June 2013 - Haskell-Cafe

Help me understand general recursion from cata- and anamorphism
by Takayuki Muranushi 23 Jun '13

23 Jun '13

In an attempt to understand why cata- and anamorphisms are considered so important, I found multiple implications that you can write any recursive functions in terms of nonrecursive functions and ana, cata (am I right here?) so I'm trying to practice the rewrite by a few functions. I'm following a recipe found here: http://lambda-the-ultimate.org/node/4290 ~~~ Given a function that recurses on itself, do a partial CPS transform so that it only ever recurses on itself with tail calls. Then, convert the recursive calls to codata returns, so that the function either returns TheAnswer or StillWorking with enough parameters to describe the recursive call / continuation state. This codata can be built with an unfold and can be collapsed back down to the final answer with a fold. ~~~ https://github.com/nushio3/practice/blob/master/lens/banana/CollatzTest.hs https://github.com/nushio3/practice/blob/master/lens/banana/FibTest.hs I find it difficult to understand the terminology, and the above attempts are only halfway done. I guess ( TheAnswer or StillWorking ) structure is the one found in iteratee/enumeratee. But I don't know how to "build a codata with unfold." I'd appreciate any advice. Best, -- Takayuki MURANUSHI The Hakubi Center for Advanced Research, Kyoto University http://www.hakubi.kyoto-u.ac.jp/02_mem/h22/muranushi.html

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data constructor names
by Brian Lewis 22 Jun '13

22 Jun '13

Say you write data Callback = Error ... | ... because one of the kinds of callbacks you need to model is an error callback. Then, later, you write data Error = ... to model some error that can happen. They're both good names, but there's a conflict. So I started thinking I should prefix my constructor names like data Callback = CallbackError ... | ... It will work, but it's not the nicest looking. I discovered I can write data Callback = Callback'Error ... | ... Where can I find the syntax reference to see what's allowed? Will people kill me if I start doing this? I could import qualified, but in my case, there would be too many modules. Thanks for any advice.

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<<loop>> error message
by Omari Norman 22 Jun '13

22 Jun '13

I compiled some code with GHC 7.6.3 that produces a simple error at runtime myProgramName: <<loop>> At which point the program exits with code 1. Is there documentation for this error anywhere? Does it mean I have some infinite loop in my code somewhere? If so, does GHC catch all infinite loops? I have never gotten this error before. Thanks.

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The promising world of Monadic formlets
by Alberto G. Corona 22 Jun '13

22 Jun '13

Hi, This is just to let you know the promising results of some experimentation: Formlets are about applicative instances, but what about monadic instances? What a Monad instance of formlets means? I recently experimented with this and the results are very interesting and powerful- It mixes the best of web forms, with the flexibility of console applications. ???!!!!!! Althoug this example is for the formlets of the MFlow <https://github.com/agocorona/MFlow>framework , it can be ported to other formlet implementations. Although the MFLow formlets include web formatting that is not supported in other formlets implementations. Static HTML templating don´t work well with monadic formlets, so it is important to include the formatting as a part of the computation: import MFlow.Wai.Blaze.Html.All dynamicForm= wform $ do (n,s) <- (,) <- p << "Who are you?" ++> getString Nothing <! hint "name" <++ br <*> getString Nothing <! hint "surname" <++ br <** submitButton "ok" <++ br flag <- b << "do you " ++> getRadio[radiob "work?",radiob "study?"] <++ br r<-case flag of "work" -> pageFlow "l" $ Left <- b << "do you enjoy your work? " ++> getBool True "yes" "no" <** submitButton "ok" <++ br "study"-> pageFlow "r" $ Right <- b << "do you study in " ++> getRadio[radiob "University" ,radiob "High School"] p << ("You are "++n++" "++s) ++> case r of Left fl -> p << ("You work and it is " ++ show fl ++ " that you enjoy your work") ++> noWidget Right stu -> p << ("You study at the " ++ stu) ++> noWidget hint s= [("placeholder",s)] onClickSubmit= [("onclick","this.form.submit()")] radiob s n= text s ++> setRadio s n <! onClickSubmit Here wform, getBool, getString , getRadio etc are formlet elements The first sentence is an applicative composition that generate a 2 tuple, to show that applicative and monadic can be mixed. the operations ++> add html to the formlet. the operatior <! add attributes to the formlet element.. noWidget is a dumb formlet that does not validate. The second monadic statement is an election between two options. The beauty of the monadic instance is that the rest of the form can vary depending on the previous answers. Since the formlets validate the input, unless the election is made, the radio will not validate, so the monadic execution will be aborted beyond any unanswered question, so nothing will appear after the question. The rest of the form will appear when the user choose one of the two options. once one or the other option is chosen, then another binary question is presented. (either he likes his work or where he study). When the questions are finised, the results are presented. I hope that you get the idea. The benefit is not only the familiar coding and presentation of a sequential console application: Since the form encloses all the fields, At any time the user can change previous inputs and the form will reflect these changes. For example if the user change from work to study (second statements) the "where do you study will appear and the work related questions and answers will disappear. That is wonderfully useful for heavily interactive applications. There is a problem however and it is the issue of the formlet identifiers. Unlike in an applicative presentation, now the number and type of the formlets will vary, so the response to a previous form create a new kind of form, and the post response can be misinterpreted. To avoid that , the pageFlow call creates fixed field labels for each branch of execution. I will release a version of MFlow that support this kind of monadic composition of fomlets, but In essence it is nothing but to add Monad instance to formlets. A single server procedure, that executes the formlet code can support all the interaction so any framework can do it. The usability of that is huge:It is possible to interact in a web page in a console style with questions and answers with the versatitly of a dynamic foms: Any modification in the form change the subsequent flow of interaction. Another application of this monadic style is to ease multistep processes such are registration, check-out and payment ad so on. Even a entire interactive dynamic application can be coded in a single page. And no javascript is needed!. To run this formlet in MFlow: main=do addMessageFlows [("" , transient $ runFlow $ ask dynamicForm )] wait $ run port waiMessageFlow This video show how the presentation of this example vary with the user input: http://youtu.be/DryBQc9agFg I hope that you find the idea interesting. If you want to experiment with this in MFlow, I have to say that the implementation of this feature is in an early stage. The code is in the head branch https://github.com/agocorona/MFlow/tree/head Alberto.

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how to debug stack overflow?
by Johannes Waldmann 22 Jun '13

22 Jun '13

What is the recommended method to find the exact reason for a stack overflow (when running a Haskell program compiled with ghc)? When I compile with -prof -auto-all, and run with +RTS -xc, I see a very short call stack, which can't be right. But that's probably because I am calling some library method that was compiled without -auto-all so it's invisible in the trace? I can avoid the overflow by +RTS -K1G or something but I want to investigate because it might indicate some inefficiency in the code (evaluations happening at the wrong time). - J.W.

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writing a function to make a correspondance between type-level integers and value-level integers
by oleg＠okmij.org 22 Jun '13

22 Jun '13

I'm late to this discussion and must have missed the motivation for it. Specifically, how is your goal, vector/tensor operations that are statically assured well-dimensioned differs from general well-dimensioned linear algebra, for which several solutions have been already offered. For example, the Vectro library has been described back in 2006: http://ofb.net/~frederik/vectro/draft-r2.pdf http://ofb.net/~frederik/vectro/ The paper also talks about reifying type-level integers to value-level integers, and reflecting them back. Recent GHC extensions (like DataKinds) make the code much prettier but don't fundamentally change the game.

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Nightly builds of GHC HEAD for OS X 10.8
by John Wiegley 21 Jun '13

21 Jun '13

Since mid-January, I’ve been running nightly builds of GHC on my Mac Pro for 10.8.x, 64-bit. I’ve decided to make these results publically downloadable here: http://ghc.newartisans.com. The installer tarballs are in dist, while the fulltest and nofib logs are in logs. According to Jenkins this build takes 8h15m minutes, so I figured this might save others some CPU heat. -- John Wiegley FP Complete Haskell tools, training and consulting http://fpcomplete.com johnw on #haskell/irc.freenode.net

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How to make this data type work?
by Magicloud Magiclouds 21 Jun '13

21 Jun '13

data ActionData = AD { oldData :: (FromJSON j, ToJSON j) => j , newData :: (FromJSON j, ToJSON j) => j} instance ToJSON ActionData where toJSON (AD o n) = object [ "oldData" .= o , "newData" .= n ] instance FromJSON ActionData where parseJSON (Object v) = AD <$> v .: "oldData" <*> v .: "newData" parseJSON _ = mzero I got when compile: No instance for (FromJSON (forall j. (FromJSON j, ToJSON j) => j)) arising from a use of `.:' Possible fix: add an instance declaration for (FromJSON (forall j. (FromJSON j, ToJSON j) => j)) In the second argument of `(<$>)', namely `v .: "oldData"' In the first argument of `(<*>)', namely `AD <$> v .: "oldData"' In the expression: AD <$> v .: "oldData" <*> v .: "newData" -- 竹密岂妨流水过山高哪阻野云飞 And for G+, please use magiclouds#gmail.com.

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TH clause Pat selection
by Brian Lewis 20 Jun '13

20 Jun '13

I want to use TH to generate functions like foo :: c -> h foo ... = ... foo ... = ... ... from lists of pairs :: [(c, h)] For example, $(genFoo ''Int ''Bool [(0,False), (1,True)]) would generate foo 0 = False foo 1 = True The problem is, I don't know how to generate the function's clauses. "foo 0 = ..." seems to be a LitP pattern. But "foo True = ..." seems to be a ConP pattern. The appropriate pattern depends on type c. Here's code with more explanation and examples: http://hpaste.org/90163

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Ann: Cascading 0.1.0, DSL for Cascading Style Sheets
by Ertugrul Söylemez 20 Jun '13

20 Jun '13

Hi there, I've just released the first version of [cascading], library for writing/generating Cascading Style Sheets (CSS) in Haskell. The API design is inspired by many other projects, including blaze-html, HSP and Yesod's Lucius. [cascading]: <http://hackage.haskell.org/package/cascading> Features: * Style sheets are monoids and can be MonadWritered. * All non-appendix properties of CSS level 2.1 are implemented and exposed through a type-safe API. It's easy enough to set properties for which no type-safe API exists. See the example below. * Easy to integrate into all monadic web frameworks, easy to interleave stylesheets with framework operations. * Builtin support for web-routes. * Color space awareness through the colour library. * Fully encoding-aware with one caveat: If you bypass PropValue and use ByteStrings as property values directly, then UTF-8 is assumed. Example: stylesheet :: (MonadWriter CSS m) => m () stylesheet = onAll $ do select ["p"] $ do margin . Edges $ [zeroLen] padding . Edges $ [_Em # 1, _Ex # 2] select ["em"] $ do fontWeight BolderWeight select ["ul", "ol"] . below ["li"] $ do borderColor (LeftEdge green) borderWidth (LeftEdge ThickWidth) vendors $ "borderRadius" $= ("3mm" :: PropValue) This results in the following stylesheet: p { margin: 0; padding: 1em 2ex } em { font-weight: bolder } ul li, ol li { border-left-color: #008000; border-left-width: thick; border-radius: 3mm; -moz-border-radius: 3mm; -ms-border-radius: 3mm; -o-border-radius: 3mm; -webkit-border-radius: 3mm } Please visit the [homepage]. Feedback is always welcome. If you find bugs, please don't hesitate to [report] them. [homepage]: <http://hub.darcs.net/ertes/cascading> [report]: <http://hub.darcs.net/ertes/cascading/issues> Greets, Ertugrul -- Not to be or to be and (not to be or to be and (not to be or to be and (not to be or to be and ... that is the list monad.

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