October 2008 - Haskell-Cafe

Improving MTL instances (was: Overlapping/Incoherent instances)
by Ryan Ingram 15 Oct '08

15 Oct '08

On Mon, Oct 13, 2008 at 2:04 AM, J. Garrett Morris <jgmorris(a)cecs.pdx.edu> wrote: > Indeed - MTL seems to have been rewritten at some point in the past to > prefer exhaustive enumeration to overlap. Indeed, and I actually think this is a weakness of the current implementation. Anyone who comes up with a new transformer that provides different functionality than what is there needs to explicitly provide all the relevant instances, instead of letting MonadTrans do its thing. Consider MonadPrompt (shameless plug, it's on hackage!) In order to be fully interoperable with the MTL I'd need to write instances for MonadState, MonadReader, MonadWriter, MonadError, and MonadCont for PromptT. These are unavoidable, although for monads with a "simple enough" interface, such as State, everything can be accomplished with "lift". But I also need to provide the same boilerplate instances for every other monad transformer in the package to give them instances of MonadPrompt. And MonadPrompt *does* have a "simple enough" interface that it could be accomplished trivially with "lift". And this ignores interacting with any other transformer library! Anyone who uses MonadPrompt along with another transformer (like DatabaseT in the PostgreSQL library) needs to write any instances they care about themselves, which adds to the difficulty in using the libraries together. Of course, the point of this message isn't just to complain. The overlap implementation was abhorrent and it *is* better now than it was before. But perhaps there is an abstraction we are missing that would allow for better interoperability. For example, the type-compose library documentation at http://haskell.org/haskellwiki/TypeCompose mentions that (f :. g) is an applicative functor if both f and g are applicative functors, which means there is a generic "transformer" for all applicative functors! The presense of >>=/join for monads make this more difficult, although there is the "product" definition: > newtype Product m n a = Prod { runProd :: m (Either a (Product n m a)) } which handles nesting joins by just nesting the monads recursively. But in this case it is up to the user to figure out how to untangle the spaghetti created, so that's no good. So, does anyone have any good ideas for improving the interoperability of MTL? -- ryan

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Flexible instances
by George Pollard 15 Oct '08

15 Oct '08

I'm a little confused. Why is this allowed: > data Blah = Blah > > instance Eq Blah where > x == y = True But not this: > class Stringable a where > toString :: a -> String > > instance Stringable [Char] where > toString = id (Resulting in:) > Illegal instance declaration for `Stringable [Char]' > (All instance types must be of the form (T a1 ... an) > where a1 ... an are distinct type *variables* > Use -XFlexibleInstances if you want to disable this.) > In the instance declaration for `Stringable [Char]' 'Blah' isn't a type variable, is it? Is my brain just not working right today?

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Very silly
by Andrew Coppin 15 Oct '08

15 Oct '08

{-# LANGUAGE FlexibleInstances #-} module Overload where class Silly s where go :: s instance Silly ([x] -> [x]) where go = reverse instance Silly (Int -> Int) where go = (+1) Don't even ask. Suffice it to say, you *can* make Haskell support arbitrary overloading of function names like C++ has, _if_ you abuse the type system violently enough. Please, won't somebody think of the children?!?

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Cairo and dialog oddities on Windows using glade.
by Jefferson Heard 15 Oct '08

15 Oct '08

Maybe I'm doing something wrong. I've created several dialog boxes in Glade, and I'm calling Gtk.runDialog on them when a user clicks the mouse in my main window. On Linux, they work mostly right, but the insertion point never shows in the Gtk.Entry areas and the dialog itself comes up without any decoration. In Windows, I cannot edit the Entry areas at all. I can click on the action buttons I added with Gtk.Dialog.addButton. Also, I'm getting an (Invalid matrix (matrix not invertible)) error from Cairo in WIndows that I'm not getting in Linux, and all I can think is that occasionally a rectangle width could be negative. Would that cause the error? -- I try to take things like a crow; war and chaos don't always ruin a picnic, they just mean you have to be careful what you swallow. -- Jessica Edwards

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Re: What I wish someone had told me...
by John Lato 15 Oct '08

15 Oct '08

> Albet Lai wrote: > John Lato wrote: >> Are you advocating introducing existential types to beginning >> Haskellers? I think something with the scary name "existential >> quantification" would greatly increase the head'splodin' on the >> learnin' slope. > > OOP(*) advocates introducing existential types to beginning programmers. Yes, because they're required to get almost anything done in OOP languages. In general they are not required in Haskell. I got a bit carried away with rhetorical flourish, but my point wasn't so much that beginners couldn't understand something as that something with such specialized usage isn't necessary for new Haskellers, and is likely to encourage bad habits from OOP-converts. > > The broken analogy between OOP interfaces and Haskell/Isabell type > classes is there because some people insist that all languages should be > like mainstream languages. You have heard it, even from reputable > leaders and pioneers: "if you know one language, picking up others > should be easy, they just differ in syntax". > I have heard it, and I used to believe it. Now I think it's only true provided the one language you know is suitably advanced (and currently non-existent, I think). John

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Network trouble: what to do?
by Creighton Hogg 15 Oct '08

15 Oct '08

So in my quest to create bindings to BlueZ in Haskell, I've hit a bit of a snag: sockets programming. In C, you can use the standard sockets library and just pass around addresses as arrays of 6 bytes instead of arrays of 4 bytes like you normally would. The problem I'm having is that in Network.Socket, there's no such wiggle room and you have to either provide a Word32 or four Word32's to represent the address. Is there a way around this that I just haven't seen, or should I write a patch to Network to add an extra constructor to SockAddr and code to handle it? Cheers, Creighton

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Type safety in foreign pointer
by Mauricio 15 Oct '08

15 Oct '08

Hi, I'm wrapping a library where functions take as parameters pointers to a few standard structs (as, well, all C libraries). I would like to ensure that only pointers of correct structs are passed to those functions. What is the "Haskell way" to do that? My idea is to do something like this: newtype SomeStruct = SomeStruct () and then foreign import ccall "my_function" myfunction :: Ptr SomeStruct -> IO int Is that the proper way to do that? Thanks, Maurício

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ICFP09: Call for Workshop Proposals
by Matthew Fluet (ICFP Publicity Chair) 15 Oct '08

15 Oct '08

CALL FOR WORKSHOP PROPOSALS ICFP 2009 14th ACM SIGPLAN International Conference on Functional Programming 31st August - 2nd September, 2009 Edinburgh, Scotland http://www.icfpconference.org/icfp2009 The 14th ACM SIGPLAN International Conference on Functional Programming will be held in Edinburgh, Scotland from 31st August to 2nd September 2009. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Proposals are invited for workshops to be affiliated with ICFP 2009 and sponsored by SIGPLAN. These workshops should be more informal and focused than ICFP itself, include sessions that enable interaction among the workshop attendees, and be fairly low cost. The preference is for one-day workshops, but other schedules can also be considered. The workshops themselves will be held between August 30th and September 5th, as capacity allows. ---------------------------------------------------------------------- Submission details Deadline for submission: 19th November 2008 Notification of acceptance: 17th December 2008 Prospective workshop organisers are invited to submit a completed workshop proposal form in plain text format to the ICFP 2009 workshop co-chairs (Chris Stone and Mike Sperber), via email to icfp09-workshops at cs.hmc.edu by 19th November 2008. Please note that this is a firm deadline. Organisers will be notified if their proposal is accepted by 17th December 2008, and if successful are required to produce a final report after the workshop has taken place that is suitable for publication in SIGPLAN Notices. The proposal form is available at: http://www.icfpconference.org/icfp2009/workshops/icfp09-workshops-form.txt Further information about SIGPLAN sponsorship is available at: http://acm.org/sigplan/sigplan_workshop_proposal.htm ---------------------------------------------------------------------- Selection committee The workshop proposals will be evaluated by a committee comprising the following members of the ICFP 2009 organising committee, together with the members of the SIGPLAN executive committee. Chris Stone Harvey Mudd College Workshops co-chair Mike Sperber DeinProgramm Workshops co-chair Graham Hutton University of Nottingham General Chair Andrew Tolmach Portland State University Program Chair ---------------------------------------------------------------------- Further information Any queries regarding ICFP 2009 workshop proposals should be addressed to the workshops co-chairs (Chris Stone and Mike Sperber), via email to icfp09-workshops at cs.hmc.edu

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I do not want to be a bitch, but ghc-6.8.3 and haskell binary policy are really horrible.
by Magicloud 15 Oct '08

15 Oct '08

1. I cannot install ghc-6.8.3 in my box until I found the old runable binary. 2. After I installed cabal, and upgraded, ghc-6.8.3 cannot rebuild itself. Because its libraries are conflict with the ones upgraded by cabal. 3. Sometimes, ghc just ignore some libs, because it does not meet its dependencies. Well, ghc does not even tell me. It knows what I want? 4. I use cabal, thinking it would make dependencies installation easier for me. Well it does not, once an error happened, nothing would work since. I cannot even remove the broken lib. 5. No more needed. The above ones, which are important enough to drive me crazy.

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Multi-line string literals are both easy /and/ elegant in Haskell
by Matt Morrow 15 Oct '08

15 Oct '08

The new QuasiQuotes extension arriving with ghc 6.10 is very exciting, and handling multi-line string literals is like stealing candy from a baby. ;) ----------------------------------------------------------------------------- -- Here.hs module Here (here) where import Language.Haskell.TH.Quote import Language.Haskell.TH.Syntax import Language.Haskell.TH.Lib here :: QuasiQuoter here = QuasiQuoter (litE . stringL) (litP . stringL) ----------------------------------------------------------------------------- ----------------------------------------------------------------------------- -- There.hs {-# LANGUAGE QuasiQuotes #-} module Main where import Here (here) main = putStr [$here| Shall I say, I have gone at dusk through narrow streets And watched the smoke that rises from the pipes Of lonely men in shirt-sleeves, leaning out of windows? I should have been a pair of ragged claws Scuttling across the floors of silent seas. |] ----------------------------------------------------------------------------- ----------------------------------------------------------------------------- [m@ganon a]$ ghc -O2 --make There.hs [1 of 2] Compiling Here ( Here.hs, Here.o ) [2 of 2] Compiling Main ( There.hs, There.o ) Loading package ghc-prim ... linking ... done. Loading package integer ... linking ... done. Loading package base ... linking ... done. Loading package syb ... linking ... done. Loading package array-0.2.0.0 ... linking ... done. Loading package packedstring-0.1.0.1 ... linking ... done. Loading package containers-0.2.0.0 ... linking ... done. Loading package pretty-1.0.1.0 ... linking ... done. Loading package template-haskell ... linking ... done. Linking There ... [m@ganon a]$ ./There Shall I say, I have gone at dusk through narrow streets And watched the smoke that rises from the pipes Of lonely men in shirt-sleeves, leaning out of windows? I should have been a pair of ragged claws Scuttling across the floors of silent seas. [m@ganon a]$ -----------------------------------------------------------------------------

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