June 2011 - Beginners

Regex woes...
by Sean Charles 10 Jun '11

10 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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can't authenticate to facebook
by Chris Bolton 09 Jun '11

09 Jun '11

anyone have any idea why the following code isn't working? http://sprunge.us/LfJf It just gives: StatusCodeException 400 (Chunk "{\"error\":{\"type\":\"OAuthException\",\"message\":\"Missing client_id parameter.\"}}" Empty) Trying to follow this: http://hackage.haskell.org/packages/archive/authenticate/0.9.0.1/doc/html/W…

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How to know which module my library belongs to ?
by Adrien Haxaire 09 Jun '11

09 Jun '11

Hello, I am writing a finite element method (FEM) library. Nothing serious at the moment, this is my home project to help me learn Haskell and add my stone to the community, as I did not find such a library. For the JSON library, in the folder of the library, there are some files for the licence and cabal, a directoy for the tests and a directory Text, in which there is the directory JSON and the file JSON.hs. I don't know which module should be the parent of my library. I think it is in Mathematics and Physics, like MEEPS for example, but I prefer to ask here to be sure. Many thanks in advance, Adrien

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Re: [Haskell-beginners] "Trinary" version of Either
by Antoine Latter 09 Jun '11

09 Jun '11

On Thu, Jun 9, 2011 at 3:58 PM, Christopher Howard <christopher.howard(a)frigidcode.com> wrote: > My preference would be: just like a regular Monad either instance from mtl, > except that the Middle value also bypasses the bound function. My semantics > would be: left value represents error, right reprents normal results, and > middle represents results that are not erroneous but that prevent further > calculations. I have some idea of how this could be used to chain together > actions and avoid messy branching in code. > >From your description, 'Either (Either errType earlyExitType) a' should do everything you want, then, with the default monad instance for either. You would need something like: > myThrowError = Left . Left > haltExecution = Left . Right Antoine > Sent from my HTC > > ----- Reply message ----- > From: "Antoine Latter" <aslatter(a)gmail.com> > Date: Thu, Jun 9, 2011 8:18 am > Subject: [Haskell-beginners] "Trinary" version of Either > To: "Christopher Howard" <christopher.howard(a)frigidcode.com> > Cc: "Haskell Beginners" <beginners(a)haskell.org> > > > On Thu, Jun 9, 2011 at 10:15 AM, Christopher Howard > <christopher.howard(a)frigidcode.com> wrote: >> Is there a Haskell package out there that provides a "trinary" version >> of Either, i.e., allowing three values instead of two? I can see how >> that would be very handy. Defining one obviously wouldn't be difficult, >> but it would be helpful if someone else had already developed Monad >> bindings and other related stuff. >> > > What would the Monad operations do? > >> -- >> frigidcode.com >> theologia.indicium.us >> >> _______________________________________________ >> Beginners mailing list >> Beginners(a)haskell.org >> http://www.haskell.org/mailman/listinfo/beginners >> > > > >

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Infinite type ... or 'not enough type arguments'...
by Sean Charles 09 Jun '11

09 Jun '11

Further to my recent attempts to scan a CSV file and build a map, I now have a foldl calling this with an empty map: --forwardRoutes :: M.Map String [String] -> Record -> M.Map forwardRoutes map row = case lookup map (row!!0) of Nothing -> M.insert (row!!0) [(row !! 1)] map Just routes -> M.insert (row!!0) (row!!1):routes map With the type declaration commented out I get this: scread.hs:102:46: Occurs check: cannot construct the infinite type: a = M.Map [(a, b)] [[(a, b)]] Expected type: M.Map [(a, b)] [[(a, b)]] Inferred type: a In the third argument of `M.insert', namely `map' In the expression: M.insert (row !! 0) [(row !! 1)] map and with it 'in#' i get this error instead... scread.hs:99:52: `M.Map' is not applied to enough type arguments Expected kind `?', but `M.Map' has kind `* -> * -> *' In the type signature for `forwardRoutes': forwardRoutes :: M.Map String [String] -> Record -> M.Map Can anybody help me to understand both problems. I thought I had declared the type signature of forwardRoutes correctly but obv. not! LOL I have seen information that says the "infinite error" message can be cured by supplying types so I did...... Thanks again. Sean

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"Trinary" version of Either
by Christopher Howard 09 Jun '11

09 Jun '11

Is there a Haskell package out there that provides a "trinary" version of Either, i.e., allowing three values instead of two? I can see how that would be very handy. Defining one obviously wouldn't be difficult, but it would be helpful if someone else had already developed Monad bindings and other related stuff. -- frigidcode.com theologia.indicium.us

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Calculations that might fail: with arrays
by Christopher Howard 09 Jun '11

09 Jun '11

I was wondering if there was a stock function or package in Haskell that allows you to map over a list, but short-circuits the mapping if any of the calculations fail. Say, something like this: -- Don't know if this is a valid signature! :) mapThatCanFail :: (b -> Either a b) -> [b] -> Either a [b] The idea being that, if any calculations fail, the function doesn't bother mapping the rest of the list, but just returns a Left value (error). Otherwise, it returns a Right value containing the new list. I suppose I could try to implement that myself, but it seems like the sort of thing that someone would have thought of already. -- frigidcode.com theologia.indicium.us

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translations PCF to LC, reduction
by ahrens 08 Jun '11

08 Jun '11

Hello, sorry for this slightly offtopic post: I am looking for information about translations from the programming language PCF to the pure lambda calculus. I am particularly interested in how those translations behave with respect to reduction in the source and target languages, such as - does a specific translation create new redexes? - are irreducible terms mapped to irreducibles? I'd be grateful for any reference regarding possible translations. Thanks, ben

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