users are encouraged to find a Haskell distribution. While these alternatives may not yet include 7.10.2, they offer tools and libraries to quickly get users up and running, as well as potentially better integration with the host operating system and package manager. See, https://www.haskell.org/downloads for more details. Background ~~~~~~~~~~ Haskell is a standard lazy functional programming language. GHC is a state-of-the-art programming suite for Haskell. Included is an optimising compiler generating good code for a variety of platforms, together with an interactive system for convenient, quick development. The distribution includes space and time profiling facilities, a large collection of libraries, and support for various language extensions, including concurrency, exceptions, and foreign language interfaces (C, whatever). GHC is distributed under a BSD-style open source license. A wide variety of Haskell related resources (tutorials, libraries, specifications, documentation, compilers, interpreters, references, contact information, links to research groups) are available from the Haskell home page (see below), https://www.haskell.org/ Supported Platforms ~~~~~~~~~~~~~~~~~~~ The list of platforms supported by GHC and the people responsible for them can be found here, https://ghc.haskell.org/trac/ghc/wiki/Platforms https://ghc.haskell.org/trac/ghc/wiki/CodeOwners Ports to other platforms are possible with varying degrees of difficulty. The Building Guide describes how to go about porting to a new platform: https://ghc.haskell.org/trac/ghc/wiki/Building Developers ~~~~~~~~~~ If you enjoy using GHC, you will likely also enjoy contributing to it! We are always looking for new contributors. Instructions on accessing our source code repository, and getting started with hacking on GHC, are available from the GHC developer site, https://ghc.haskell.org/trac/ghc/ Mailing lists ~~~~~~~~~~~~~ We run mailing lists for GHC users and bug reports; to subscribe, use the web interfaces at https://www.haskell.org/mailman/listinfo/glasgow-haskell-users https://www.haskell.org/mailman/listinfo/glasgow-haskell-bugs There are several other haskell and ghc-related mailing lists on www.haskell.org; for the full list, see https://www.haskell.org/mailman/listinfo/ Some GHC developers hang out on #haskell on IRC, too: https://www.haskell.org/haskellwiki/IRC_channel Please report bugs using our bug tracking system. Instructions on reporting bugs can be found here: https://www.haskell.org/ghc/reportabug Hashes & Signatures ~~~~~~~~~~~~~~~~~ In https://downloads.haskell.org/~ghc/7.10.2/ you will find a signed copy of the SHA256 hashes for the tarballs, using my GPG key, Benjamin Gamari <ben(a)well-typed.com> Fingerprint: FFEB 7CE8 1E16 A36B 3E2D ED6F 2DE0 4D4E 97DB 64AD

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ANN: hsqml-datamodel(-vinyl) = data model for QML
by Marcin Mrotek 29 Jul '15

29 Jul '15

Hello, I've just uploaded a package that enables using Haskell records in QML table- or listviews: http://hackage.haskell.org/package/hsqml-datamodel There's also an implementation for Vinyl: http://hackage.haskell.org/package/hsqml-datamodel-vinyl Best regards, Marcin Mrotek

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haskell-src-exts maintenance
by Matthew Pickering 28 Jul '15

28 Jul '15

Dear List, There are a large number of untriaged pull requests and issues currently open on the issue tracker for haskell-src-exts. The tool is widely used by tool writers but the last commit to the project is now over 6 months ago. I think it is important to get this work merged and update the parser to support modern language features introduced in GHC 7.8 and 7.10. I would personally be capable of helping with this effort but before doing so I would appreciate a comment from the current maintainers as to the current status of the project. So, 1. Does anyone know if Niklas or Peter plan to return to the project? 2. Is there anyone else who is interested in updating the HSE parser? Matt

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Names in a record changes read behavior?
by Mike Meyer 27 Jul '15

27 Jul '15

Ok, this all works as expected: λ> data Point = Point Float Float deriving (Show, Read) λ> Point 1.0 1.0 Point 1.0 1.0 λ> read "Point 1.0 1.0" *** Exception: Prelude.read: no parse λ> read "Point 1.0 1.0" :: Point Point 1.0 1.0 But now we add names to the values in the Point, and things change: λ> data Point = Point {x :: Float, y :: Float } deriving (Show, Read) λ> Point 1.0 1.0 Point {x = 1.0, y = 1.0} λ> read "Point 1.0 1.0" *** Exception: Prelude.read: no parse λ> read "Point 1.0 1.0" :: Point *** Exception: Prelude.read: no parse λ> read "Point {x = 1.0, y = 1.0 }" :: Point Point {x = 1.0, y = 1.0} OK, why won't read accept the same syntax as the REPL? I can see wanting Read and Show to be inverses of each other, though I think it's a bit misguided. But if we're going to be that strict about them being inverses, shouldn't we also insist that the READ eval print loop only accept what read will accept?

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Re: [Haskell-cafe] Strange "ambiguity" problems thanks to GADTs
by wren romano 27 Jul '15

27 Jul '15

On Tue, Jul 7, 2015 at 3:35 AM, Simon Peyton Jones <simonpj(a)microsoft.com> wrote: > If you post a repro case someone (even me) might help. The devil is always in the details. The smallest repro I can come up with (which is still related to the actual problem) is as follows. In case it matters for the solution, there are a number of important bits I've erased. Most importantly, the inferType function is actually mutually recursive with a checkType function, though that doesn't seem to be the cause of the problem. {-# LANGUAGE DataKinds, PolyKinds, GADTs #-} data Type = Fun Type Type data Sing :: Type -> * where SFun :: Sing i -> Sing j -> Sing (Fun i j) data AST :: (Type -> *) -> Type -> * where App :: ast (Fun a b) -> ast a -> AST ast b class ABT (abt :: Type -> *) where view :: abt a -> AST abt a unview :: AST abt a -> abt a class WellTypedABT (abt :: Type -> *) where unviewWT :: Sing a -> AST abt a -> abt a typeOf :: abt a -> Sing a data TypeCheckMonad a = TCM { unTCM :: a } instance Functor TypeCheckMonad where fmap f = TCM . f . unTCM instance Applicative TypeCheckMonad where pure = TCM TCM f <*> TCM x = TCM (f x) instance Monad TypeCheckMonad where return = pure TCM x >>= k = k x inferType :: (ABT abt, WellTypedABT abt') => abt a -> TypeCheckMonad (abt' a) inferType e0 = case view e0 of App e1 e2 -> do e1' <- inferType e1 case typeOf e1' of SFun typ2 typ3 -> do e2' <- inferType e2 return . unviewWT typ3 $ App e1' e2' -- Live well, ~wren

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Munich Haskell Meeting, 2015-07-29 @ 19:30
by Christian Neukirchen 27 Jul '15

27 Jul '15

Dear all, This week, our monthly Munich Haskell Meeting will take place again on Wednesday, July 29 at Cafe Puck at 19h30. For details see here: http://www.haskell-munich.de/dates If you plan to join, please add yourself to this dudle so we can reserve enough seats! It is OK to add yourself to the dudle anonymously or pseudonymously. https://dudle.inf.tu-dresden.de/haskell-munich-jul-2015/ Everybody is welcome! cu, -- Christian Neukirchen <chneukirchen(a)gmail.com> http://chneukirchen.org

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