- Beginners - Haskell.org

Type constructors sharing a common field
by Lorenzo Tabacchini 30 Apr '14

30 Apr '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 29 Apr '14

29 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 29 Apr '14

29 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 28 Apr '14

28 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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Clocks, scheduling, jack and supercollider
by martin 27 Apr '14

27 Apr '14

Hello all, I just installed the haskell-supercollider bindings ("hsc3") and I am able to send events to supercollider and hear sound. But I have no idea how to schedule events to be sent in the future - a prerequisite to play a song. Ideally I would sync to an external clock source, like a time code (SMPTE/MIDI). Also it would be nice to listen to jack-transport, so I can fast-forward and rewind from another application. But the jack bindings for Haskell do not mention jack-transport. Do you have any advice where I should look next.

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easy question: transform Float -> Int how?
by John M. Dlugosz 25 Apr '14

25 Apr '14

If I have a value of type Float, and I want to pass it (approximately rounded) to a function wanting an Int, how do I do that?

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How to build a package?
by John M. Dlugosz 24 Apr '14

24 Apr '14

I used cabal unpack to get the source of a package to read. Now I want to make a change. How do I tell Haskell Platform to make that (the same way cabal knows how to) and do so with the provided files rather than downloading into temp. I got a package to work before using Leksah, but I'd like to do it directly since the cabal file is present and works just fine. A related issue: what should I do about the local copy of a module name that is also installed? Can I make my project use that instead, without having to globally change the name in all the files?

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why do classes require the type variable in type signatures?
by Dimitri DeFigueiredo 23 Apr '14

23 Apr '14

Hello All, Why does this compile class Special a where isSpecial :: a -> Bool whereas, GHC 7.6 complains about this class AlsoSpecial a where isAlsoSpecial :: b -> Bool This is the error message I get: The class method `isAlsoSpecial' mentions none of the type variables of the class AlsoSpecial a When checking the class method: isAlsoSpecial :: forall b. b -> Bool In the class declaration for `AlsoSpecial' My question is: Why must I use the type variable of the class declaration (i.e. *a*) in the type signature for the associated method? Is there a fundamental reason for this or is it just a GHC specific limitation? I didn't see a need for it when watching this video http://channel9.msdn.com/posts/MDCC-TechTalk-Classes-Jim-but-not-as-we-know… that explains the translation that GHC does to turn type classes into core. Thanks! Dimitri

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How would you write...
by John M. Dlugosz 22 Apr '14

22 Apr '14

chain1 = [ translate x y $ color red $ Circle r | (x,y,r) <- pappus 100 [1..10] ] The above is not right, as the comprehension syntax doesn't see the input range buried in an argument. What's the right way to express pap1 = translate x y $ color red $ Circle r where (x,y,r) = pappus 100 1 where 1 is [1..10] and I get a list of results? (pap1 does work as expected) (1) how can the list comprehension syntax manage it, (2) what's the recommended way to express that (not necessarily via list comprehension syntax) ? —John

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