November 2011 - Beginners

multi-haskell management
by Rustom Mody 08 Nov '11

08 Nov '11

Ive just finished building haskell from source. [Thanks to all the folks who've made this so easy] 1. What is the official way for ensuring that some command like $ cabal install vector goes into my new haskell and not the old? 2. If I modify PATH so that the new ghc is found before the system one, will not system-ly things break (when for example I mess around in the haskell sources and break it?) [BTW I am on debian testing/gnome and I gather that an increasing amt of sysad stuff is being done in haskell. So I certainly do not want that I play around inside haskell sources and synaptic for example stops working :-) ]

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Re: [Haskell-beginners] Yesod umlaut issue
by Chaddaï Fouché 05 Nov '11

05 Nov '11

On Sat, Nov 5, 2011 at 3:59 PM, Obscaenvs <obscaenvs(a)gmail.com> wrote: > > As for a related issue, it isn't immediately obvious from the Yesod book how > I should go about producing "Γ" for instance in the html output -- if > I do > > gammaString = "Γ" > ...later... > #{gammaString} > > ...then this will result in "&Gamma;" being output in the html source. > Should I in this case also produce Html directly? It's pretty good having > these as shortcuts, as long as I do not have a keyboard that can easily > switch to, say, Greek encoding... > Well, I guess those shortcuts are only present in your own code and strings, so using something like > #{preEscapedText myStringWithEntities} is probably not too much of a security risk, just be more cautious if you're getting outside content... -- Jedaï

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Yesod umlaut issue
by Obscaenvs 05 Nov '11

05 Nov '11

Hello, friends. I come in peace. I would like there to be an "umlaut-translator"-function for Yesod (i.e. "ö" -> "ö"), but I have yet to find it. Furthermore the "#{}"-syntax does escaping of stuff like '<', '>' and, a more troubling '&' which is used in the umlaut-syntax. So any umlaut-translator would have to do this *after* the application of toHtml by the "#{}"-syntax which complicates writing ones own umlaut-converter. I may be totally wrong here; if so, feel free to correct me. I am also rather new to Yesod, but I am intent on giving it a good evaluation. I am going to write several web applications in the near future for different clients, and Yesod seems like the promising tool. However, I need this conversion tool -- I *think*; maybe I have the wrong angle altogether. /Fredrik

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Cabal: Upgrading to the latest version of library
by Hugo Ferreira 04 Nov '11

04 Nov '11

Hello, Recently I had to update and install a newer version of a library. I executed the "cabal update", which downloads the latest package list. I had assumed that this command would also detect, build and install those libraries that have been upgraded. However this does not seem to be the case. I had to manually build the library and then use GHC's package manager do unregister the old version. My question is: how does one check for and upgrade the cabal libraries automatically? TIA, Hugo F.

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Selecting single result of function application to list
by Hugo Ferreira 04 Nov '11

04 Nov '11

Hello, Apologies the simpleton question but I would like to know how it is done in Haskell. I have a list of values, and I am applying a function to each of these elements. The result is a Maybe. I would like to return the first occurrence which is not a Nothing. I am considering something like: selectOne f = take 1 . filter (\e -> case e of Just _ -> True _ -> False ) . map f I this how it is done? TIA, Hugo F.

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Stack space overflow: using strict accumulator still fails
by Hugo Ferreira 01 Nov '11

01 Nov '11

Hello, Have a stack overflow but cannot see why (read up on [1], may be missing something trivial). Once again using the http://nlpwp.org/ book code. If I call the following function, it blows its top: scoreRule :: TransformationRule -> Z.Zipper (Tag, Tag) -> Int scoreRule r z = nCorrect - nIncorrect where (nCorrect, nIncorrect) = scoreRule_ r z scoreRule_ :: TransformationRule -> Z.Zipper (Tag, Tag) -> (Int, Int) scoreRule_ r = Z.foldlz' (scoreElem r) (0, 0) where scoreElem r s@(nCorrect, nIncorrect) z = case ruleApplication r z of Just tag -> if tag == correct then (nCorrect + 1, nIncorrect) else (nCorrect, nIncorrect + 1) Nothing -> s where (correct, _) = Z.cursor z however I see that the eager version of foldlz is being used. I also though that maybe ruleApplication my not be executing immediately. But I cannot see why (added definition below for reference). Can anyone point out why this is not strict? TIA, Hugo F. [1] http://www.haskell.org/haskellwiki/Stack_overflow ruleApplication :: TransformationRule -> Z.Zipper (Tag, Tag) -> Maybe Tag ruleApplication (NextTagRule (Replacement old new) next) z = do (_, proposed) <- Z.safeCursor z (_, nextProposed) <- rightCursor z if proposed == old && nextProposed == next then Just new else Nothing ruleApplication (PrevTagRule (Replacement old new) prev) z = do (_, proposed) <- Z.safeCursor z (_, prevProposed) <- leftCursor z if proposed == old && prevProposed == prev then Just new else Nothing ruleApplication (SurroundTagRule (Replacement old new) prev next) z = do (_, proposed) <- Z.safeCursor z (_, nextProposed) <- rightCursor z (_, prevProposed) <- leftCursor z if proposed == old && prevProposed == prev && nextProposed == next then Just new else Nothing

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