- Beginners - Haskell.org

How Interactive can Haskell Ultimately be?
by Michael Carpenter 08 Oct '12

08 Oct '12

Yep, I'm verbose in my questions. I put the core question(s) in bold. Everything else is elaboration for clarity. *- How interactive and "live" can GHCI make Haskell? What are the limits?**By "live", I mean on-the-fly coding. **- Would it be possible to make a Haskell plugin for something like the Light Table IDE even if it required some behind-the-scenes trickery?** **- How dependent is Haskell's interactivity on it's structure as a language and how much of it is dependent on the given compiler?**Is it that GHC could never provide on-the-fly coding or is it that the idea of highly interactive coding is completely incompatible with the core principles that make Haskell work? (static type system vs. dynamic type system) **- Could GHC/GHCi be used or extended to provide a more functional reactive Haskell similar to Elm such that you could play with your code the same way you can with SuperCollider or Chuck?* By interactive, I mean that the language allows for the programmer to edit the program's source as it's running and see the results of the edit play out in the current running session without having to restart the program. A lot of languages use this for debugging (GHCi), but I know that this feature is typically very limited unless you have a dynamic, reactive, and/or interpreted language. There seems to be many tricks out there for at least giving the illusion of "live coding" such as interpreting the files being edited and leaving everything that's not dependent on it compiled. I've also read about having the compiler and interpreter actively dance around each other in the source, with the interpreter managing the files that were recently edited, providing the slow but interactive functionality with only the code local to the programmer's current activity, and the compiler recompiling files after they've been left alone for a while. I totally don't know what I'm talking about here with regard to compiler behavior, but is this interactive behavior even possible with GHC/GHCI given how much optimization takes place at compile time? Can you cherry pick like that? Can you even distinguish between files anymore once it's been melted down into C--? I'm really taking to Haskell for general purpose programming and I'm finding little to complain about, but there is one thing that Haskell lacks in that other expressive high-level languages like Lisp, Python, and Ruby seem to have. Interactivity. I like that Haskell has both a good compiler and a nice interpreter, but I don't know whether (in the future) the two could work together to provide GHCI's human-centric interactivity with GHC's computer-centric optimization. Can I have good performance and eat my cake too? I was incredibly inspired by the prototype of the Light Table IDE project that got funding recently. I, like many, really liked how it gave very quick feedback in multiple ways to what you are doing. That helps me tremendously and its probably the most powerful feature a language could ever include imo. I have no doubt that such a project can become a successful reality, but the language that Light Table was performing this interactivity with was Clojure and it seems like only dynamic interpreted languages will work. Is Haskell, despite its differences from Lisp and the fact that it is a mostly compiled language, disqualified from granting the coder a true interactive environment? GHCi is REALLY powerful and helpful in this regard, but it seems limited to a debugger and a useful tool for tutorials. I'm asking this because Haskell seems to have everything I would want in a language that is not C. The only competitor to it as a functional language, in my opinion, is Lisp because of this interactivity. Haskell seems to be doing everything right as a modern, high-level, functional language, but it would be perfect (for me) if it further reduced the barrier between the coder and the code by supporting this interactive paradigm. Interactivity is a killer feature for me and I love things like the friendly interactive shell for their focus on it. I've used Vim plugins before for compiled languages where my changes were automatically compiled as I made them and a live preview presented after each compilation. In this sense, I know that virtually any language can act like you are directly affecting your software during run-time, but I'm talking about the sweet spot between that fake method and something like SuperCollider where musicians can modify their code in real-time as their code runs in real-time and not suffer from any interruptions or sudden delays - the source language is the same language that you use to interface with it. I think this is the ideal way to program and I believe it is an important feature for any language that aims to serve the human and not the machine. I want to use Haskell's high-level, expressive, and modular language as the way to play with the code I've written. Whether it's for debugging, experimentation, scripting, or learning and exploring. If this is possible (even if it's just theory and speculation for now), Haskell will be that language that lets me code the way I want to. Sorry for the long message, I didn't want any ambiguity to my question. - Michael

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IO action on a list of [IO a]
by Manfred Lotz 07 Oct '12

07 Oct '12

Hi all, I want to do an IO action on a list of [IO a] like this: myfor :: (a -> IO () ) -> [IO a] -> IO () myfor _ [] = return () myfor f (x:xs) = do x' <- x f x' myfor f xs Is there a library function doing just this? -- Thanks, Manfred

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Questions on type declarations
by thorsopia＠lavabit.com 06 Oct '12

06 Oct '12

Hi, Could you comment on the following type declarations? - getArgs :: IO [String] It can get several params, but its type declaration looks like it gets none. - dispatch :: [(String, [String] -> IO ())] [1] I don't know how to read this one. It looks bizarre. [1] http://learnyouahaskell.com/input-and-output#files-and-streams

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ACID-state - how does it evaluate data
by Konrad Szyc 04 Oct '12

04 Oct '12

Dear Haskell-Beginners I have a problem understanding how ACID-state stores its data. To be more specific, I would like to know how does it deal with lazy evaluation. I suppose it tries to "strictly" evaluate functions before serializing, but then how does it handle infinitely-recursive structures? Regards, kombat

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stream of monadic calculations
by Christopher Howard 04 Oct '12

04 Oct '12

Say I've got a stream of monadic calculations, like so: code: -------- do a' <- f a a'' <- g a' a''' <- h a'' return a''' -------- There is a cleaner way to do that, yes? (Without using all the tick marks?) -- frigidcode.com indicium.us

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Netwire fromRational
by Nathan Hüsken 03 Oct '12

03 Oct '12

Hey, I noticed than when using this code: (-1.0) . isKeyPressed leftKeyCode <|> 0.0 instead of (pure (-1.0)) . isKeyPressed leftKeyCode <|> (pure 0.0) than "fromRational" gets called. I am wondering, if this is not pretty inefficient. I am noticing this, because when I compile a netwire test program with haste, the first code segment fails during runtime because of the primOp newByteArray#. Regards, Nathan

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looking for an haskell job
by Why NotSmile 03 Oct '12

03 Oct '12

Hi all, even if I'm a newbie into the haskell world I'd like to start my next job with it. If anyone is searching for such a profile please contact me. My zipped resume (more details in private): - 7 yrs of experience - c++, c# - agile - available to relocate cheers

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computing percentile
by Dennis Raddle 01 Oct '12

01 Oct '12

Just thought I would toss the list a problem I need to solve and see what I can learn from the solutions here. My problem is that I need to take a list of Double and map them into "percentile" form (not sure if I have that terminology correct) - so that the lowest number is mapped to 0, the highest number is mapped to 1, the median is mapped to 0.5, and in general given a fixed delta D and an interval (x, x +D) in the percentile domain, the same number of elements get mapped into that interval for any x. Except there aren't an infinity of values and there may be ties, so it gets a little fuzzier how to define it. so I need: toPercentile :: [Double] -> [Double] I shall now work on it and report back. Dennis

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Using my first map instance
by Darren Grant 30 Sep '12

30 Sep '12

Hi all, I'm working on project euler problem 14, whose solution is the maximum Collatz chain length for all natural numbers less than 1-million. The naive approach takes too long to execute: collatz 1 = [1] collatz n = let next x | even x = x `div` 2 | otherwise = 3*x+1 in n:collatz (next n) result = maximum [length (collatz x) | x <- [1..999999]] I know there are a handful of approaches to take to reduce computation time, but in this case I am focusing on exploiting fast recollection of previously computed sub-chain lengths in a map. So I wrote the following instead: import qualified Data.Map as M type CollatzSubMap = M.Map Int [Int] collatz :: (Int, CollatzSubMap) -> ([Int], CollatzSubMap) collatz (1,m) = ([1], m) collatz (n,m) = let next x | even x = x `div` 2 | otherwise = 3*x+1 in case M.lookup n m of Nothing -> let (ns,m') = collatz (next n, m) in (n:ns, M.insert n (n:ns) m') Just ns -> (ns,m) result = maximum [length $ fst $ collatz (x, M.empty) | x <- [1..999999] :: [Int]] Where I'm currently stumped is in feeding the resulting map from one call to collatz into the next iteration in the list comprehension; that M.empty should carry the end result of previous iterations. Can anyone point me in the right direction? Other criticisms of the code are also welcome. Cheers, Darren

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Making sense of currying in the context of the Hask Category
by Lino Rosa 29 Sep '12

29 Sep '12

Hi, I'm a Haskell newbie and I couldn't quite make sense of how currying maps to the the Hask Category. How would I map, for instance (+) to a Hask 'arrow'? If objects are types on Hask, then would a -> a -> a be the first object on this chain? In that case, for the first arrow, would I have as many arrows as there are possible partial applications on this type? In other words, would I have (+) 1, (+) 2, (+) 3 ... all those transitioning to the second object ( a -> a ) Or, do I have ONE arrow only, like (+) a ? In either case, what happens after 'm left with the object a -> a? What function (arrow) mutates it to the final value 'a'? That's the function resulting from the previous partial application of (+), but that fuction only exists at run time, after you apply the first one. I guess the question is, if you'd have to write a diagram for this, what would you write beside each object and beside each arrow?

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