- Haskell-Cafe - Haskell.org

Tutorial for creating haskell apps in the browser easy
by Alberto G. Corona 05 Nov '14

05 Nov '14

Hi Just in case you don´t noticed it, this is the first tutorial for creating Browser applications in Haskell for non haskellers. http://www.airpair.com/haskell/posts/haskell-tutorial-introduction-to-web-a… -- Alberto.

2 3

installing haskell on ubuntu using apt-get??
by Vasili I. Galchin 05 Nov '14

05 Nov '14

My config: runghc 7.4.1 3.2.0-61-generic #93-Ubuntu I installed Haskell using Ubuntu's "apt-get install" ======================================================== Please see results below ... (should I instead install Haskell from haskell.org??) vasili@vasili-VirtualBox:~/biolib$ cabal configure Resolving dependencies... Configuring bio-0.5.2... Warning: This package indirectly depends on multiple versions of the same package. This is highly likely to cause a compile failure. package text-1.2.0.0 requires bytestring-0.10.4.0 package tagsoup-0.13.3 requires bytestring-0.10.4.0 package parsec-3.1.7 requires bytestring-0.10.4.0 package bio-0.5.2 requires bytestring-0.10.4.0 package binary-0.4.5 requires bytestring-0.10.4.0 package unix-2.5.1.0 requires bytestring-0.9.2.1 vasili@vasili-VirtualBox:~/biolib$

4 3

Debugging Wreq/http-client https requests
by Cody Goodman 05 Nov '14

05 Nov '14

I have a program that is POST'ing some data to a remote server. However, some part of the data I'm sending is wrong I believe. If this were plain http without encryption, wireshark would allow me to see the exact data being sent over the wire. However, with https it is encrypted. Is there an inbuilt way to debug requests sent by wreq or more likely, a way to output debug info for http-client? Alternatively, is there a way to use NSS support with either of these libraries: http://security.stackexchange.com/questions/35639/decrypting-tls-in-wiresha… Thanks in advance to all for your time.

4 5

[ANN] Simulation library aivika-3.0
by David Sorokin 04 Nov '14

04 Nov '14

Hi Cafe, I’m glad to announce a new release of my simulation library Aivika written in Haskell. It is focused on DES and System Dynamics. I strongly recommend to upgrade to this version for everyone who uses my library. It contains an important fix in that how the TimingStats statistics is gathered. A more simple SamplingStats is usually used. But in the recent versions I started using more widely TimingStats too. This is a kind of (immutable) statistics which is bound up to the simulation time. For example, the queue size statistics is collected as TimingStats, while other queue properties are usually collected as SamplingStats. The SamplingStats is used to be working. But I felt that there was something wrong with TimingStats. Now TimingStats is finally fixed. As a consequence, the queueRate function gives us a result, which suits very well to Little’s rule with good precision in the examples included in the distributive. So, please upgrade to a new version of Aivika (and related libraries) as possible. Besides, I added debugging facilities to the library. These are functions traceEvent, traceProcess, traceStream and so on. They allow tracing the behavior of the simulation model in time. Finally, the Aivika simulation library can be definitely translated to JavaScript with help of the Haste compiler and it works fine! Only I had to use random numbers different from that one, which is specified with help of the SimpleGenerator data constructor. Please read the notes [2] on the wiki page of my project on GitHub. Thanks, David [1] http://hackage.haskell.org/package/aivika <http://hackage.haskell.org/package/aivika> [2] https://github.com/dsorokin/aivika/wiki/Compiling-to-JavaScript

1 0

Re: [Haskell-cafe] How to use randomized algorithm within the implementation of pure data structures?
by Hiromi ISHII 04 Nov '14

04 Nov '14

Hi there, Thank you in advance for many people suggesting solution for this problem! On 2014/11/01 20:59, Travis Cardwell <travis.cardwell(a)extellisys.com> wrote: > If it works with the algorithm, you could use a pseudo-random number > generator with a fixed seed. For example, here is a program to estimate > the value of π (purely) using a Monte Carlo simulation: I got it. But, in my case, fixing initial seed might cause inefficiency, so I can't take this way in this case. By the way, this approach seems works well for other cases which is not so quality sensitive. On 2014/11/02 1:02, Carter Schonwald <carter.schonwald(a)gmail.com> wrote: > Hrm, you could make a num instance of a newtype wrapped state monad thats threading around your math! > > newtype MyNum = MN (State StdGen TheNumberType) This is almost the same as what I mean by "CPS-ing". We can omit generator argument by this approach, but we have to pass generator whenever we want to get the result or inspect the intermediate value. On 2014/11/02 1:15, Jun Inoue <jun.lambda(a)gmail.com> wrote: > Just an idea here, but would implicit-params work? It only gives you > Reader-monad capabilities, but you can always split random generators. > There might be repercussions for the quality of the generated numbers, > though, for which I have no idea. I think this is virtually the same as Travis's approach, because it returns same seed whenever we split the global value. And, yes, this causes repercussions for the random quality, and not suitable for my case. Perhaps we can parametrize `IORef StdGen` instead of `StdGen` so that we can change the state, but we have to call `unsafePerformIO` internally whenever accessing random generator, and then this appoarch became almost the same as my initial approach. By the way, we can control initial value with this implicit parameter approach, this might improve the random quality slightly than splitting global generator beforewards. -- Hiromi ISHII konn.jinro(a)gmail.com

4 5

Why does this work: 'instance Num (Show a) where'
by Tom Ellis 04 Nov '14

04 Nov '14

This is accepted by GHC 7.6.3. Is this a bug? What does it mean? instance Num (Show a) where

3 3

Obscure, important concepts
by Jeffrey Brown 04 Nov '14

04 Nov '14

What concepts relatively unique to Haskell are powerful, widely applicable, and not mentioned in most introductory texts? (By "relatively unique to Haskell" I mean maybe they're part of Lisp or Scheme, but not, say, Java.) Some of the concepts important to Haskell, such as lambda expressions and recursion, are also present in more popular languages. Many others, though, either have no equivalent in (most) other languages, or else are very unlike the equivalent: types, classes and kinds higher-order functions evaluation partial application tail recursion laziness currying pattern matching application and composition operations contexts functors, applicatives, monads monad transformers * lenses* * :def macros* * arrows* * continuation passing style* * software transactional memory* Most of those topics appear to be covered by introductory texts, but the last five are not. I found each of them by accident, and each kind of blew my mind. They all strike me as powerful, widely applicable, and obscure. Are there others?

3 2

Monads and scope
by Jeffrey Brown 04 Nov '14

04 Nov '14

*Question 1: Are monad stacks "transparent" -- that is, can one use a layer in the middle of a monad stack without thinking about other layers?* In learning about how to stack multiple monads (or more literally, how to stack monad transformers on top of a monad), I was expecting that in order to reach into the middle of the stack, one would have to stay aware of the order in which the stack was created, and unwrap outer transformers to get there. That appears to be false. For instance, the program UglyStack.hs in Chapter 18 (Monad Transformers) of Real World Haskell <http://book.realworldhaskell.org/read/monad-transformers.html> includes a function called "constrainedCount" that, in a ReaderT (StateT IO) stack, calls "ask", "get" and some IO functions as if no layer of the stack interferes with any of the others. Is that true in general? That is, if I build on top of a monad M a stack of monad transformers T1 T2 T3 ... TN M, and each of the Tn have different functions associated with them, call I call those functions without even remembering the order in which the transformers were stacked? Does the order of the stack constrain the programmer in how they chain execution functions like runReader, but not otherwise? *Question 2: Does a monad's scope extend maximally upward through the call stack?* All the programs I have seen that use monads keep them at the top scope: The "main" function will use them, and perhaps functions it calls, but if one descends low enough into the call stack, one escapes from the monad context into pure functional code. Can one ever escape a monad in the reverse direction? Stated differently: Monadic code can call pure code. Can pure code ever call monadic code?

4 3

[ANN]json-autotype: Tool generating JSON parsers and type declarations from example documents...
by Michal J Gajda 04 Nov '14

04 Nov '14

Dear Fellow Haskellers, I've just published a beta version of a tool facilitating generation of Haskell type declarations from example JSON documents. It is supposed to simplify implementing large JSON-based interfaces, without necessity of writing any boilerplate code. USAGE: ====== After installing with `cabal install json-autotype`, you might generate stub code for the parser: $ json-autotype input.json -o MyFormat.hs Then you might test the parser by running it on an input file: $ runghc MyFormat.hs input.json If everything is correct, then feel free to inspect the data structure generated automatically for you! The goal of this program is to make it easy for users of big JSON APIs to generate entries from example data. Occasionally you might find a valid JSON for which `json-autotype` doesn't generate a correct parser. You may either edit the resulting file _and_ send it to the author as a test case for future release. HOW IT WORKS: ============== The program uses union type unification to trim output declarations. The types of same attribute tag and similar attribute set, are automatically unified using recognition by attribute set matching. (This option can be optionally turned off, or a set of unified types may be given explicitly.) Either alternatives is used to assure that all JSON inputs seen in example input file are handled correctly. Details on official releases are on Hackage [1] 1. https://hackage.haskell.org/package/json-autotype Patches and suggestions are most welcome. -- Best regards Michał J. Gajda

3 3

Inlinable vs inline
by Charles Durham 04 Nov '14

04 Nov '14

Hi, I wanted to get some clarifications on how inlinable and inline behave. One particular case is where you have a module exporting some functions with helper functions that are not exported. You go ahead and add inlinable to the exported functions so that the call site can specialize on them. Is there any penalty to adding inline vs inlinable to the helper functions? For example, are there cases where a helper function will not inline and thus can not be specailized at the call site and it would be better to declare the helper function inlinable? I would be interested to hear any nuances related to the differences and practical consequences of these pragmas. Thanks! Charlie Durham

1 0