January 2015 - Beginners

typeclass woes...how to constain a typeclass to be "closed" under an operation....
by Nicholls, Mark 07 Jan '15

07 Jan '15

> {-# LANGUAGE TypeFamilies, MultiParamTypeClasses, FlexibleContexts, FlexibleInstances, GADTs #-} lets say I have a data type > data S a = S a deriving Show and a nice simple typeclass > class Foo a where > inc :: a -> S a make Integer an instance and.... > instance Foo Integer where > inc x = S x brilliant... now lets say I want my typeclass to enforce that any result of inc is also of a type that is an instance of the typeclass I'll follow my nose...which causes me a problem that makes me realise I maybe I don't understand whats going on. > class Bar a where > inc' :: (Bar (S a)) => a -> S a follow it to.... > instance Bar Integer where > inc' x = S x this WORKS!...even though "S Integer" is not an instance of Bar!...maybe I don't understand. > -- x = inc' (1 :: Integer) This would go BOOM ...."no instance of (Bar (S Integer))" ...so that makes sense.... How do I force Haskell to check the constraint before I evaluate an expression? follow my nose? > -- class (Baz (S a)) => Baz a where > -- inc'' :: a -> S a BOOM ...."Cycle in class declaration".... booo..but I suppose thats understandable.... so a) is my use of typeclasses correct?...or am I missing something? b) if so, how do I force the compiler to make sure my typeclass operations is nicely closed (under the typeclass)? CONFIDENTIALITY NOTICE This e-mail (and any attached files) is confidential and protected by copyright (and other intellectual property rights). If you are not the intended recipient please e-mail the sender and then delete the email and any attached files immediately. Any further use or dissemination is prohibited. While MTV Networks Europe has taken steps to ensure that this email and any attachments are virus free, it is your responsibility to ensure that this message and any attachments are virus free and do not affect your systems / data. Communicating by email is not 100% secure and carries risks such as delay, data corruption, non-delivery, wrongful interception and unauthorised amendment. If you communicate with us by e-mail, you acknowledge and assume these risks, and you agree to take appropriate measures to minimise these risks when e-mailing us. MTV Networks International, MTV Networks UK & Ireland, Greenhouse, Nickelodeon Viacom Consumer Products, VBSi, Viacom Brand Solutions International, Be Viacom, Viacom International Media Networks and VIMN and Comedy Central are all trading names of MTV Networks Europe. MTV Networks Europe is a partnership between MTV Networks Europe Inc. and Viacom Networks Europe Inc. Address for service in Great Britain is 17-29 Hawley Crescent, London, NW1 8TT.

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Relatively simple undirected graph library?
by Stuart Hungerford 06 Jan '15

06 Jan '15

Hi, I've recently started learning Haskell and I'm looking for a library to create and manipulate undirected graphs for a small learning project. Ideally I'd like to be able to: - represent undirected or directed graphs with vertex type a and edge type b - not need to manually maintain mappings between vertex values and vertices assumed to be integers - be able to add and remove vertices and edges by their values with appropriate checks for missing vertices etc - find the connected components of a graph Looking on Hackage I can see some very sophisticated graph libraries including Data.Graph and Data.Graph.Inductive and others, but they all seem to need vertices to be integers or not support changing the graph once constructed. This may well be my ignorance of both graph theory and Haskell but can someone point me to a library that meets my needs or should I extend the learning process and create one myself? Thanks, Stu

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Fwd: cis194 hw7 question - getting example to work
by Dustin Lee 06 Jan '15

06 Jan '15

(sorry if dup. I sent first before confirming my subscription) >From the homework description I've tried running: dlee@morty ~/repos/cis194/week7 $ runhaskell StringBufEditor.hs Editor.hs:11:8: Could not find module `Control.Monad.State' Perhaps you meant Control.Monad.ST (from base) Control.Monad.ST.Safe (from base) Control.Monad.Fix (from base) Use -v to see a list of the files searched for. Is my haskell install messed up? Have the libraries changed since the homework was created? Thanks, dustin -- Dustin Lee qhfgva=rot13(dustin)

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Re: [Haskell-beginners] Beginners Digest, Vol 76, Issue 18
by ebi anomaro 05 Jan '15

05 Jan '15

iPhoneから送信 2014/10/20 21:00、"beginners-request(a)haskell.org" <beginners-request(a)haskell.org> のメッセージ: > Send Beginners mailing list submissions to > beginners(a)haskell.org > > To subscribe or unsubscribe via the World Wide Web, visit > http://www.haskell.org/mailman/listinfo/beginners > or, via email, send a message with subject or body 'help' to > beginners-request(a)haskell.org > > You can reach the person managing the list at > beginners-owner(a)haskell.org > > When replying, please edit your Subject line so it is more specific > than "Re: Contents of Beginners digest..." > > > Today's Topics: > > 1. Re: Is working with Haskell easier on Linux than on a Mac? > (Brandon Allbery) > 2. Re: Is working with Haskell easier on Linux than on a Mac? > (Brandon Allbery) > 3. Re: Is working with Haskell easier on Linux than on a Mac? > (Michael Martin) > 4. GhcDynamic and DYNAMIC_GHC_PROGRAMS (Jeremy) > > > ---------------------------------------------------------------------- > > Message: 1 > Date: Sun, 19 Oct 2014 20:36:56 -0400 > From: Brandon Allbery <allbery.b(a)gmail.com> > To: The Haskell-Beginners Mailing List - Discussion of primarily > beginner-level topics related to Haskell <beginners(a)haskell.org> > Subject: Re: [Haskell-beginners] Is working with Haskell easier on > Linux than on a Mac? > Message-ID: > <CAKFCL4VbGJrYcyNDW7L5SRa+a=ybwBHyPXo2yBkrC9gRTUStgQ(a)mail.gmail.com> > Content-Type: text/plain; charset="utf-8" > > On Sun, Oct 19, 2014 at 8:34 PM, Michael Martin <mmartin4242(a)gmail.com> > wrote: > >> Signals.h provides an interface to OS process signals > > > That is <signal.h>, not "Signals.h". And while case may be fungible on OS > X, the trailing "s" is not. > > -- > brandon s allbery kf8nh sine nomine associates > allbery.b(a)gmail.com ballbery(a)sinenomine.net > unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net >

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Haskell way of defining and implementing OO interfaces
by Thomas Koch 04 Jan '15

04 Jan '15

Hi, I'm writing a password manager that implements a dbus-api using the dbus[1] package. I'd like to separate the code that implements from the dbus api from the code that stores and retrieves the secrets (passwords). In Java I'd use an interface, e.g.: interface PasswordStore { void store(Path path, String secret, Map metadata); (String secret, Map metadata) retrieve(Path path); (String secret, Map metadata) search(Map criteria); } And the dbus-api would export this interface: dbusClient.export(PasswordStore store) What would be a Haskell way to do the same? My only idea is to define a record: data PasswordStore { store :: Path -> Secret -> MetaData -> IO () , retrieve :: Path -> IO (Secret, MetaData) , search :: Criteria -> IO (Secret, MetaData) } Thank you for any suggestions! Thomas Koch [1] http://hackage.haskell.org/package/dbus-0.10.9

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Generate API implementations from XML
by Thomas Koch 04 Jan '15

04 Jan '15

Hi, as written in my previous mail, I'm implementing a dbus interface in Haskell. The interface definition is given in XML[1] according to a given dtd[2]. For every dbus method one need to invoke[3] export :: Client -> ObjectPath -> [Method] -> IO () What would be a good way to autogenerate all this export calls and Method instances? All necessary informations are given in the dbus spec xml. One could write a script that writes Haskell code. Or one could use template haskell that reads the xml file at compile time? Or? Do you know other examples to learn from that autogenerate Haskell code from interface definitions? Thank you, Thomas Koch [1] http://code.metager.de/source/xref/freedesktop/xdg/specs/secret-service/org… [2] http://standards.freedesktop.org/dbus/1.0/introspect.dtd [3] http://hackage.haskell.org/package/dbus-0.10.9/docs/DBus-Client.html#v:expo…

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Foldable of Foldable
by Julian Birch 02 Jan '15

02 Jan '15

Apologies if this isn't clear, I suspect if I understood the terminology better I'd already have solved the problem. I've got a foldable of a foldable of a. (Specifically `[Set a]`) and I want to be able to express the concept that I can treat `(Foldable f1, Foldable f2) => f1 (f2 a)` as `f3 a` i.e. a foldable of a foldable of a can be newtyped to a foldable of a. At least, I think that's right. Sadly, my attempts at actually expressing this concept have met with incomprehension from GHC, could someone help me out, please? Thanks, Julian.

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Errors package
by Andrew Bernard 02 Jan '15

02 Jan '15

When attempting to install the package ‘errors’ I get the following: $ cabal install errors Resolving dependencies... In order, the following would be installed: exceptions-0.6.1 (reinstall) changes: mtl-2.1.2 -> 2.2.1, transformers-0.3.0.0 -> 0.4.2.0 transformers-base-0.4.3 (reinstall) changes: transformers-0.3.0.0 -> 0.4.2.0 monad-control-1.0.0.1 (new version) transformers-compat-0.3.3.4 (reinstall) changes: transformers-0.3.0.0 -> 0.4.2.0 contravariant-1.2 (reinstall) changes: transformers-0.3.0.0 -> 0.4.2.0 distributive-0.4.4 (reinstall) changes: transformers-0.3.0.0 -> 0.4.2.0 comonad-4.2.2 (reinstall) changes: transformers-0.3.0.0 -> 0.4.2.0 semigroupoids-4.2 (reinstall) changes: transformers-0.3.0.0 -> 0.4.2.0 bifunctors-4.2 (reinstall) profunctors-4.3.2 (reinstall) changes: transformers-0.3.0.0 -> 0.4.2.0 free-4.10.0.1 (new version) either-4.3.2.1 (new version) errors-1.4.7 (new package) cabal: The following packages are likely to be broken by the reinstalls: temporary-1.2.0.3 ghc-mod-5.2.1.1 resourcet-1.1.2.3 yaml-0.8.9.3 stylish-haskell-0.5.11.0 persistent-sqlite-1.3.0.5 persistent-1.3.3 persistent-template-1.3.2.2 monad-logger-0.3.8 hoogle-4.2.36 conduit-extra-1.1.4.2 conduit-1.2.3 either-4.3.2 monad-journal-0.5.0.1 resource-pool-0.2.3.1 monad-control-0.3.3.0 lifted-base-0.2.3.0 io-choice-0.0.5 HTF-0.12.2.3 free-4.9 Use --force-reinstalls if you want to install anyway. I am using the ghc platform version 7.6.3. Is this package obsolete? Why are so many packages likely to be broken? With Haskell, how does one know what is a proper up to date package and what is not? Andrew

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Continuations
by Cary Cherng 01 Jan '15

01 Jan '15

I read (http://en.wikibooks.org/wiki/Haskell/Continuation_passing_style) that in some circumstances, CPS can be used to improve performance by eliminating certain construction-pattern matching sequences (i.e. a function returns a complex structure which the caller will at some point deconstruct). And that attoparsec is an example of this. I don't see exactly how CPS gives rise to concrete examples of performance gains. Moreover how does this arise in parsing for example with attoparsec as mentioned. I also encountered various web frameworks such as mflow that are based on continuations. How a typical http restful system is made into something based around continuations is not something that is obvious to me. And what is gained from that?

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