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[Haskell-Beginners] Just
by John Sampson 14 Jun '11

14 Jun '11

What does the word "Just" mean in Haskell code? What is its purpose? Regards _John Sampson_

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Rechnung
by Ertugrul Soeylemez 14 Jun '11

14 Jun '11

Und nochmal. -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://ertes.de/

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(no subject)
by Fernando Henrique Sanches 13 Jun '11

13 Jun '11

http://maipenarai.com/lindex02.html

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Does Haskell have a function that creates a list corresponding to the nodes in a tree, from the leaf node to the root node?
by Costello, Roger L. 13 Jun '11

13 Jun '11

Hi Folks, I am implementing the following function in another language. I want to see if Haskell already has a function that does this; if so, I want to use the Haskell function name rather than inventing my own. Here is the function that I am implementing: consider a tree data structure. I pass to the function one of its leaf nodes. The function returns a list containing that leaf node as the first element of the list, the leaf node's parent as the second element of the list, ..., the tree's root node as the n'th element of the list. (The list consists of all the nodes from the leaf node to the root node) Does Haskell have a function that does this? If not, and you were to define a function for this, what name would you give to the function? /Roger

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Graph algorithms
by . 13 Jun '11

13 Jun '11

Hello Haskellers, I would like to implement a graph based algorithm, the sum-product algorithm, in Haskell. I have looked at the FGL package, but I am not sure whether this is really what I need. For example, I need to know whether I am at a leaf (assuming the first version I implement is the "original" algorithm which works for trees). I would also need to store numerical values as well as functions at nodes and edges. The FGL provides for "labels" at nodes and edges, but these appear to have to be instances of classes such as Eq or Ord, which does not work well for functions. I could of course store node values and edge functions externally, e.g. in a Map. All this seems a bit awkward, but I may still just not have enough experience in Haskell. Can anyone give me a hint, or is there maybe even already a factor graph implementation out there which I have missed? Thanks, Christian

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Memory usage patterns for server-like processes
by Sean Charles 13 Jun '11

13 Jun '11

Does anybody know of any reading material that I can ingest that will give me a good feel for how Haskell memory allocation may (or may not) spike at run-time. I want to write a server process (yes, another web server) as part of my continued learning and I just wondered how lazy evaluation might affect a long running server process under a heavy "slashdot" type of load. I know there are lazy and strict alternatives to a lot of functions, I understand 9at least in principle) that you can use strict forms in Data.Map et al. to ensure that the contents aren't thunked, and I know that you can use $! to enforce a return result to be evaluated rather than thunked but are there still issues to be wary of or am I needlessly scaring myself? Has anybody got any long-term use cases with Yesod, HappStack, Snapp and such like. It would be interesting to know! :) Thanks, Sean Charles.

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[Updated] Summary of Chapter 1 of Pearls of Functional Algorithm Design
by Costello, Roger L. 12 Jun '11

12 Jun '11

Hi Folks, I completed my Powerpoint slides which summarize the material in chapter 1 of "Pearls of Functional Algorithm Design": http://www.xfront.com/Pearls-of-Functional-Algorithm-Design/Chapter1/Find-t… /Roger

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Regex woes...
by Sean Charles 11 Jun '11

11 Jun '11

I did a cabal update then: cabal install regex-base cabal install regex-compat cabal install regex-posix Everything built and installed, there were no errors as far as I could tell. However, on attempting to get into regex on page 199 of my Real World Haskell book I get this error when trying to use [String] as the return type and I don't know why... GHCi, version 6.12.3: http://www.haskell.org/ghc/ :? for help Loading package ghc-prim ... linking ... done. Loading package integer-gmp ... linking ... done. Loading package base ... linking ... done. Loading package ffi-1.0 ... linking ... done. Prelude> :m Text.Regex.Posix Prelude Text.Regex.Posix> Prelude Text.Regex.Posix> "this is the string" =~ "th" :: [String] <interactive>:1:0: No instance for (RegexContext Regex [Char] [String]) arising from a use of `=~' at <interactive>:1:0-27 Possible fix: add an instance declaration for (RegexContext Regex [Char] [String]) In the expression: "this is the string" =~ "th" :: [String] In the definition of `it': it = "this is the string" =~ "th" :: [String] Prelude Text.Regex.Posix> I am trying to write a page scraper, I've already sucked pages with Network.HTTP and I thought it'd be easy to get the anchor tags!

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Simple warp-based server
by Michael Xavier 10 Jun '11

10 Jun '11

Hi Haskell Beginners, I am writing a fairly simple app in haskell that will be used to support some Ruby on Rails apps at work. The gist of it is that it takes a JSON request and produces a JSON response. I've figured that strapping it to a web server will be the easiest way to allow our Rails apps to talk to it without worrying about IPC and all of that. I looked at yesod and snap and both seemed to be too much for what I'm asking. It seems to me that the best approach would be to write up an WAI interface, strap it to a Warp server and call it a day. 1) Is my reasoning about the approach to take sound or is there an alternative I haven't considered? My app will have 1 path (or rather won't care about the request path) and will take its input by just reading the request body. 2) Could anyone recommend what I need to read to take this approach? I started looking at Warp and WAI documentation and found I was well over my head as I have no knowledge of Iterators/Iteratees/Enumerators nor Builders, but I'm definitely willing to learn if that's what it takes. -- Michael Xavier http://www.michaelxavier.net

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Why do class instance functions behave differently when executed directly in WinGHCi than when loaded from a file?
by Costello, Roger L. 10 Jun '11

10 Jun '11

Hi Folks, I created a file, and in the file I entered this type class and an instance: class MyEqualityOperator a where isEqual :: a -> a -> Bool instance MyEqualityOperator Double where isEqual x y = x == y To test the isEqual function I entered this: test = isEqual 3 4 When I loaded my file into WinGHCi, I got this error: ---------------------------------------------------------------------------- Ambiguous type variable `t' in the constraints: `Num t' arising from the literal `3' at simpleTypeClassTest.hs:11:14 `MyEqualityOperator t' arising from a use of `isEqual' at simpleTypeClassTest.hs:11:5-16 Probable fix: add a type signature that fixes these type variable(s) ---------------------------------------------------------------------------- Why is it "ambiguous"? Interestingly, when I typed this at the WinGHCi command prompt: isEqual 3 4 I got no error and I got the expected result (False). Why is this? What would I need to do, in my file, to make this work properly: test = isEqual 3 4 /Roger

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