- Haskell-Cafe - Haskell.org

partially indeterminate?
by Christopher Howard 16 Jul '13

16 Jul '13

Hi. For learning, I was doing the "phone words" problem, where a function translates the digits of a phone number into all possible words. I am trying to connect this idea to the idea of list comprehensions / list monads (sort of the same thing, yes?) I know it is easy to do this: w = do two <- "ABC" three <- "DEF" four <- "GHI" -- and the other digits [[two, three, two, four]] -- for example But what if you don't know in advance what the digits will be? I'm not sure how to insert that deterministic component into this idea. So far, I have a framework like so: p dx = do undefined where m = map (\d -> case d of 2 -> "ABC" 3 -> "DEF" 4 -> "GHI" 5 -> "JKL" 6 -> "MNO" 7 -> "PRS" 8 -> "TUV" 9 -> "WXY" otherwise -> show d) dx I would appreciate any guidance.

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Re: [Haskell-cafe] Non-recursive let [Was: GHC bug? Let with guards loops]
by oleg＠okmij.org 16 Jul '13

16 Jul '13

I'd like to emphasize that there is a precedent to non-recursive let in the world of (relatively pure) lazy functional programming. The programming language Clean has such non-recursive let and uses it and the shadowing extensively. They consider shadowing a virtue, for uniquely typed data. Richard A. O'Keefe wrote > >> let (x,s) = foo 1 [] in > >> let (y,s) = bar x s in > >> let (z,s) = baz x y s in ... > I really wish you wouldn't do that. > ... > I find that that when the same name gets reused like > that I get very confused indeed about which one I am > looking at right now. > ... > If each instance of the variable is labelled with a > sequence number, I don't get confused because each > variable has a different name and I can *see* which > one this is. > > Yes, sequence numbering variable states is a chore for > the person writing the code, but it's a boon for the > person reading the code. Let me point out the latest Report on the programming language Clean http://clean.cs.ru.nl/download/doc/CleanLangRep.2.2.pdf specifically PDF pages 38-40 (Sec 3.5.4 Let-Before Expression). Let me quote the relevant part: Many of the functions for input and output in the CLEAN I/O library are state transition functions. Such a state is often passed from one function to another in a single threaded way (see Chapter 9) to force a specific order of evaluation. This is certainly the case when the state is of unique type. The threading parameter has to be renamed to distinguish its different versions. The following example shows a typical example: Use of state transition functions. The uniquely typed state file is passed from one function to another involving a number of renamings: file, file1, file2) readchars:: *File -> ([Char], *File) readchars file | not ok = ([],file1) | otherwise = ([char:chars], file2) where (ok,char,file1) = freadc file (chars,file2) = readchars file1 This explicit renaming of threaded parameters not only looks very ugly, these kind of definitions are sometimes also hard to read as well (in which order do things happen? which state is passed in which situation?). We have to admit: an imperative style of programming is much easier to read when things have to happen in a certain order such as is the case when doing I/O. That is why we have introduced let-before expressions. It seems the designers of Clean have the opposite view on the explicit renaming (that is, sequential numbering of unique variables). Let-before expressions have a special scope rule to obtain an imperative programming look. The variables in the left- hand side of these definitions do not appear in the scope of the right-hand side of that definition, but they do appear in the scope of the other definitions that follow (including the root expression, excluding local definitions in where blocks. Notice that a variable defined in a let-before expression cannot be used in a where expression. The reverse is true however: definitions in the where expression can be used in the let before expression. Use of let before expressions, short notation, re-using names taking use of the special scope of the let before) readchars:: *File -> ([Char], *File) readchars file # (ok,char,file) = freadc file | not ok = ([],file) # (chars,file) = readchars file = ([char:chars], file) The code uses the same name 'file' all throughout, shadowing it appropriately. Clean programmers truly do all IO in this style, see the examples in http://clean.cs.ru.nl/download/supported/ObjectIO.1.2/doc/tutorial.pdf [To be sure I do not advocate using Clean notation '#' for non-recursive let in Haskell. Clean is well-known for its somewhat Spartan notation.] State monad is frequently mentioned as an alternative. But monads are a poor alternative to uniqueness typing. Granted, if a function has one unique argument, e.g., World, then it is equivalent to the ST (or IO) monad. However, a function may have several unique arguments. For example, Arrays in Clean are uniquely typed so they can be updated destructively. A function may have several argument arrays. Operations on one array have to be serialized (which is what uniqueness typing accomplishes) but the relative order among operations on distinct arrays may be left unspecified, for the compiler to determine. Monads, typical of imperative programs, overspecify the order. For example, do x <- readSTRef ref1 y <- readSTRef ref2 writeSTRef ref2 (x+y) the write to ref2 must happen after reading ref2, but ref1 could be read either before or after ref2. (Assuming ref2 and ref1 are distinct -- the uniqueness typing will make sure of it.) Alas, in a monad we cannot leave the order of reading ref1 and ref2 unspecified. We must choose the order, even if it is irrelevant. Overspecification inhibits the possible optimizations. Why Clean is relatively unknown? Well, why is Amiga?

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[GSoC] Interactive-diagrams GSoC progress report
by Daniel F 15 Jul '13

15 Jul '13

Hello everyone, I am participating in Haskell.org's Google Summer of Code, working on the interactive-diagrams [1] project. The mid-term evaluations are approaching and I am doing some writeup on the work I did so far: http://parenz.wordpress.com/2013/07/15/interactive-diagrams-gsoc-progress-r… A beta-version of the site is also available to the public: http://paste.hskll.org Please note that this is not a final product and you may experience cryptic error messages, some downtime. The security is pretty tight though :) Any suggestions/comments are welcome. [1] https://google-melange.appspot.com/gsoc/proposal/review/google/gsoc2013/dif… -- Sincerely yours, -- Daniil Frumin

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Homotopy Type Theory IRC Channel
by Darin Morrison 15 Jul '13

15 Jul '13

Hello, I know there are many haskellers interested in dependent type theory so I wanted to mention that there is now an IRC channel on freenode for discussing Homotopy Type Theory (HoTT), e.g., the recent book, possible implementations, dependent type theory in general, etc. The channel is ##hott (NOTE the *two* hashes!) and you can connect directly through freenode's web interface if you don't already have a client: https://webchat.freenode.net/?channels=%23%23hott Currently we have around 50 members but the more the merrier! If you don't know much about IRC but would like to learn more: * http://www.irchelp.org — how IRC works, software clients, FAQs, and other info * http://freenode.net — info about the freenode IRC network specifically Cheers, Darin Morrison

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Re: [Haskell-cafe] What have happened to haskell.org?
by Daniel F 15 Jul '13

15 Jul '13

The web site is migrating. IRC says: Topic for #haskell: haskell.org in the middle of migration; expect turbulence; use www.haskell.org On Mon, Jul 15, 2013 at 6:19 PM, Kirill Zaborsky <qrilka(a)gmail.com> wrote: > On URL http://haskell.org/ I get starting Apache page and 404 on > http://haskell.org/hoogle/ > URL with starting www - http://www.haskell.org/ seems to be working but > http://www.haskell.org/hoogle/ responds with some ELF file. > > Kind regards, > Kirill Zaborsky > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe > -- Sincerely yours, -- Daniil -- Sincerely yours, -- Daniil

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What have happened to haskell.org?
by Kirill Zaborsky 15 Jul '13

15 Jul '13

On URL http://haskell.org/ I get starting Apache page and 404 on http://haskell.org/hoogle/ URL with starting www - http://www.haskell.org/ seems to be working but http://www.haskell.org/hoogle/ responds with some ELF file. Kind regards, Kirill Zaborsky

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Compiling haskell-platform-2013.2.0.0 fails
by Roger Mason 15 Jul '13

15 Jul '13

Hello, I'm attempting to compile haskell-platform on an Arch Linux system. It fails very soon after invoking 'make': Configuring HUnit-1.2.5.2... Setup: Use of GHC's environment variable GHC_PACKAGE_PATH is incompatible with Cabal. Use the flag --package-db to specify a package database (it can be used multiple times). If it makes a difference, I'm using GHC 7.6.3 from the Arch package repo. The envar GHC_PACKAGE_PATH mentioned in the error message is unset. Thanks for any help, Roger This electronic communication is governed by the terms and conditions at http://www.mun.ca/cc/policies/electronic_communications_disclaimer_2012.php

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Hoogle website not working?
by Ivan Perez 15 Jul '13

15 Jul '13

Hi cafe, I'm seeing a binary executable file every time I try to access haskell.org/hoogle. I don't know if the maintainers are aware of this. Cheers. Ivan

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Workshop on Termination (deadline: July 22)
by Johannes Waldmann 15 Jul '13

15 Jul '13

Dear all. Still one week to go: submit your results and questions on all things related to termination and bounded resource consumption (of user programs, of type checkers, etc.) to the Workshop on Termination. We are specifically encouraging contributions from the Haskell (and Agda, and Idris) community. - Johannes. ################################################################# 13th International Workshop on Termination (WST) ( full CfP see http://www.imn.htwk-leipzig.de/WST2013/ ) Centro Residenziale Universitario di Bertinoro (near Bologna, Italy). submission: July 22, 2013 notification: July 25, 2013 final version: August 10, 2013 workshop: August 29 - 31, 2013 The Workshop on Termination traditionally brings together, in an informal setting, researchers interested in all aspects of termination, whether this interest be practical or theoretical, primary or derived. The workshop also provides a ground for cross-fertilisation of ideas from term rewriting and from the different programming language communities. The friendly atmosphere enables fruitful exchanges leading to joint research and subsequent publications.

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OpenGL extensions survey results
by Brian Lewis 15 Jul '13

15 Jul '13

Hi, here are the results of the recent OpenGL extensions survey: https://github.com/bsl/opengl-extensions-survey If I did anything dumb, please send me mail or a pull request. Thanks to everyone who contributed!

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