January 2015 - Haskell-Cafe

Representation of lenses
by David Turner 28 Jan '15

28 Jan '15

Hi, I'm planning on discussing lenses with some colleagues shortly so I'm consolidating some of my notes into a coherent story. At one point I found myself describing lenses as a way of packaging up a getter and a setter into a single thing. However, this raises the question: why not just use a pair (getter, setter)? More precisely, I believe that the types (a -> s, s -> a -> a) and (forall f. Functor f => (s -> f s) -> a -> f a) are isomorphic. Is that right? I see that one advantage of the lens type is that you can use (.) to compose them since they're just functions, but that doesn't bother me much and it seems I could define another operator to compose (getter, setter) lenses and the rest of the machinery would work either way. It's also possible that you can't get the full generality of (forall f. (s -> f t) -> a -> f b) lenses with getter/setter pairs, although I haven't worked through the details yet so don't know either way. So, my question is: what is the advantage of representing lenses in the way that they are? Many thanks, David

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Using GHC API to compile Haskell sources to CORE and CORE to binary
by Wojciech Danilo 28 Jan '15

28 Jan '15

Hello All! :) Recently I've came across a problem and hopefully you could help me with it. Basically I and some people here are running a startup and we have created a language for processing images. This language has 2 interchangeable representations - textual and visual, dataflow one. Right now we are compiling it to haskell sources, but we want to move over GHC CORE. Getting into GHC has a high learning curve, as far as we see, and we would be very thankfull for any help with a minimal working example of compiling haskell sources to CORE and then CORE to binaries. I've posted couple days ago a detailed question on StackOverflow, here: http://stackoverflow.com/questions/28059669/using-ghc-api-to-compile-haskel… I would be very thankful for any help or hints! All the best, Wojciech

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Contracting Haskell dev role at Standard Chartered
by Don Stewart 28 Jan '15

28 Jan '15

The Strats team at Standard Chartered is hiring a developer for a 1 year contracting role in London. The role is to develop and extend our parsing and validation library for FpML, using the FpML Haskell library to parse and build financial product data into our internal Haskell data types. You will extend our library to support more products, as well as building testing and validation tools (e.g. in QuickCheck) to confirm the soundness and completeness of the implementation. This is a rare entry level Haskell role, and you'll join a very experienced team. More details at: https://donsbot.wordpress.com/2015/01/28/contracting-haskell-development-ro…

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runSubStateT
by Christopher Done 28 Jan '15

28 Jan '15

runSubStateT :: Monad m => (s -> s') -> (s' -> s) -> StateT s' m a -> StateT s m a runSubStateT to from m = StateT (\s -> liftM (\(a,s') -> (a,from s')) (runStateT m (to s))) Anyone ever needed something like this? Does it already exist in some form in the standard libraries? Ciao

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Tooling for equational reasoning in Haskell
by Christopher Done 28 Jan '15

28 Jan '15

Hi I quite enjoy doing equational reasoning to prove that my functions satisfy some laws, I like to type out each substitution step until I get back what I started with. The only trouble is that it's rather manual and possibly error prone. Is there any tooling anywhere out there that can take in a Haskell expression and reduce it by one step? I only know of stepeval: http://bm380.user.srcf.net/cgi-bin/stepeval.cgi?expr=foldr+%28%2B%29+0+%5B1… But it's just a prototype and works on a teeny weeny subset of Haskell. As much as I like doing tooling, my bandwidth for this area is already full. It seems quite hard to implement such a tool with existing tooling. Real compilers and interpreters tend to be distinctly far away from a simple substitution model or retaining the original source code and being able to print valid source back out. If such a tool existed, though, it'd be super handy and you could probably include it as another check for your build process like your type checking, your quickcheck properties and your unit tests. I would personally invest a little bit of time to add interactive Emacs support for such a tool. Ciao

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Re: [Haskell-cafe] Drastic Prelude changes imminent
by adam vogt 27 Jan '15

27 Jan '15

On Tue, Jan 27, 2015 at 11:03 AM, Johan Tibell <johan.tibell(a)gmail.com> wrote: > Looking at the generalizations in Data.List I find them pretty odd now when > I think about it. I'd expect Data.List functions to be monomorphic to lists, > just like I expect functions in Data.Map to be monomorphic to maps. Now > there might be generalized versions of these functions in e.g. the Prelude, > but generalizing Data.List means that I don't even have the monomorphic > versions available if I want to resolve ambiguity by using them*. > > * It turns out resolving ambiguity is pretty annoying. The type signatures > are awkward to write so I end having to write something akin to > > mymap :: (a -> b) -> [a] -> [b] > mymap = map Unless you're using -XOverloadedLists, you can throw in a (`asTypeOf` []) to disambiguate. Or is that the annoyance you were talking about?

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Error trying to re-upload to the Haskell wiki
by Henk-Jan van Tuyl 27 Jan '15

27 Jan '15

L.S., I am trying to upload a file to the Haskell wiki again, after I deleted it (the description was wrong and there is no option to edit the description). Every time I try this, I get the message: Remote server or file not found You tried to access the address https://wiki.haskell.org/Special:Upload, which is currently unavailable. The file is not visible in the upload log. How can I upload this file again? Regards, Henk-Jan van Tuyl -- Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://folding.stanford.edu/ http://Van.Tuyl.eu/ http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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Help finding one-sided ordered list implementation
by David Feuer 27 Jan '15

27 Jan '15

In "Numerical Representations as Higher-Order Nested Datatypes", Ralf Hinze describes an implementation of one-sided ordered lists as 2-3 search trees under the left spine view. I'm wondering if anyone's actually coded those up somewhere I could download them, or if I'll have to dig into all the details in the paper and write them myself. Thanks, David

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Wrapping a C++ library
by Richard Lewis 27 Jan '15

27 Jan '15

Hi there, There is a C++ library for which I'm working on an FFI wrapper. I've written a translation unit which defines several functions with an extern "C" API. Each of the functions does a little bit of C++ to operate the third party library, but the point is that I now have a C API that I can wrap with the FFI. The next thing I did was to try and get Cabal to compile this translation unit for me. I eventually hacked together a Setup.hs that I barely understand that calls gcc to compile the .cpp file and then calls `ar` to, um, well do whatever it is that ar does with .o files, make some sort of archive, or something. Next, I need to get my hsc file (in which I define some imported functions and some data types with Storable instances for shovelling data through the FFI) to compile. This file uses the hsc2hs #include macro to include the .h file of my translation unit. In turn, that .h includes some of the C++ header files from the third party, and eventually an STL header: vector. At this point, the compiler, gcc, complains that it can't find vector. The gcc command that hsc2hs (I think?) is creating does not include -lstdc++, and nethier does it use g++, either of which would, I think, result in a successful compilation of the hsc2hs .c file. In my cabal file I have: extra-libraries: stdc++ ghc-options: -pgml g++ -Wall But these don't seem to fix anything. Perhaps they don't have any effect on how hsc2hs operates? There's a specific question here which goes something like: how do I get hsc2hs to compile code that uses STL headers? But there's also a more general question as to whether my whole approach of creating an extern "C" wrapper is actually going to work? Any answers would be very appreciated. Richard

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Drastic Prelude changes imminent
by Augustsson, Lennart 27 Jan '15

27 Jan '15

If you didn't know that, e.g., the type of foldr is changing, check out the thread https://www.haskell.org/pipermail/libraries/2015-January/024777.html. This email and any attachments are confidential and may also be privileged. If you are not the intended recipient, please delete all copies and notify the sender immediately. You may wish to refer to the incorporation details of Standard Chartered PLC, Standard Chartered Bank and their subsidiaries at http://www.standardchartered.com/en/incorporation-details.html Insofar as this communication contains any market commentary, the market commentary has been prepared by a sales and/or trading desk of Standard Chartered Bank or its affiliate. It is not and does not constitute research material, independent research, recommendation or financial advice. Any market commentary is for information purpose only and shall not be relied for any other purpose, and is subject to the relevant disclaimers available at http://wholesalebanking.standardchartered.com/en/utility/Pages/d-mkt.aspx. Please visit http://wholesalebanking.standardchartered.com/en/capabilities/financialmark… for important information with respect to derivative products.

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