April 2014 - Beginners

Data.Stream interleave implementation question
by Bryan Brady 19 Aug '14

19 Aug '14

I'm in the process of learning Haskell and am implementing my own Stream type. ( I'm working my way through homework problems I found at http://www.seas.upenn.edu/~cis194/hw/06-laziness.pdf ) One of the questions is: "Deﬁne the stream ruler :: Stream Integer which corresponds to the ruler function 0, 1, 0, 2, 0, 1, 0, 3, 0, 1, 0, 2, 0, 1, 0, 4, . . . where the nth element in the stream (assuming the ﬁrst element corresponds to n = 1) is the largest power of 2 which evenly divides n. " I got pretty close, but ended up looking at Data.Stream to nail down my mistake. My original solution was: ----- data Stream a = Cons a (Stream a) deriving (Eq, Ord) streamRepeat :: a -> Stream a streamRepeat a = Cons a (streamRepeat a) interleaveStreams :: Stream a -> Stream a -> Stream a interleaveStreams (Cons a as) (Cons b bs) = Cons a (Cons b (interleaveStreams as bs)) ruler :: Stream Integer ruler = foldr1 interleaveStreams (map streamRepeat [0..]) ----- Attempting to evaluate 'ruler' in ghci does not finish. I knew something was wrong with my implementation of interleaveStreams because it worked when I expanded it out manually for a few steps. The way Data.Stream implements interleave is: interleaveStreams (Cons a as) bs = Cons a (interleaveStreams bs as) I don't understand why this works and why my original implementation doesn't. I figure if the latter works, the former should as well. Here is my reasoning: In the latter definition, Cons a (interleaveStreams bs as), (interleaveStreams bs as) is a thunk. The thunk should only be evaluated when it is needed. In my original definition, (interleaveStreams as bs) is a thunk. The difference is an extra Cons (e.g., Cons b). It seems like the additional Cons is causing problems, I just don't understand why. Can anyone point out what I'm missing? thanks, bb

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Functors in Haskell
by Dimitri DeFigueiredo 07 Jul '14

07 Jul '14

Hi All, I'm trying to merge in my head the concepts of: - functors from category theory; with, - the Functor typeclass in haskell. Here's what I have come up with: A Functor F in haskell is a type constructor that provides a functor map (fmap) and that maps Hask (the category of haskell types) into a category generated by the Functor. For example, Maybe maps the category of all haskell types "a" (i.e. Hask) into the category of all types of the form "Maybe a" by providing an implementation of fmap. Neglecting the fact that apparently Hask isn't a category, does this sound reasonable? I think the insight for me here is that functors in haskell are type constructors mapping Hask into another category. Is that a good way to think about it? Thanks! Dimitri

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Problem installing EclipseFp in Eclipse
by Robert Weisser 01 May '14

01 May '14

I decided to try out Eclipse with the EclipseFP plug-in for Haskell. I downloaded Eclipse, and then started to install EclipseFP. At some point, I got a screen which said that I needed to install buildwrapper and scion-browser. It offered to install both of them, and optionally install hoogle, hlint, stylish-haskell, SourceGraph, and cabal-dev. The option to install the additional items was pre-checked. I left it checked. The installation of buildwrapper, etc. took a long time. It seemed to be going well until I saw the messages below, which say that installing SourceGraph will likely break several other packages, most of which were part of the EclipseFP installation. I am not a complete novice with Haskell but I am a complete novice with Cabal. I'm not sure what to do. Here are some options I have considered: 1. Use --force-reinstalls and hope for the best. 2. Try to resolve the problem with Cabal, although I don't know to do this. 3. Use EclipseFP without SourceGraph and cabal-dev. 4. Use EclipseFP without SourceGraph, but install cabal-dev using cabal on the command line. 5. Forget about Eclipse altogether, and go back to using vim. Any advice would be appreciated. By the way, I am using Haskell Platform 7.6.3. Here are the messages I saw in the Eclipse console window: <terminated> Installing executable SourceGraph Resolving dependencies... In order, the following would be installed: asn1-types-0.2.3 (new package) asn1-encoding-0.8.1.3 (new package) asn1-parse-0.8.1 (new package) bktrees-0.3.1 (new package) byteable-0.1.1 (new package) cereal-0.4.0.1 (new package) colour-2.3.3 (new package) cpphs-1.18.2 (new version) crypto-pubkey-types-0.4.2.2 (new package) cryptohash-0.11.4 (new package) digest-0.0.1.2 (new package) dlist-0.5 (new version) aeson-0.7.0.3 (reinstall) changes: dlist-0.7.0.1 -> 0.5 data-default-instances-dlist-0.0.1 (reinstall) changes: dlist-0.7.0.1 -> 0.5 data-default-0.5.3 (reinstall) cookie-0.4.1.1 (new package) haskell-src-exts-1.13.5 (new version) hslua-0.3.12 (new package) mime-types-0.1.0.4 (new package) multiset-0.2.2 (new package) pandoc-types-1.12.3.2 (new package) pem-0.2.2 (new package) polyparse-1.8 (new version) publicsuffixlist-0.1 (new package) http-client-0.3.2 (new package) regex-pcre-builtin-0.94.4.8.8.34 (new package) highlighting-kate-0.5.6.1 (new package) resourcet-0.4.10.2 (new version) securemem-0.1.3 (new package) crypto-cipher-types-0.0.9 (new package) cipher-aes-0.2.7 (new package) cipher-rc4-0.1.4 (new package) crypto-random-0.0.7 (new package) cprng-aes-0.5.2 (new package) crypto-numbers-0.2.3 (new package) crypto-pubkey-0.2.4 (new package) socks-0.5.4 (new package) temporary-1.1.2.5 (new package) text-stream-decode-0.1.0.5 (new package) conduit-1.0.17.1 (new version) http-client-conduit-0.2.0.1 (new package) wl-pprint-text-1.1.0.2 (new package) graphviz-2999.16.0.0 (new package) x509-1.4.11 (new package) x509-store-1.4.4 (new package) x509-system-1.4.5 (new package) x509-validation-1.5.0 (new package) tls-1.2.6 (new package) connection-0.2.1 (new package) http-client-tls-0.2.1.1 (new package) http-conduit-2.0.0.10 (new package) xml-1.3.13 (new package) texmath-0.6.6.1 (new package) yaml-0.8.8.2 (reinstall) changes: conduit-1.1.1 -> 1.0.17.1, resourcet-1.1.2 -> 0.4.10.2 zip-archive-0.2.2.1 (new package) pandoc-1.12.3.3 (new package) Graphalyze-0.14.0.2 (new package) SourceGraph-0.7.0.5 (new package) cabal: The following packages are likely to be broken by the reinstalls: stylish-haskell-0.5.10.0 scion-browser-0.3.1 persistent-template-1.3.1.3 persistent-sqlite-1.3.0.5 persistent-1.3.0.6 buildwrapper-0.8.0 dynamic-cabal-0.3.1 Use --force-reinstalls if you want to install anyway.

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A little explanation!
by Gilberto Melfe 01 May '14

01 May '14

Hello everybody ! Could someone explain me exactly how this function works? elementAt_w'pf = (last .) . take . (+ 1) It's a posted solution to: 99 Haskell problems, problem 3. I'm having trouble with the "(last .) ." thing! Hope someone can help! Thank You! Gilberto

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How to Update Cabal?
by Ari King 01 May '14

01 May '14

Hi, I installed the haskell platform for ubuntu, which contains cabal-install version 1.16.0.2 I'd like to use sandboxes, which as I understand is available in versions >= 1.18. Is it best to clone the cabal git repo and install as described here<http://coldwa.st/e/blog/2013-08-20-Cabal-sandbox.html>? In which case, should I remove the existing cabal installation from "/usr/bin"? Best, Ari

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Type constructors sharing a common field
by Lorenzo Tabacchini 01 May '14

01 May '14

Imagine the following data type: data Person = Child { childAge :: Int, school :: School } | Adult { adultAge :: Int, job :: Job } Both the alternatives share an "age" field, but in order to access it we are obliged to match all the constructors: personAge :: Person -> Int personAge (Child {childAge = age}) = age personAge (Adult {adultAge = age}) = age Is there a way to define a common field in a data type (somehow like inheritance in the OOP world)? It would be practical if we could define "age :: Int" as a common field and do something like: personAge (_ {age = age}) = age An alternative solution could be extracting the varying fields in a different type: data WorkingPerson = Child School | Adult Job data Person = Person { age :: Int, workingPerson :: WorkingPerson } Or to make Person a type class (which would probably require existential types in order to be useful). But the first one looks simpler and more natural. Does this feature exist (maybe in other forms) and, if not, is there a reason? Would it make sense to have a GHC extension (or even to make a Template Haskell hack) for this feature?

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DB + CSV + Mail
by Ari King 30 Apr '14

30 Apr '14

Hi, I'd like to port a python script that queries a DB, writes the results to a CSV file, and emails that file. I'd appreciate suggestions on libraries and/or approaches that can help me achieve this. For DB querying I was thinking of using HDBC. Best, Ari

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development workflow ?
by John M. Dlugosz 30 Apr '14

30 Apr '14

If you're working on a Haskell project that includes packages, and the packages are in subdirectories of the source tree, how do you "build"? In my case, I want to alter the "gloss" package so I unpacked it, changed the name to "customgloss" in the .cabal file, and installed. Meanwhile, I used the package-quallified import GHC feature. So I have to go into that subdirectory and run cabal install, let it recompile everything in there and copy it to the repository, then chdir back out and build/run my main program. How about a "make"-like way that knows if a source file in that package directory has changed, only recompiles the changed parts, and can put the output files in my directory tree with the source rather than copying to the machine repository? How do you normally go about developing a new version of a package? —John

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Building a tree
by Norbert Melzer 29 Apr '14

29 Apr '14

Hi! I have to implement a solver for the game 2048 and decided to use Haskell from the pool of possible languages. My current implementation has a fixed recursion depth of 5 and simply sums up the possible score the best line of moves would give. One problem with this implementation is, that in some cases obvious defends are favoured because of the points that the dead end grants in the short term. So I now want to build up a tree of moves and pick that move that has the deepest subtree. My problem now is, that I haven't worked with trees before, so I don't have any idea how to build it up recursively. Also, since I don't consider new stones that would spawn after doing my move, I might create infinite subterranean just swiping left and right or up and down or anything else, how would I be able to consider such subtrees as a game over on the first repetition? TIA Norbert

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annoying precedence of unary negate
by John M. Dlugosz 28 Apr '14

28 Apr '14

[ ⋯ , translate (-50) 0 $ color green $ Circle 50 , ⋯] The parens around -50 are necessary. Is there a more elegant way to write it? Without the parens, I don't understand what the compiler sees it as, based on the error message. How does the compiler parse it in that case? Also, is the comma in a list different from the operator comma which is noted as right-associative precedence 5? I had thought that the item separator in the list was special syntax that had very low precedence. After all, I can write f$g as a list item without parens, and $ is lower than comma. What is comma (as an operator) used for?

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