April 2015 - Haskell-Cafe

Haskell on BBC Radio 4
by Colin Adams 10 Apr '15

10 Apr '15

Haskell featured strongly today in the last Episode of Radio 4's "Codes that changed the world": http://www.bbc.co.uk/programmes/b05prkh7

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Re: [Haskell-cafe] PrimMonad for Control.Monad.ST.Lazy?
by oleg＠okmij.org 10 Apr '15

10 Apr '15

Ken Takusagawa II wrote: > What I Am Really Trying To Do: get a purely lazy stream of random values Streams of random data is actually somewhat popular structure, and has been asked about before. What was also found out that such lazy streams are fraught with problems. Some are described on this page: http://okmij.org/ftp/continuations/PPYield/index.html#randoms The page also shows two solutions, which are robust and easy to reason about. Lazy evaluation does promote compositionality on one hand, and it also inhibits compositionality. Reasoning about lazy programs is inherently non-compositional, breaking abstraction boundaries, as the page demonstrates. Overall, lazy evaluation has not worked out as it was originally hoped for. There are alternatives that are more compositional and easier to reason about, including reasoning about performance.

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Re: [Haskell-cafe] Generalized Newtype Deriving not allowed in Safe Haskell
by Simon Peyton Jones 10 Apr '15

10 Apr '15

| prefer, such as only exporting the Coerce instance if all the | constructors are exported, it seems that the ship sailed on these Coercible is relatively recent; I don't think we should regard it as cast in stone. But yes, the Coerbible instance of a newtype is only available when the data constructor for the newtype is lexically in scope. Simon | -----Original Message----- | From: davidterei(a)gmail.com [mailto:davidterei@gmail.com] On Behalf Of | David Terei | Sent: 10 April 2015 09:38 | To: Simon Peyton Jones | Cc: Omari Norman; haskell Cafe; ghc-devs(a)haskell.org | Subject: Re: Generalized Newtype Deriving not allowed in Safe Haskell | | I'll prepare a patch for the userguide soon. | | As for something better, yes I think we can and should. It's on my todo | list :) Basically, the new-GND design has all the mechanisms to be | safe, but sadly the defaults are rather worrying. Without explicit | annotations from the user, module abstractions are broken. This is why | we left GND out of Safe Haskell for the moment as it is a subtle and | easy mistake to make. | | If the module contained explicit role annotations then it could be | allowed. The discussion in | https://ghc.haskell.org/trac/ghc/ticket/8827 has other solutions that I | prefer, such as only exporting the Coerce instance if all the | constructors are exported, it seems that the ship sailed on these | bigger changes sadly. | | Cheers, | David | | On 9 April 2015 at 00:56, Simon Peyton Jones <simonpj(a)microsoft.com> | wrote: | > There is a long discussion on | > https://ghc.haskell.org/trac/ghc/ticket/8827 | > about whether the new Coercible story makes GND ok for Safe Haskell. | > At a type-soundness level, definitely yes. But there are other | > less-clear-cut issues like “breaking abstractions” to consider. The | > decision on the ticket | > (comment:36) seems to be: GND stays out of Safe Haskell for now, but | > there is room for a better proposal. | > | > | > | > I don’t have an opinion myself. David Terei and David Mazieres are in | > the driving seat, but I’m sure they’ll be responsive to user input. | > | > | > | > However, I think the user manual may not have kept up with #8827. | The | > sentence “GeneralizedNewtypeDeriving — It can be used to violate | > constructor access control, by allowing untrusted code to manipulate | > protected data types in ways the data type author did not intend, | > breaking invariants they have established.” vanished from the 7.8 | > user manual (links below). Maybe it should be restored. | > | > | > | > Safe Haskell aficionados, would you like to offer a patch for the | manual? | > And maybe also a less drastic remedy than omitting GND altogether? | > | > | > | > Simon | > | > | > | > From: Omari Norman [mailto:omari@smileystation.com] | > Sent: 09 April 2015 02:44 | > To: haskell Cafe | > Subject: Generalized Newtype Deriving not allowed in Safe Haskell | > | > | > | > When compiling code with Generalized Newtype Deriving and the | > -fwarn-unsafe flag, I get | > | > | > | > -XGeneralizedNewtypeDeriving is not allowed in Safe Haskell | > | > | > | > This happens both in GHC 7.8 and GHC 7.10. | > | > | > | > I thought I remembered reading somewhere that GNTD is now part of the | > safe language? The GHC manual used to state that GNTD is not allowed | > in Safe | > Haskell: | > | > | > | > https://downloads.haskell.org/~ghc/7.6.3/docs/html/users_guide/safe- | ha | > skell.html#safe-language | > | > | > | > But this language on GNTD not being part of the safe language was | > removed in the 7.8 manual: | > | > | > | > https://downloads.haskell.org/~ghc/7.8.2/docs/html/users_guide/safe- | ha | > skell.html#safe-language | > | > | > | > The GHC release notes don't say anything about this one way or the | other. | > Thoughts? | > | > | > _______________________________________________ | > ghc-devs mailing list | > ghc-devs(a)haskell.org | > http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs | >

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ANN: exact-pi 0.1
by Douglas McClean 10 Apr '15

10 Apr '15

I'm announcing the release of the new exact-pi package. It provides a type that exactly represents all rational multiples of integer powers of pi. Because it's closed under multiplication and taking of reciprocals, it's useful for computing exact conversion factors between physical units. In order to provide full Num and Floating instances there is also a representation for approximate values. I'm not sure if this will be of use to anyone else, but it is nice and self-contained so I thought I would put it out there. -Doug McClean

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PrimMonad for Control.Monad.ST.Lazy?
by Ken Takusagawa II 10 Apr '15

10 Apr '15

I notice that the strict ST monad has an instance for PrimMonad but the lazy ST monad does not. Is there a reason why, or is merely an oversight? (What I Am Really Trying To Do: get a purely lazy stream of random values out of mwc-random.) --ken

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Cabal hell
by Patrick Redmond 09 Apr '15

09 Apr '15

Since learning Haskell I've had the pleasure of finding my way out of a cabal hell or two. I've developed some knowledge to cope with it [1], but mostly concluded that if I can avoid burdening my projects with dependencies that have many of their own dependencies, then cabal hell can be averted. This puts somewhat of a damper on the joy of haskell's composability. With the new release of GHC I've observed a flurry of discussion on haskell mailing lists and from Linux distro maintainers about all the fixing and patching required to keep the haskell ecosystem going. Meanwhile I've learned other languages and used other tools that don't seem to have this problem that haskell does. For example, in the elm-lang community the package management tool enforces strict api-versioning, and in the clojure ecosystem people talk about "repeatability" and achieve it by using mostly exact-version requirements, even including the language (the language version is a dependency of a project, rather than a constraint of the environment). I guess I'm wondering why we don't try something simpler to solve the haskell cabal hell problem. How about using minimum version dependencies only, not ranges, since we can't accurately guess about the future compatibility of our projects. How about automatic api-versioning of our projects to give the version numbers some rigid semantics with regard to package compatibility? [1] http://f06code.com/post/90205977959/cabal-usage-notes

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help w/ improving custom Stream for parsec
by Maurizio Vitale 09 Apr '15

09 Apr '15

I need a custom stream that supports insertion of include files and expansions of macros. I also want to be able to give nice error messages (think of clang macro-expansion backtrace), so I cannot use the standard trick of concatenating included files and expanded macros to the current input with setInput/getInput (I think I can't maybe there's a way of keeping a more complex "position" and since the use in producing an error backtrac is rare, it migth be worth exploring; if anybody has ideas here, I'm listening) Assuming I need a more compelx stream, this is what I have (Macro and File both have a string argument, but it will be more compicated, a list of expansions for Macro for instance). Is there a better way for doing this? What are the performance implications with backtracking? I'll be benchmarking it, but if people see obvious problems, let me know. Thanks a lot, Maurizio {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE FlexibleContexts #-} {-# LANGUAGE InstanceSigs #-} {-# LANGUAGE MultiParamTypeClasses #-} module Parsing where import Text.Parsec type Parser s m = ParsecT s () m data VStream = File String | Macro String deriving Show newtype StreamStack = StreamStack [VStream] deriving Show instance (Monad m) ⇒ Stream VStream m Char where uncons ∷ VStream -> m (Maybe (Char, VStream)) uncons (File (a:as)) = return $ Just (a, File as) uncons (File []) = return Nothing uncons (Macro (a:as)) = return $ Just (a, File as) uncons (Macro []) = return Nothing instance (Monad m) => Stream StreamStack m Char where uncons (StreamStack []) = return Nothing uncons (StreamStack (s:ss)) = case uncons s of Nothing → uncons $ StreamStack ss Just Nothing → uncons $ StreamStack ss Just (Just (c, File s')) → return $ Just (c, StreamStack (File s': ss)) Just (Just (c, Macro s')) → return $ Just (c, StreamStack (Macro s':ss))

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VACANCIES : 3x PhD position in Fuctional Programming
by Johan Jeuring 09 Apr '15

09 Apr '15

The research group of Software Technology is part of the Software Systems division of in the department of Information and Computer Science at the Utrecht University. We focus our research on functional programming, compiler construction, tools for learning and teaching (serious games, intelligent tutoring systems), program analysis, validation, and verification. Financed by the Technology Foundation STW, the EU, and Utrecht University we currently have job openings for: ** 3x PhD researcher (PhD student) in Functional Programming ** We are looking for PhD students to develop functional programming techniques related to parsing, rewriting, property-based testing, dependently typed programming, or program analysis, and to apply these techniques in several applications, such as distributed systems, applied games, dialogue management systems, or assessment tools. Besides research, the successful candidate will be expected to help supervise MSc students and assist courses. We prefer candidates to start no later than September 2015. --------------------------------- What we are looking for --------------------------------- The candidate should have an MSc in Computer Science with good grades, be highly motivated to pursue a PhD, and speak and write English well. Knowledge of functional programming, such as Haskell or ML is essential. --------------------------------- What we offer --------------------------------- The candidate is offered a full-time salaried position for four years. The salary is supplemented with a holiday bonus of 8% and an end-of-year bonus of 8,3% per year. In addition we offer: a pension scheme, partially paid parental leave, flexible employment conditions. Conditions are based on the Collective Labour Agreement Dutch Universities. The research group will provide the candidate with necessary support on all aspects of the project. More information is available on the website: Terms and employment: http://bit.ly/1elqpM7 Salary starts at € 2,083.- and increases to € 2,664.- gross per month in the fourth year of the appointment. Utrecht is a great place to live, having been ranked as one of the happiest places in the world, according to BBC travel. Living in Utrecht: http://bitly.com/HdbL0X --------------------------------- In order to apply --------------------------------- To apply please attach a letter of motivation, a curriculum vitae, and (email) addresses of two referees. Make sure to also include a transcript of the courses you have followed (at bachelor and master level), with the grades you obtained, and to include a sample of your scientific writing, such as your master thesis. It is possible to apply for this position if you are close to obtaining your Master's. In that case include a letter of your supervisor with an estimate of your progress, and do not forget to include at least a sample of your technical writing skills. Applications are accepted until the positions are filled. Send your application via email to Johan Jeuring: J.T.Jeuring(a)uu.nl --------------- Contact person --------------- For further information you can direct your inquiries to: Johan Jeuring phone: +31 (0)30 253 4115/ (0) 6 40010053 e-mail: J.T.Jeuring(a)uu.nl website: http://www.staff.science.uu.nl/~jeuri101/

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Re: [Haskell-cafe] indexed writer monad
by Doug McIlroy 09 Apr '15

09 Apr '15

I infer from the following dialog that ghci nontransparently caches knowledge about the contents of the library. This is doubtless helpful for the flurry of imports that a single "load" or interactive "import" may cause. But the time saved by caching from one such top-level action to the next is probably imperceptible. The hours wasted in the present instance and others like it (the very quick response on cafe suggests that it's not unusual) probably outweigh all the milliseconds that the cache normally saves. Non-transparent caching is inherently evil. While the risk may be justifiable in some cases (e.g. stdio buffers in C), it is very questionable at top level in ghci. Doesn't this warrant a ticket? Doug >> I install the indexed package by using cabal >> >> Cabal install indexed >> >> https://hackage.haskell.org/package/indexed >> >> Seemed to do something. >> >> Then I want to use that package... >> >>> import Control.Monad.Indexed >> >> Boom...unknown... > > Possibly silly question but... did you restart `ghci` after installing indexed? Genius Maybe this Haskell lark isnt as hard as I thought....

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Generalized Newtype Deriving not allowed in Safe Haskell
by Omari Norman 09 Apr '15

09 Apr '15

When compiling code with Generalized Newtype Deriving and the -fwarn-unsafe flag, I get -XGeneralizedNewtypeDeriving is not allowed in Safe Haskell This happens both in GHC 7.8 and GHC 7.10. I thought I remembered reading somewhere that GNTD is now part of the safe language? The GHC manual used to state that GNTD is not allowed in Safe Haskell: https://downloads.haskell.org/~ghc/7.6.3/docs/html/users_guide/safe-haskell… But this language on GNTD not being part of the safe language was removed in the 7.8 manual: https://downloads.haskell.org/~ghc/7.8.2/docs/html/users_guide/safe-haskell… The GHC release notes don't say anything about this one way or the other. Thoughts?

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