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Using existential types from TAPL book in Haskell
by Dominik Bollmann 14 Apr '16

14 Apr '16

Hi all, I'm currently reading Prof. Pierce's TAPL book. Reading the chapter on existential types, I've been wondering how to use existentials in Haskell. In particular, in the TAPL book there is a section on purely functional objects, which are encoded (e.g.) as follows: counter = {*Nat, { state = 5 , methods = { get = \x:Nat. x , inc = \x:Nat. succ x } } } as Counter where Counter = {∃X, { state : X , methods = { get : X -> Nat , inc : X -> X } } } That is, a Counter consists of some internal state as well as methods `get` to retrieve its state and `inc` to increment it. Internally, a Counter is just implemented with a natural number. However, from the outside, we hide this fact by making its state an existential type X. Thus, we can only modify a Counter instance (e.g., counter) using its exposed methods. For example, let {X, body} = counter in body.methods.get (body.state) (see Ch 24.2 on Existential Objects for further details) I've been wondering how to write such counters in Haskell. In particular, how to write a Counter instance like the above counter that uses a concrete representation type Nat, while exposing its state to the outside world abstractly with an existential type X. I've been trying to do this, but so far I haven't succeeded... If anyone could hint me at how to do this, would help me understanding existentials better, I think :-). Cheers, Dominik.

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Simple Interpreter with State Monad
by Jake 13 Apr '16

13 Apr '16

I'm currently in a graphics class where, in order to provide a standard interface to all of our graphics libraries, we have to process small scripts that look like this: line 0 0 0 1 1 1 circle 0 0 10 scale 0 0 3 save pic.png I successfully wrote a parser with attoparsec that parses the file into a list of Commands. Now I'm trying to process that list to produce an IO action, and I thought the State monad would be useful to keep track of the persistent state of the shapes to draw and the transformations on them. I'm confused about how exactly to do this though. What should the type of my State be? Right now I have an execute function that is execute :: Command -> State ParseState (IO ()) where ParseState is a tuple of stuff. ParseState also includes an IO () because I wanted to be able to create multiple pictures one after another, and I couldn't figure out how to access the previous result value from State to add on to it in the next one. So can anyone offer advice on my specific situation, or maybe a simpler example of how to go about writing an interpreter with the State monad? Thanks, Jake

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how ghc finds libgmp.so when linking
by Johannes Waldmann 13 Apr '16

13 Apr '16

Hi. I have some obsolete libgmp in /usr/lib, and the correct one in /usr/local/lib, but ghc does not find it, resp., finds the /usr/lib one earlier, and this is missing some symbols /usr/local/lib/ghc-7.10.3/integ_2aU3IZNMF9a7mQ0OzsZ0dS/libHSinteger-gmp-1.0.0.0-2aU3IZNMF9a7mQ0OzsZ0dS.a(wrappers.o): In function `integer_gmp_mpn_xor_n': wrappers.c:(.text+0x1761): undefined reference to `__gmpn_xor_n' when I put ghc ... -optl -Wl,--verbose I see that: SEARCH_DIR("/usr/i486-linux-gnu/lib32"); SEARCH_DIR("/usr/local/lib32"); SEARCH_DIR("/lib32"); SEARCH_DIR("/usr/lib32"); SEARCH_DIR("/usr/i486-linux-gnu/lib"); SEARCH_DIR("/usr/local/lib"); SEARCH_DIR("/lib"); SEARCH_DIR("/usr/lib"); but that's apparently not what's happening. Is ghc changing this, and how can I work around it? - J.W.

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Finding way around libraries (was parsec reorganization)
by Rustom Mody 13 Apr '16

13 Apr '16

On Wed, Mar 30, 2016 at 10:06 PM, Joe Hillenbrand <joehillen(a)gmail.com> wrote: > Parsec isn't as maintained as Megaparsec > <https://mrkkrp.github.io/megaparsec/>. > > I recommend checking out the tutorials here > <https://mrkkrp.github.io/megaparsec/tutorials.html>. > So under the advice to use megaparsec instead of parsec, I tried the examples here: https://mrkkrp.github.io/megaparsec/tutorials/fun-with-the-recovery-feature… Got a page full of errors starting: fpl.hs:42:20: Not in scope: ‘<$>’ … Perhaps you meant one of these: On a hunch I changed import Control.Applicative (empty) to import Control.Applicative and then it started working. I can only expect that if megaparsec's own official tutorials are thus out of sync with current haskell, then parsec's will be even more so. Also I find no megaparsec on hoogle/hayoo So how does one go about handling the dilemma: - megaparsec : in development but undocumented on hayoo/hoohle - parsec : documented but bitrotten What/how do haskell pros go about handling such Thanks Rusi

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LumiGuide is hiring
by Bas van Dijk 13 Apr '16

13 Apr '16

Dear Haskellers, We're hiring a Haskell engineer to join our software engineering team. LumiGuide develops parking guidance systems for cars and bicycles. Our technology is based on optical detection techniques (Computer Vision). A nice example of one of our projects is the P-route Bicycle <https://bicycledutch.wordpress.com/2015/06/09/bicycle-parking-guidance-syst…> in Utrecht City. Functional Programming is used extensively at LumiGuide. Haskell, GHCJS and the Nix stack (Nix, NixOS, hydra and nixops) are some of the technologies we're using. I recently gave a talk <http://foswiki.cs.uu.nl/foswiki/pub/NlFpDay2016/WebHome/LumiGuide_FP-Dag-20…> at the Dutch Functional Programming Day that highlights some of the products and tools we're developing. We're looking for an experienced Haskell developer who wants to join our team full-time and will work from our office in Nijmegen, The Netherlands. As one of the first engineers you'll have a big impact on the culture and the technological direction of the company. If you're interested, please send a mail to bas(a)lumiguide.nl and tell me a bit about your experience (previous jobs, open source projects, things that you made that you're proud of, etc.). Best Regards, Bas van Dijk CTO LumiGuide

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Re: [Haskell-cafe] Proposal: Add functions to get consecutive elements to Data.List
by David Turner 13 Apr '16

13 Apr '16

Hi there, The idiom I normally use for this is 'zip xs (drop 1 xs)'. By using 'drop 1' instead of 'tail' it handles the empty case without partiality or needing an extra equation. Does that work for you? Cheers, David On 12 Apr 2016 21:55, "Johan Holmquist" <holmisen(a)gmail.com> wrote: I propose adding two new functions to Data.List: zipConsecutives :: [a] -> [(a,a)] zipConsecutives xs = zip xs (tail xs) zipConsecutivesWith :: (a -> a -> b) -> [a] -> [b] zipConsecutivesWith f xs = zipWith f xs (tail xs) (with possibly more efficient implementations) The Trac feature request is at https://ghc.haskell.org/trac/ghc/ticket/11815 . Why These functions are useful when one wants to process consecutive pairs of elements in a sequence, which is not uncommon. The ticket lists some examples, reiterated here: -- diff consecutive elements: diffs = zipConsecutivesWith (flip (-)) -- determine if list is ascending (similar for descending and strict): isAscending = and . zipConsecutivesWith (<=) -- an old friend of ours: fibs = 1 : 1 : zipConsecutivesWith (+) fibs -- get the edges of a closed path defined by points (ps): edges ps = zipConsecutivesWith makeEdge (ps ++ take 1 ps) Why add to Data.List (and base) The definition must either use an extra equation for the empty list case: zipConsecutives [] = [] zipConsecutives xs = zip xs (tail xs) which makes it non practical to define inline at each use site. Or one may omit the empty list equation, which is safe (thanks to laziness), but that may not be immediately obvious. (It wasn't to me anyway.) The tail function is generally considered unsafe so it is not desirable to force the user to use it. The proposed functions would offer an alternative. The Data.List module is often imported unqualified, so new identifiers may cause collissions. I do find it rather unlikely that the proposed names would cause such problems, however. Deadline for discussion: 2016-05-31. _______________________________________________ Libraries mailing list Libraries(a)haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/libraries

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Are libraries expected to enforce safe usage of thread-unsafe functions?
by Thomas Koster 13 Apr '16

13 Apr '16

Hi friends, I am writing a library that has Handle-like values for resources that are not thread-safe (foreign state from a non-thread-safe C library). In other words, a resource can only be "used" by one Haskell thread at a time. Unless I take steps, if one shares the resource between concurrent Haskell threads, bad things will happen at runtime (even with a single-threaded runtime). Naturally, I want consumers to use my library safely, so I am considering making it safe for all by pessimistically enforcing safe usage with MVars. Is it common/expected that Haskell libraries enforce safe access like this? MVars are not free from cost, but are they cheap (i.e. almost as fast as no forced synchronization) as long as no blocking/rescheduling occurs? Thanks, -- Thomas Koster

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Is it safe to postulate () has one inhabitant?
by David Feuer 13 Apr '16

13 Apr '16

The () kind has infinitely many inhabitants: the type '() and any stuck or looping types with appropriate kinds. However, I've not been able to find a way to break type safety with the postulate unitsEqual :: (a :: ()) :~: (b :: ()) in GHC 7.10.3 (it was possible using a now-rejected data family in 7.8.3). Is there some way I haven't found yet, or is this safe? If it's *not* safe, is the equivalent postulate using Void instead of () safe? For my purposes, () would be nicer, but it feels riskier.

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Emacs (was stack and the Atom editor)
by Rustom Mody 13 Apr '16

13 Apr '16

On Sun, Apr 10, 2016 at 7:29 AM, Thomas Koster <tkoster(a)gmail.com> wrote: > > If I can't make Atom work for me, I will drop it just as fast as I > dropped Emacs. Spacemacs was suggested to me off-list; if it addresses > my complaints about Emacs, I might give it a chance. But Atom is the > topic of this thread, so lest this thread descend into a text editor > flame war, I suggest we stop talking about Emacs (or Vim, or Eclipse, > or anything else). > > [Note the changed subject line :-) ] Ive been having a great deal of confusion/trouble with emacs haskell-mode of late. No this is not a gripe about haskell-mode Just that I find myself confused over 1. The old comint interface and new λ one -- when does which kick in 2. Basic setups -- is it called haskell-mode/haskell-emacs or what? 3. I dont quite know when things stopped working/being comfortable/familiar but I remember it as having nothing to do with haskell -- updating org mode or some such Just that I am supposed to be (at least out here!!) an emacs and haskell expert and while showing others how to get started I am often finding my own setups embarrassingly broken Any tips for an old emacs-geezer?

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Re: [Haskell-cafe] Emacs (was stack and the Atom editor)
by Johannes Waldmann 13 Apr '16

13 Apr '16

> ... failed experiments like code folding So, what *is* the recommended method for folding (in emacs)? Yes, ideally we don't need this, because we do not write deeply nested code. But then we have a lot of top-level declarations, which I still want to fold. Yes, if there are too many declarations in one module, then this should be split. But still, that whole mess of declarations needs to be navigated, no matter how it's mapped to files (modules). The question is how it's mapped to the screen real estate. - J.W.

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