- Beginners - Haskell.org

learning about exceptions
by Jake Penton 22 May '11

22 May '11

I am working my way through Real World Haskell. I just finished chapter 8, and am moving through chapter 9. I note that RWH uses deprecated "old" exceptions. I am currently importing the old style exceptions as I work through the code, but would like to learn the current (new) exception stuff. Is there a good tutorial on thissomewhere ? Please bear in mind that at my state of knowledge it is still rather difficult to get this knowledge by reading the library documentation alone. I am not all that great at inferring what is going on just by inspecting signatures, etc. Is ch.19 of RWH (which covers exceptions in more depth) still relevant to "new" exceptions? Thanks. - Jake -

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code review?
by Chris Bolton 22 May '11

22 May '11

Hey guys. Pretty new to haskell here. I've started off by writing a small command-line interface to plurk.. I was just wondering if anyone would be willing to give everything a look-over and lemme know what kinds of things I should be looking to improve upon, style-wise. Not sure I'm currently doing things in the 'haskell-way' so to speak :P. Thanks a bunch! https://github.com/saiko-chriskun/hermes (note: the JSON and Plurkell submodules are also mine.)

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Re: [Haskell-beginners] Load functions from broken source code files?
by Christopher Howard 21 May '11

21 May '11

On 05/21/2011 01:54 PM, Tom Murphy wrote: > I'm sorry if this is obvious, but it's only 8 keystrokes to comment > out everything before and after the function you're testing: > {- > ... > -} > f x = ... > {- > ... > } > > Tom Ha! Provided, of course, your editor knows that you need to first move your cursor to the point just before the function, then to the point just after your function, then to the beginning of the file (after any required imports) then to the end of the file. Otherwise, that might be more like a 12 stroke action, granting that your function is only one line long and that you happen to be on the same line as your function. Unless, of course, you are using a graphical editor and use your mouse to set cursor position, in which case 8 keystrokes is possible, but then the time required is proportional to the size of your source code file and the speed of text scrolling. And, of course, you will eventually need to uncomment the code, which will require additional keystrokes. ;) -- frigidcode.com theologia.indicium.us

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Load functions from broken source code files?
by Christopher Howard 21 May '11

21 May '11

Is there some way in GHCi to load a single function from a broken source code file? E.g, you're writing a program, the main function is incomplete, and half the other functions are broken, but you are developing one function and you just want to load in the current version of that, reload it after some editing, and so forth. I can see how normally that would not be a good idea, but I was wondering if there was some hackish way to do it that was fairly easy and quick. -- frigidcode.com theologia.indicium.us

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"pure" versus "impure" code
by Costello, Roger L. 21 May '11

21 May '11

Hi Folks, Is there a web site that defines "pure" versus "impure" code? My understanding is that: - Pure code is code that does no I/O - Impure code is code that does I/O Is that correct? Also, it is my understanding that good software design is to isolate/separate the impure code from the pure code. Is that correct? Does that principle apply to all programming languages, or just Haskell? /Roger

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Extension for "Pearls of Functional Algorithm Design" by Richard Bird, 2010, page 25 #Haskell
by KC 21 May '11

21 May '11

Extension for "Pearls of Functional Algorithm Design" by Richard Bird, 2010, page 25 #Haskell ------------------------------------------------------------------------------- ------------------------------------------------------------------------------- module SelectionProblem where import Data.Array import Data.List ------------------------------------------------------------------------------- ------------------------------------------------------------------------------- -- Question: is there a way to get the type signature as the following: -- smallest :: (Ord a) => Int -> [Array Int a] -> a ------------------------------------------------------------------------------- ------------------------------------------------------------------------------- -- Works on 2 finite ordered disjoint sets represented as sorted arrays. smallest :: (Ord a) => Int -> (Array Int a, Array Int a) -> a smallest k (xa,ya) = search k (xa,ya) (0,m+1) (0,n+1) where (0,m) = bounds xa (0,n) = bounds ya -- Removed some of the "indexitis" at the cost of calling another function. search :: (Ord a) => Int -> (Array Int a, Array Int a) -> (Int,Int) -> (Int,Int) -> a search k (xa,ya) (lx,rx) (ly,ry) | lx == rx = ya ! (k+ly) | ly == ry = xa ! (k+lx) | otherwise = case (xa ! mx < ya ! my) of (True) -> smallest2h k (xa,ya) ((lx,mx,rx),(ly,my,ry)) (False) -> smallest2h k (ya,xa) ((ly,my,ry),(lx,mx,rx)) where mx = (lx+rx) `div` 2 my = (ly+ry) `div` 2 -- Here the sorted arrays are in order by their middle elements. -- Only cutting the leading or trailing array by half. -- Here xa is the first array and ya the second array by their middle elements. smallest2h :: (Ord a) => Int -> (Array Int a, Array Int a) -> ((Int,Int,Int),(Int,Int,Int)) -> a smallest2h k (xa,ya) ((lx,mx,rx),(ly,my,ry)) = case (k<=mx-lx+my-ly) of (True) -> search k (xa,ya) (lx,rx) (ly,my) (False) -> search (k-(mx-lx)-1) (xa,ya) (mx+1,rx) (ly,ry) ------------------------------------------------------------------------------- ------------------------------------------------------------------------------- -- Works on 3 finite ordered disjoint sets represented as sorted arrays. smallest3 :: (Ord a) => Int -> (Array Int a, Array Int a, Array Int a) -> a smallest3 k (xa,ya,za) = -- On each recursive call the order of the arrays can switch. search3 k (xa,ya,za) (0,bx+1) (0,by+1) (0,bz+1) where (0,bx) = bounds xa (0,by) = bounds ya (0,bz) = bounds za -- Removed some of the "indexitis" at the cost of calling another function. search3 :: (Ord a) => Int -> (Array Int a, Array Int a, Array Int a) -> (Int,Int) -> (Int,Int) -> (Int,Int) -> a search3 k (xa,ya,za) (lx,rx) (ly,ry) (lz,rz) | lx == rx && ly == ry = za ! (k+lz) | ly == ry && lz == rz = xa ! (k+lx) | lx == rx && lz == rz = ya ! (k+ly) | lx == rx = search k (ya,za) (ly,ry) (lz,rz) | ly == ry = search k (xa,za) (lx,rx) (lz,rz) | lz == rz = search k (xa,ya) (lx,rx) (ly,ry) | otherwise = case (xa ! mx < ya ! my, xa ! mx < za ! mz, ya ! my < za ! mz) of (True, True, True) -> smallest3h k (xa,ya,za) ((lx,mx,rx),(ly,my,ry),(lz,mz,rz)) -- a<b<c (True, True, False) -> smallest3h k (xa,za,ya) ((lx,mx,rx),(lz,mz,rz),(ly,my,ry)) -- a<c<b (False, True, True) -> smallest3h k (ya,xa,za) ((ly,my,ry),(lx,mx,rx),(lz,mz,rz)) -- b<a<c (False, False, True) -> smallest3h k (ya,za,xa) ((ly,my,ry),(lz,mz,rz),(lx,mx,rx)) -- b<c<a (True, False, False) -> smallest3h k (za,xa,ya) ((lz,mz,rz),(lx,mx,rx),(ly,my,ry)) -- c<a<b (False, False, False) -> smallest3h k (za,ya,xa) ((lz,mz,rz),(ly,my,ry),(lx,mx,rx)) -- c<b<a where mx = (lx+rx) `div` 2 my = (ly+ry) `div` 2 mz = (lz+rz) `div` 2 -- Here the sorted arrays are in order by their middle elements. -- Only cutting the leading or trailing array by half. -- Here xa is the first array, ya the second array, and za the third array by their middle elements. smallest3h :: (Ord a) => Int -> (Array Int a, Array Int a, Array Int a) -> ((Int,Int,Int),(Int,Int,Int),(Int,Int,Int)) -> a smallest3h k (xa,ya,za) ((lx,mx,rx),(ly,my,ry),(lz,mz,rz)) = case (k<=mx-lx+my-ly+mz-lz) of (True) -> search3 k (xa,ya,za) (lx,rx) (ly,ry) (lz,mz) (False) -> search3 (k-(mx-lx)-1) (xa,ya,za) (mx+1,rx) (ly,ry) (lz,rz) -- -- Regards, KC

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"In several chapters there is a penetrating conceptual analysis of the matter at hand, on which a superstructure of code is erected."
by KC 21 May '11

21 May '11

Interesting comment on the book "The Fun of Programming" in JOOP. "In several chapters there is a penetrating conceptual analysis of the matter at hand, on which a superstructure of code is erected." -- -- Regards, KC

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Is there a place where we can have a discussion on the merits of various GUI's & various graphics? And then vote?
by KC 20 May '11

20 May '11

:) -- -- Regards, KC

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question about Yesod
by Michael Litchard 20 May '11

20 May '11

I admire what's been done with Yesod, especially in the example given by the Haskellers website. It's time I start making design decisions for my own website project, and I thought it would be a great idea to just steal from Haskellers liberally. My initial approach was to just gut Haskellers until everything I don't need is gone. This isn't working out as I had hoped. If it's not a huge pain in the butt, could someone put an example on github that does the following: I'm looking for the look of Haskellers. The tabs, and navbars is fine. The little bar that tracks what part of the site you are on would be bonus. Thanks for any help on this. Michael Litchard

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Class and Instance Question
by Adam C. Emerson 20 May '11

20 May '11

Good morning, As my first "real" toy project (something trivial but with some pretense of usefulness at least to me) in Haskell, I had been trying to write an XDR encoder/decoder based on top of Data.Binary. As such, I have a typeclass: class Encodable t where -- | Encode a value in the Put monad. put :: t -> Put -- | Decode a value in the Get monad get :: Get t And I have various instances, all of which seem to work, except for two. My thought was to treat a list of Chars as a string and a list of "encodable" types as a counted array. Thus, I tried: instance Encodable [Char] where put s = put $ runPut (putUTF8str s) get = do bs <- get return (runGet getUTF8str bs) and instance (Encodable e) => Encodable [e] where put l = if (length l) > xdrmaxlen then fail "Length of data exceeds XDR maximum for arrays." else (put (length l) >> putFixed (length l) l) get = do n <- get getFixed n This failed, and the compiler suggested I try adding: {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE OverlappingInstances #-} But even with these two statements, I get: *Data.XDR.Encodable> encode [1, 2, 3] <interactive>:1:0: Overlapping instances for Encodable [t] arising from a use of `encode' at <interactive>:1:0-15 Matching instances: instance [overlap ok] (Encodable e) => Encodable [e] -- Defined at /home/azure/programming/xdr/src/Data/XDR/Encodable.hs:226:9-38 instance [overlap ok] Encodable [Char] -- Defined at /home/azure/programming/xdr/src/Data/XDR/Encodable.hs:160:9-24 (The choice depends on the instantiation of `t' To pick the first instance above, use -XIncoherentInstances when compiling the other instance declarations) In the expression: encode [1, 2, 3] So, the question I have, more than wondering how to get this to work (I suspect I shouldn't even be doing this, and instead I should newtype XDRInt and XDRString and so on), is why the two instances overlap. I have no instance for Char, so the first instance should apply to lists of Char (which isn't Encodable.) And the second should apply to lists of Encodable things (which Char isn't.) Is there a fairly comprehensible source I should read to understand typeclasses and instances better? (I've read my way through Real World Haskell, the Wikibook, and A Gentle Introduction, though it's possible they covered this and I just missed it.) Thank you.

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