July 2012 - Beginners

Re: [Haskell-beginners] [Haskell-cafe] arbitrary rank polymorphism and ghc language pragmas
by rickmurphy 05 Jul '12

05 Jul '12

Thanks Francesco. And I did verify that ExplicitForAll does in fact allow Rank 1 Types in functions like the following ... f :: (forall a. a -> a) -- Rick On Thu, 2012-07-05 at 16:28 +0100, Francesco Mazzoli wrote: > At Thu, 05 Jul 2012 11:18:00 -0400, > rickmurphy wrote: > > data T = TC (forall a b. a -> b -> a) > > The type of `TC' will be `(forall a b. a -> b -> a) -> T', a Rank-2 > type. > > -- > Francesco * Often in error, never in doubt > > _______________________________________________ > Haskell-Cafe mailing list > Haskell-Cafe(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell-cafe >

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arbitrary rank polymorphism and ghc language pragmas
by rickmurphy 05 Jul '12

05 Jul '12

Hi All: I've been working through some details in these papers [1], [2] and noticed a language pragma configuration that I hope you can confirm. When using explicit foralls in a data constructor, it appears that GHC 7.4.2 requires Rank2Types in the Language pragma for what the papers consider rank 1 types. Here's an example: data T = TC (forall a b. a -> b -> a) Am I correct, or is there another extension? The ExplicitForAll does not appear to support rank 1 types in data constructors. 1. Practical Type Inference for Arbitrary-Rank Types. 2. A Direct Algorithm for Type Inference in the Rank 2 Fragment of the Second-Order Lambda Calculus. -- Rick

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Using tls-extra for simple smtp
by Sarfraz K. 04 Jul '12

04 Jul '12

I am trying to write a simple script to send a mail via my gmail account. But I am a beginner so it is not that simple. I tryed google but exept for hackage, there is no help or examples at all. The problem is that I did not find the way to use tls-extra(or tls) to initiate the STARTTLS exchange. Ok, here is the code: import Network import Network.TLS.Extra import System.IO import Text.Printf server = "smtp.gmail.com" port = 25 --that has to chage, I think forever a = a >> forever a main = test1 write :: Handle -> String -> IO () write h s = do hPrintf h "%s\r\n" s printf "> %s\n" s listen :: Handle -> IO () listen h = forever $ hGetLine h >>= putStrLn test1 = do h <- connectTo server (PortNumber (fromIntegral port)) hSetBuffering h NoBuffering write h "EHLO" write h "STATTLS" listen h Another thing is that I use the listen function to know what is going on. But I cannot figure out how to use it along with write. That is to say, I would like to be able to see what is going on server-side while sending some data. I found two functions that may resolve my problems: connectionClient :: CryptoRandomGen g => String -> String -> TLSParams -> g -> IO (TLSCtx Handle) forkIO :: IO () -> IO ThreadId The first for tls and the second to send and receive at the same time. But can't seem to make them work. I hope I am not to messy here, any help would be appreciated. PS: English is not my native. -- Sarfraz K.

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What's this [f| data |] ??
by Carlos J. G. Duarte 03 Jul '12

03 Jul '12

Hi. I'm trying to get into haskell in my free time. I have already covered some syntax, but there's plenty to do yet, and when I'm consulting other people's stuff, I find lots of unknown constructs to me, which turns harder to lookup for, due to the very "symbolic" nature of Haskell. For instance, on this http://www.yesodweb.com/blog/2012/04/yesod-js-todo they have a few constructs like this: |mkYesod "App" [parseRoutes| /HomeR GET /todoTodosR GET PUT /todo/#TodoId TodoR GET DELETE |]| It seems that the inline text is going to be fed to parseRoutes. How does that constructs work (links?)? I already know list comprehensions which appear to be related with this. Thanks.

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Problems with installing lambdabot
by Antoras 03 Jul '12

03 Jul '12

I tried to install labdabot with 'cabal install lambdabot' and after some minutes cabal aborts with an error message (see end of mail). I use ArchLinux 64Bit and the following software versions: $ cabal -V cabal-install version 0.14.0 using version 1.14.0 of the Cabal library $ ghc --version The Glorious Glasgow Haskell Compilation System, version 7.4.2 $ gcc -v Using built-in specs. COLLECT_GCC=gcc COLLECT_LTO_WRAPPER=/usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.1/lto-wrapper Target: x86_64-unknown-linux-gnu [...] gcc version 4.7.1 (GCC) Does someone know what's the problem here? Error message: $ cabal install lambdabot Resolving dependencies... Configuring lambdabot-4.2.3.2... Building lambdabot-4.2.3.2... Preprocessing executable 'lambdabot' for lambdabot-4.2.3.2... [...] [28 of 79] Compiling Plugin.Activity ( Plugin/Activity.hs, dist/build/lambdabot/lambdabot-tmp/Plugin/Activity.o ) Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package Boolean-0.0.1 ... linking ... done. Loading package array-0.4.0.0 ... linking ... done. Loading package deepseq-1.3.0.0 ... linking ... done. Loading package containers-0.4.2.1 ... linking ... done. Loading package semigroups-0.8.3.2 ... linking ... done. Loading package void-0.5.6 ... linking ... done. Loading package MemoTrie-0.5 ... linking ... done. Loading package NumInstances-1.0 ... linking ... done. Loading package vector-space-0.8.1 ... linking ... done. Loading package transformers-0.3.0.0 ... linking ... done. Loading package mtl-2.1.2 ... linking ... done. Loading package bytestring-0.9.2.1 ... linking ... done. Loading package unix-2.5.1.1 ... linking ... done. Loading package unlambda-0.1 ... linking ... done. Loading package extensible-exceptions-0.1.1.4 ... linking ... done. Loading package old-locale-1.0.0.4 ... linking ... done. Loading package time-1.4 ... linking ... done. Loading package random-1.0.1.1 ... linking ... done. Loading package pretty-1.1.1.0 ... linking ... done. Loading package template-haskell ... linking ... done. Loading package QuickCheck-2.5 ... linking ... done. Loading package dlist-0.5 ... linking ... done. Loading package smallcheck-0.6.1 ... linking ... done. Loading package syb-0.3.6.2 ... linking ... done. Loading package show-0.4.1.2 ... linking ... done. Loading package filepath-1.3.0.0 ... linking ... done. Loading package old-time-1.1.0.0 ... linking ... done. Loading package directory-1.1.0.2 ... linking ... done. Loading package process-1.1.0.1 ... linking ... done. Loading package readline-1.0.1.0 ... <command line>: can't load .so/.DLL for: /usr/lib/gcc/x86_64-unknown-linux-gnu/4.7.1/../../../../lib/libncurses.so (-lncursesw: cannot open shared object file: No such file or directory) cabal: Error: some packages failed to install: lambdabot-4.2.3.2 failed during the building phase. The exception was: ExitFailure 1 cabal install lambdabot 15.19s user 0.67s system 96% cpu 16.413 total

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Help with Why Do Monads Matter blog post understanding
by Matt Ford 03 Jul '12

03 Jul '12

Hi, I've been reading the following blog post https://cdsmith.wordpress.com/2012/04/18/why-do-monads-matter/ And I think I like it. But there's a part that I don't get. "For a set A, we will define the set Pref(A) to be the set of functions from application settings to the set A. Now watch closely: a function in context from A to B is just an ordinary function from A to Pref(B). In other words, you give it a value from the set A, and it gives you back another function that maps from application settings to the set B." This is in the "functioning with dependency" section and is talking about a procedure that uses outside info from preferences or application settings. If I set my prefs as follows configvar = 3 and define a function as follows add x = configvar + 6 So add’s signature is add: int -> int What does prefs(int) look like? Is that even the right thing to ask? What happens to the add function? By substituting the B for Prefs(B) and returning now only functions from Pref(B) don't we lose the rest of the mapping for add i.e., " + 6"? Matt.

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FRP and a set of pairwise interacting (colliding) objects
by Nathan Hüsken 02 Jul '12

02 Jul '12

Hi, I want to simulate as set of 2D objects, which can collide pairs wise with each other. In an OOP language, I would do this: for (o1 in objects) { for (o2 in objets) { if (testCollision(o1, o2)) { CollData cd = getCollisionData(o1,o2); o1.reactToCollision(cd); o2.reactToCollision(cd): } } } Now I want to do the same thing in Haskell with FRP. Normally in FRP (correct me if I am wrong) I have for my objects a Signal (or whatever it is called in the specific library), which gets as input the collision events for this object (and probably more data, but let's assume collision events are enough): object :: Signal (Event CollData) ObjectState The CollData events themself are generated at another place: collisions :: Signal [ObjectState] (Event CollData) But now the collisions are generated at one place, and processed at another. This means that CollData must be somehow tagged to the objects it belongs to (an ID for example). This again means that some function must take the pool of all collision datas and distribute them to the "object" Signals. When I have a lot of objects, this means a significant overhead! Now I am wondering if there is a nicer approach which avoids this overhead. Thanks! Nathan

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Improving Performance
by Robert Heumüller 01 Jul '12

01 Jul '12

Hello, in order to tune my brain to "functional thinking" I've decided to start writing some "real" programs. After having spent a couple of days figuring out how to translate an imperative algorithm into stateless haskell I beleive I've now managed a simple implementation of the DBSCAN clustering algorithm. http://privatepaste.com/e6bb4fb665 This code has several drawbacks (and probably it doesn't work correctly after all) but I would like to tune its performance. Obviously the code calls several functions multiply with the same arguments. Haskell, being stateless should thus always yield the same resuts, correct? In this case performing the same calculations multiply seems pointless. My question: How would you go about improving this code? In any imperative language I would simply cache distances/neighborhoods in a matrix. Obviously this is not what the way to go in haskell? Thank you very much

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