November 2010 - Haskell-Cafe

Re: [Haskell-cafe] Re: Functional dependencies and Peano numbers (and hoogle-bug?)
by Oscar Finnsson 03 May '11

03 May '11

Thanks for the great feedback. The bijective example was especially interesting. While reading "Fun with Type Functions" I notices GNum as an interesting alternative to the Num type class but I couldn't find any such package on hackagedb. Do anyone know if there is anything like GNum on hackagedb? On an unrelated note: I hoogled "to" (i.e. http://haskell.org/hoogle/?hoogle=to) and just got a blank page. Nothing. Nil (not even <html>...</html>). Is this a bug or a "feature"? :) The reason I hoogled it was because I'm searching for something like explicit casting found in many other languages, something similar to > class To f t where > to :: f -> t > > instance To a a where > to x = x > > instance (Real a, Fractional b) => To a b where > to = realToFrac > > instance (Read a) => To String (Maybe a) where > to = maybeRead -- not from cgi :) so I can write something like > ("23.2" `to`) :: Maybe Double or > ((42 :: Integer) `to`) :: Float Anyone made a module/package that solves this problem already? I cannot be the first that needs generic type safe conversion... . -- Oscar

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ANNOUNCE: Multiplate 0.0.1
by roconnor＠theorem.ca 25 Mar '11

25 Mar '11

Multiplate is a lightweight generic library for mutually recursive data types that won't make Conor lose his lunch. Multiplate is an alternative extension of the Uniplate/Compos core design to support mutually recursive datatypes in a way that is as powerful as Compos, almost as easy to use as Uniplate, and more portable than both of them. Multiplate does not require you to rewrite your data type, does not require run-time reflection, does not require GADTs, and does not even require multi-parameter type classes. It only requires rank 3 polymorphism. <http://hackage.haskell.org/package/multiplate-0.0.1> A more detailed paper is forthcoming, but the library is available to try right now. -- Russell O'Connor <http://r6.ca/> ``All talk about `theft,''' the general counsel of the American Graphophone Company wrote, ``is the merest claptrap, for there exists no property in ideas musical, literary or artistic, except as defined by statute.''

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Anyone recommend a VPS?
by Michael Snoyman 21 Mar '11

21 Mar '11

Hi all, I'm working with a client right now on deploying an app. I was wondering if anyone had some recommendations for a Haskell-friendly host. I'm inclined to go with a VPS for this setup, but if there's reliable shared hosting, that would do as well. This client is fairly price-sensitive, so I don't think anything past the $20/month range to start off with. The only other requirement for this project is PostgreSQL. As far as wants: * Linux is preferable * A server that isn't on everyone's spam list would be better. Of course, I can route through an outside e-mail provider, but local would be nicer. * Have room to grow. For now, this is a fairly small app, but the hope of course is for it to expand. Thanks in advance. Michael

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Downloading Haskell repos from GitHub
by Gwern Branwen 20 Mar '11

20 Mar '11

Along the lines of http://blog.patch-tag.com/2010/03/13/mirroring-patch-tag/ for downloading all patch-tag.com repositories, I've begun to wonder how to download all Github repositories since more and more people seem to be using it. Nothing in http://develop.github.com/ seems especially useful for grabbing the git:// URLs of all repos by language - just by user. The only real list of repos by language seems to be gotten at via http://github.com/languages/Haskell/updated or http://github.com/languages/Haskell/created . (You might think http://github.com/languages/Haskell would be good, but no, it's just a few random repos by interest and not a full listing.) I looked at the HTML, and it looks possible to use tagsoup to get all 98 pages and then parse the entries to get the HTTP URLs of the repos, and then turn *that* into git:// URLs suitable for shelling out to 'git clone', but I can't help but wonder if maybe there's a better approach someone more familiar with Github would know. -- gwern

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Infinite types should be optionally allowed
by Job Vranish 22 Feb '11

22 Feb '11

There are good reasons against allowing infinite types by default (mostly, that a lot of things type check that are normally not what we want). An old haskell cafe conversation on the topic is here: http://www.nabble.com/There%27s-nothing-wrong-with-infinite-types!-td771373… However, I think infinite types should be allowed, but only with an explicit type signature. In other words, don't allow infinite types to be inferred, but if they are specified, let them pass. I think it would be very hard to shoot yourself in the foot this way. Newtype is the standard solution to situations where you really need an infinite type, but in some cases this can be a big annoyance. Using newtype sacrifices data type abstraction and very useful type classes like Functor. You can use multiparameter type classes and functional dependencies to recover some of the lost abstraction, but then type checking becomes harder to reason about and the code gets way more ugly (If you doubt, let me know, I have some examples). Allowing infinite types would fix this. I'm imagining a syntax something like this: someFunctionThatCreatesInfiniteType :: a -> b | b = [(a, b)] Thoughts? Opinions? Am I missing anything obvious? - Job

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Splittable random numbers
by Simon Peyton-Jones 01 Feb '11

01 Feb '11

Hi Cafe A while back there was a thread<http://www.mail-archive.com/haskell-cafe@haskell.org/msg79633.html> about a good implementation of a (pseudo) random number generator with a good "split" operation. There's lots of material on generators that generate a linear sequence of random numbers, but much less on how to generate a tree of random numbers, which is what Haskell's System.Random API requires. I happened to meet Burton Smith recently, who wrote some early papers about this stuff (eg "Pseudo random trees in Monte-Carlo<http://portal.acm.org/citation.cfm?id=1746034>"), so I asked him. His reply is below, along with some follow-up comments from his colleagues Tolga Acar and Gideon Yuval. The generator uses crypto functions, so it's probably more computationally expensive than common linear-sequence generators, but in exchange you get robust splitting. Does anyone feel like taking the idea and turning it into a Haskell library? (Or even a Haskell Wiki page?) I'm taking the liberty of cross-posting to the libraries list. Simon From: Burton Smith Sent: Tuesday, November 02, 2010 3:58 PM To: Simon Peyton-Jones Cc: Gideon Yuval (Gideon Yuval); Tolga Acar Subject: Random number generation With some help from Gideon and Tolga, I think the solution to the "arbitrary tree of random numbers problem" is as follows: The generator G is a pair comprising a crypto key G.k and an integer counter (the "message") G.c. The (next G) operation returns a pair: 1. a random integer r obtained by encrypting G.c with G.k, and 2. a new generator G' such that G'.k = G.k and G'.c = G.c + 1. The (split G) operation is similar, returning the same G', except that instead of returning a random integer r it returns a third generator G'' such that G''.k = r and G''.c = 0. A suitable block cipher system might be 128-bit AES (Rijndael). Unencumbered implementations exist in a variety of languages, and performance is pretty good and will improve dramatically as hardware support improves. I'd pick both crypto key size and the size of the result r to be 128 bits, and employ a 64 bit counter c. Other crypto options exist. From: Simon Peyton-Jones Sent: Wednesday, November 03, 2010 3:11 AM To: Burton Smith; Gideon Yuval (Gideon Yuval) Cc: Tolga Acar; Simon Peyton-Jones Subject: RE: Random number generation Burton, Gideon, Tolga Aha, that's interesting. I'd never seen a random number generator based on crypto, but it seems like an attractive idea. As I understand it, successive calls to 'next' will give you encrypt(0), encrypt(1), encrypt(2), encrypt(3),.... Is this standard? Does it have provably good randomness properties, (cycle length, what else?) like other RNGs? Or does it simply seem very plausible? Can I send it round to the Haskell mailing list, in the hope that someone will turn the idea into a library? (Ideally I'd like to make claims about the randomness properties in doing so, hence my qns above.) From: Gideon Yuval (Gideon Yuval) Sent: Wednesday, November 03, 2010 7:15 AM To: Simon Peyton-Jones; Burton Smith Cc: Tolga Acar Subject: RE: Random number generation As long as the key, and the non-counting part of the counter, are kept" secret", anyone who can distinguish these pseudorandoms from real random, in less than 2^128 steps, has a nice paper for crypto-2011 (this is known as "provable security") concerning a weakness in AES128. One exception: real randoms have a birthday paradox; the pseudorandoms suggested do not. If you care, you can: (1) Limit the counter to 2^32 steps (paradox has 2^-64 probability) or even 2^16 (2^-96), then rekey; or (2) XOR 2 such encrypted counters, with different keys; or (3) XOR 3 successive values for the same counter (just possibly cheaper; top-of-head idea). More hard-core: swap the position of key & message: encrypting a constant "secret" with 1,2,3,4.... Gives pseudorandoms with no birthday paradox. From: Tolga Acar Sent: 03 November 2010 15:50 To: Gideon Yuval (Gideon Yuval); Simon Peyton-Jones; Burton Smith Subject: RE: Random number generation Simon, The general idea is not really that new in the crypto area with constraints Gideon describes, of course. That is typically called a PRNG - Pseudo Random Number Generator, or in another parlance, Deterministic Random Bit Generators (DRBG). The DRBG constructions based on hash functions and block ciphers are even standardized in NIST publication SP800-90 (even though I may not recommend every one of them). As for the construction below, that is based on the AES block cipher, that essentially takes advantage of the PRP (Pseudo Random Permutation) property of the AES block cipher, as each block cipher ought to be. So, as Gideon outlines below, if you fix the key, the PRP gives you a random-looking (or, in other terms, indistinguishable from random) output that no one without the secret key and the "state" can generate or easily predict. Assuming an ideal cipher (and AES is a close approximation to it), the probability is believed to be the birthday paradox - no counterexample or a proof exists without assumptions; so we stick to the birthday bound. There ought to be papers on this somewhere. If not, that condensed information is spread across many papers and is part of the crypto folklore, I'd say. From: Burton Smith Sent: 03 November 2010 19:03 To: Simon Peyton-Jones Cc: Gideon Yuval (Gideon Yuval); Tolga Acar Subject: RE: Random number generation Just two points of further clarification: 1. All PRNGs used in the technical computing space fail the birthday paradox criterion (i.e. have full period), so we really need not worry about this. Also, there are other mitigating factors, e.g. suppose we are using the PRNG to generate pseudorandom residues mod n << 2^128; the paradox is happily present there. 2. The big innovation in this scheme is that the rekeying operation done by split creates a new generator with independence guaranteed by "provable security" in the sense Gideon mentioned - if someone can find something nonrandom in the correlation between G' and G'', say, then it amounts to a weakness in AES128 and is publishable. So it's yet another example of reducibility, common in our field: "if you can easily transform a known/famous hard problem P into this other problem Q, Q must be hard".

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windows network programming
by Michael Litchard 22 Jan '11

22 Jan '11

I took a quick look on hackage for an interface to windows networking function calls, and didn't find anything that worked. I may have overlooked something. What's the state of windows network programming? Any recommendations for a good package?

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A question regarding cmdargs package
by Ben Franksen 31 Dec '10

31 Dec '10

How can I disable the standard arguments 'help' and 'version'? Cheers

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what is the status of haskell's mail libraries?
by Günther Schmidt 29 Dec '10

29 Dec '10

Hi all, I was just looking for mail libraries on hackage. You know libraries where I can construct an email, or retrieve on from the server. With retrieving an email I mean something with a bit more structure than a String, or, God help me, a ByteString. Where are we on this subject? I really cannot tell from the first glance. Günther

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Compiling a DSL on the shoulders of GHC
by Patai Gergely 26 Dec '10

26 Dec '10

Hello all, I'm trying to reuse as much of the sweat and tear put into GHC as possible to derive a compiler for a language highly similar to Haskell in many aspects. The difference is that instead of constructing an expression of type IO (), the programmer has to provide a stream processor on the top level - whether this is an arrow structure or just a pure function mapping input to output is not decided yet. In other words, I want to allow the programmer to use full Haskell as a metaprogramming language to create an expression of a much more limited language. My idea was to move the relevant types into a module, and make my combinators simple data constructors behind the scene, so the Haskell phase would effectively build an AST. The tricky bit is that I don't necessarily want to make everything an explicit data constructor. For instance, I'd like to allow Haskell functions in the tree. Using the GHC API, I can get the Core representation of the metaprogram. What I'd like at this point is to evaluate the top level expression and still have a representation open to analysis (traversal, pretty printing, interpretation etc.), including lambdas; if this evaluation takes more time than some well-defined limit, I could treat it as a compile-time error. Basically, I want the constant that is the main program to be folded completely if possible during compile time, and signal an error in case it isn't or doesn't seem to be. The resulting structure would be transformed further by a separate application instead of the existing GHC backends. Unfortunately the GHC API is not very approachable for the uninitiated, so it's not clear to me whether this is even possible. Or is it simply such a bad idea that it's not even worth considering in the face of more conventional approaches? Gergely -- http://www.fastmail.fm - Same, same, but different...

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