April 2009 - Haskell-Cafe

How to define a common return and bind?
by Bas van Dijk 11 Apr '09

11 Apr '09

Hello, Suppose you have defined a monad transformer such as: > newtype T1 m a = T1 { unT1 :: A1 m a } Where 'A1 m' is an arbitrary monad of your choosing. For this discussion we just take the identity: > type A1 m a = m a -- (can be any monad) If you want to define a Monad instance for 'T1 m' you generally do this: instance Monad m => Monad (T1 m) where return = T1 . return m >>= f = T1 $ unT1 m >>= unT1 . f (I know I can use the 'GeneralizedNewtypeDeriving' language extension to automatically derive a Monad but suppose that isn't available) Now when I define a new monad transformer: > newtype T2 m a = T2 { unT2 :: A2 m a } Where 'A2 m' is again an arbitrary monad of your choosing but for now just the identity: > type A2 m a = m a -- (can be any monad) The Monad instance for it is almost completely identical to the former: instance Monad m => Monad (T2 m) where return = T2 . return m >>= f = T2 $ unT2 m >>= unT2 . f Note that the only differences are: * a function to convert from the outer monad _to_ the inner monad: 'unT1' and 'unT2' * a function to convert _from_ the inner monad to the outer monad: 'T1' and 'T2' The common parts seem to be: liftReturn from = from . return liftBind from to m f = from $ to m >>= to . f My question is: can these be given suitable and general enough types so that they can be used to define Monad instances for monad transformers? In other words can I use them to write: instance Monad m => Monad (T1 m) where return = liftReturn T1 (>>=) = liftBind T1 unT1 and: instance Monad m => Monad (T2 m) where return = liftReturn T2 (>>=) = liftBind T2 unT2 Thanks, Bas

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cabal haddock and Paths_Xxx
by Martijn van Steenbergen 11 Apr '09

11 Apr '09

Hello, cabal generates a Paths_Xxx file for me which I import and use, but cabal haddock doesn't seem to like it much. If I don't specify the generated module at all in my cabal file, cabal haddock generates visible documentation for the module, which is not what I want: the module should stay internal. If I specify it in either Exposed-Modules or Other-Modules, 'cabal configure; cabal haddock' fails with: > cabal: can't find source for Paths_Xxx in ., dist/build/autogen While 'cabal configure; cabal build; cabal haddock' fails with: > cabal: can't find source for module Paths_Xxx Can I have the best of both, i.e. generate docs without problems and use the generated module? Thanks, Martijn.

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ANN: MSem replacement for QSem
by ChrisK 11 Apr '09

11 Apr '09

"Grasshopper, try to take the stone from my hand" Hello all, After looking at the code for QSem [1], I realize it is not exception safe. And it does more work than needed. I have a proposed replacement module MSem at http://haskell.org/haskellwiki/SafeConcurrent on the wiki. I claim this is a safer, more correct, and more efficient replacement for QSem. But I may be tragically wrong which is where you come in. Please try and find some strange concurrency bug that I have missed. Once there is reviewed replacement code I will propose changing the base package. Exception safe replacements for QSemN and SampleVar are also needed. The MSem code I have can be generalized into an "MSemN", but the semantics are be different from "QSemN". This MSemN only considers the needs of the first blocked waiter instead of all of them. The code MSemN is on the same wiki page, perhaps someone will find this variant useful. Cheers, Chris [1] http://www.haskell.org/ghc/docs/latest/html/libraries/base/src/Control-Conc…

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ANN xmobar-0.9.2
by Andrea Rossato 11 Apr '09

11 Apr '09

Hi, I'm glad to announce xmobar-0.9.2. What's new: - Norbert Zeh tracked and fixed a longstanding bug that made xmobar leak X resources. This alone makes it worth this release and its public announcement, with the advice to users to upgrade.[*] - Norbert also overhauled the template parser to allow nested color definitions; - Norbert, once again, added the possibility to add color definitions in the monitors' templates. Many thanks to Norbert!! I've read that xmobar lists in the top 25 most downloaded Hackage packages. This is very cool: thanks to all the xmobar users. Hope you'll enjoy. Andrea [*] I tend to believe the same issue affects xmonad, even though it should be visible only after a huge number of restarts. Things get far worse when it comes to the xmonad-contrib library, especially if you use fancy decorations in your layouts: if you see colors you are probably leaking them.

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Hac5 is almost upon us!
by Sean Leather 11 Apr '09

11 Apr '09

In six days, tens of crazy/obsessed, type-safe, functional programmers will be converging on Utrecht to commence execution of... ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The 5th Haskell Hackathon April 17 - 19, 2009 Utrecht, The Netherlands ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ The local organizing team welcomes you all and looks forward to all of the new developments that come out of everyone's undying quest to write more and better code. As always, you can find lots of useful information on the wiki: http://haskell.org/haskellwiki/Hac5 == Sponsors == We have sponsors! The following companies were very gracious to provide us with support during these tough economic times. * Galois - http://galois.com/ * Microsoft Research - http://research.microsoft.com/ * Tupil - http://tupil.com/ The organizers are incredibly grateful to Galois, MSR, and Tupil for helping to encourage and promote more Haskell hacking. == More To Come == Hopefully, you have already registered, booked your room, and bought your ticket. If not, don't forget that you can keep in touch with the attendees via IRC on Freenode: #haskell-hac5 All registered attendees will soon be receiving an email with more details. Let us know if you have any questions. Happy Hacking! == Organizers == Andres Löh, Utrecht University (UU) José Pedro Magalhães, UU Sean Leather, UU Eelco Lempsink, UU + Tupil Chris Eidhof, UU + Tupil ... and more ...

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Re: [Haskell-cafe] Referential Transparency and Monads
by Brandon S. Allbery KF8NH 10 Apr '09

10 Apr '09

On 2009 Apr 9, at 11:47, Mark Spezzano wrote: > How exactly do monads “solve” the problem of referential > transparency? I understand RT to be such that a function can be > replaced with a actual value. > > Since a monad could potentially encapsulate any other value—say, > data read from a keyboard—doesn’t that violate the assumption of RT > on monads? Monads provide a way to carry extra data or operations around with their values. IO passes an opaque "world state" around in the background, conceptually I/O operations modify the "world state" and it is in fact always valid to replace the monadified version with the unwrapped version --- ignoring IORefs, IO is just a simple state monad. The "world state" insures that operations in the IO monad are constrained to an ordered list. This gives us a referentially transparent *model* that behaves the way we need it to; the fact that actual I/O is not referentially transparent doesn't matter because the model insures that computations in IO behave properly. (Unless you use unsafePerformIO or unsafeInterleaveIO, which is why they're unsafe; you've violated the model, unexpected things can happen as a result.) -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery(a)kf8nh.com system administrator [openafs,heimdal,too many hats] allbery(a)ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH

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Re: Referential Transparency and Monads
by Heinrich Apfelmus 10 Apr '09

10 Apr '09

Mark Spezzano wrote: > How exactly do monads “solve” the problem of referential transparency? I > understand RT to be such that a function can be replaced with a actual > value. > > Since a monad could potentially encapsulate any other value—say, data read > from a keyboard—doesn’t that violate the assumption of RT on monads? > > Or does RT ignore the encapsulated data and just view the “action” performed > by the monad as the “value” of the monad? The term "referential transparency" refers to functions and means that "equal arguments give equal results". For instance, there cannot be a function getChar :: () -> Char that reads input from the keyboard because referential transparency demands that the character returned must always be the same. Similarly, a function putChar :: Char -> () that prints a character on the screen cannot exist because we could replace every say putChar 'a' with () which does not output anything. Both values are equal, after all. However, functions getChar :: () -> IO Char putChar :: Char -> IO () are entirely feasible; the side effect has been pushed into the IO type constructor. In other words, getChar () :: IO Char is always the same IO action (reading a character from the keyboard) and putChar 'c' :: IO () is always the same IO action (printing 'c' on the screen). In other words, we have decomposed "function with side effect" = "pure function" + "side effect" -> + IO Regards, apfelmus -- http://apfelmus.nfshost.com

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Trying to write 'safeFromInteger'
by Kannan Goundan 10 Apr '09

10 Apr '09

Here's my code (in file "Test.hs") safeFromInteger :: (Num a, Integral a, Bounded a) => Integer -> Maybe a safeFromInteger i = if i > (toInteger maxBound) then Nothing else Just (fromInteger i) Here's the error from GHCi 6.10.1: Test.hs:3:19: Ambiguous type variable `a' in the constraints: `Bounded a' arising from a use of `maxBound' at Test.hs:3:19-26 `Integral a' arising from a use of `toInteger' at Test.hs:3:9-26 Probable fix: add a type signature that fixes these type variable(s) It's almost like GHC thinks that the type variable "a" can't be part of the "Bounded" lass and the "Integeral" class. I plan to use "safeFromInteger" for converting from Word8, Word16, Word32, etc.

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compilation related questions
by minh thu 10 Apr '09

10 Apr '09

Hi, There was a 'I want to write a compiler' thread recently. I wonder if there is such kind of pointers for compilation of strict functional languages (perhaps with lazy evaluation annotations) . Although GRIN is designed for lazy languages, would it be ok to use it for a strict one (and leverage its thunk representation for the lazy part of the language)? The 'whole program' optimization aspect is a plus. On a related note, I have another question. Say we have some data structure, for instance a list, some functions on this data structure (probably defined with some well known functions, such as map or fold), and a program using them. Is there any research trying to rewrite the program, and the data structure, to optimize them ? A contrived example is the length of a list : instead of traversing a list to know its length, the list can have an additional field which is incremented at each cons. Thanks, Thu

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Ann: Yogurt-0.4
by Martijn van Steenbergen 10 Apr '09

10 Apr '09

Bonsoir café, It is my pleasure to announce version 0.4 of Yogurt, a functional MUD client. Version 0.4 makes Yogurt available as a standalone executable that is able to dynamically load and reload Yogurt scripts. Here is a small example of such a script: > module Minimal where > > import Network.Yogurt > > newmoon :: Session > newmoon = session > { hostName = "eclipse.cs.pdx.edu" > , portNumber = 7680 > , mudProgram = \reload -> do > mkCommand "reload" reload > } Valid scripts define at least one session which is used to connect to the MUD. mudProgram fields are provided with a reload action that when invoked reloads the script without interrupting the MUD connection. Of course, Yogurt also still offers hooks, timers, logging, variables and more. The executable is released as a separate package so that developers wishing to use just the "pure" machinery can do that without inheriting dependencies on the GHC API or readline library. To install the executable, run: $ cabal update $ cabal install Yogurt -freadline $ cabal install Yogurt-Standalone Yogurt's new home page can be found at: http://code.google.com/p/yogurt-mud/ And on hackage: http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Yogurt http://hackage.haskell.org/cgi-bin/hackage-scripts/package/Yogurt-Standalone I would love to hear your feedback! Suggestions, complaints, comments, bug reports, experiences et cetera are all welcome. Kind regards, Martijn.

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