August 2012 - Haskell-Cafe

Adding to / subtracting from a LocalTime?
by Adde Nilsson 19 Aug '12

19 Aug '12

Hi. How do you add to or subtract from a LocalTime? I'm trying to subtract a second from a LocalTime value but the only API's I can find act only on the TimeOfDay part. subSec :: LocalTime -> LocalTime subSec (LocalTime d t) = LocalTime d $ timeToTimeOfDay ((timeOfDayToTime t) - (secondsToDiffTime 1)) I'm obviously on the wrong track as this looks way too complex for what it's trying to achieve. Calling with '2012-08-18 00:00:00' gives '2012-08-18 -01:59:59' which isn't very helpful. How do I get to '2012-08-17 23:59:59'? Thanks, Adde

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Compositional Compiler Construction, Oberon0 examples available
by Doaitse Swierstra 19 Aug '12

19 Aug '12

On Aug 19, 2012, at 10:40 , Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > Doaitse Swierstra wrote: >> Over the years we have been constructing a collection of Embedded >> Domain Specific Languages for describing compilers which are >> assembled from fragments which can be compiled individually. In this >> way one can gradually ``grow a language'' in a large number of small >> steps. The technique replaces things like macro extensions or >> Template Haskell; it has become feasible to just extend the language >> at hand by providing extra modules. The nice thing is that existing >> code does not have to be adapted, nor has to be available nor has to >> be recompiled. >> Recently we have been using (and adapting) the frameworks such that >> we could create an entry in the ldta11 (http://ldta.info/tool.html) >> tool challenge, where one has to show how one's tools can be used to >> create a compiler for the Oberon0 language, which is used a a running >> example in Wirth's compiler construction book. >> We have uploaded our implementation to hackage at: >> http://hackage.haskell.org/package/oberon0. >> More information can be found at the wiki: >> http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo >> You may take a look at the various Gram modules to see how syntax is >> being defined, and at the various Sem modules to see how we use our >> first class attribute grammars to implement the static semantics >> associated with the various tasks of the challenge. >> We hope you like it, and comments are welcome, > > Awesome! > > I have a small question: Last I remember, you've mainly been using your UUAGC preprocessor to write attribute grammars in Haskell, especially for UHC. Now that you have first-class attribute grammars in Haskell ("achievement unlocked"), what do you intend to do with the preprocessor? How do these two approaches compare at the moment and where would you like to take them? > > > Best regards, > Heinrich Apfelmus On the page http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo there is a link (http://www.fing.edu.uy/~mviera/papers/VSM12.pdf) to a paper we presented at LDTA (one of the ETAPS events) this spring. It explains how UUAGC can be used to generate first class compiler modules. We have also a facility for grouping attributes, so one can trade flexibility for speed. The first class approach stores list of attributes as nested cartesian products, access to which a clever compiler might be able to optimize. This however would correspond a form of specialisation, so you can hardly say that we have really independent modules; as always global optimisation is never compositional). From the point of view of the first class approach such grouped non-termionals are seen as a single composite non-terminal. Doaitse > > -- > http://apfelmus.nfshost.com > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

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Predictive parser combinators need four values to report errors
by John Lask 19 Aug '12

19 Aug '12

I am unable to find a copy of the following paper: Andrew Partridge, David Wright: Predictive parser combinators need four values to report errors. Journal of Functional Programming 6(2): 355-364, 1996 does any one have a copy of, that they could send me ?

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3 level hierarchy of Haskell objects
by Patrick Browne 18 Aug '12

18 Aug '12

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Does someone have a compiled binary of the LLVM for Windows 7 or must one compile the source?
by KC 18 Aug '12

18 Aug '12

I just want to get started on some matrix operations with REPA. Or is there a library (package?) like REPA without using the LLVM? -- -- Regards, KC

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Regex string replacement
by José Romildo Malaquias 18 Aug '12

18 Aug '12

Hello. I am looking for a substring replacement based on Perl like regular expressions that would let me use part of the original string in the replacement string. Something like: reReplace "name is (.*)\\." "hi \1!" "my name is john." which would result in "hy john!" Any help? Romildo

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Re: [Haskell-cafe] phantom types
by Florian Hartwig 17 Aug '12

17 Aug '12

> Hi, > > I am currently reading documentation on Generalized Algebraic Data Types: > > http://en.wikibooks.org/wiki/Haskell/GADT > > I have a question concerning this page. Let us consider the following code > proposed in the page: > > ---------------------------------- > -- Phantom type variable a (does not appear in any Expr: it is just a > -- dummy variable). > data Expr a = I Int > | B Bool > | Add (Expr a) (Expr a) > | Mul (Expr a) (Expr a) > | Eq (Expr a) (Expr a) > deriving (Show) > > -- Smart constructors > add :: Expr Int -> Expr Int -> Expr Int > add = Add > > i :: Int -> Expr Int > i = I > > b :: Bool -> Expr Bool > b = B > > eval :: Expr a -> a > eval (I n) = n > ---------------------------------- > > I obtain the following error: > > Phantom.hs:27:14: > Couldn't match type `a' with `Int' > `a' is a rigid type variable bound by > the type signature for eval :: Expr a -> a at Phantom.hs:27:1 > In the expression: n > In an equation for `eval': eval (I n) = n > > The wiki page explains: > > """ > But alas, this does not work: how would the compiler know that encountering > the constructor I means that a = Int? > """ > > I don't understand. When we write "eval (I n) = n", as I is a constructor > which takes an Int as argument, we are able to deduce that the type of n is > Int; so the type of eval should be in this case "Expr Int -> Int". > What do I miss? Since the example uses phantom types, the type of n is not actually related to the type of (I n). It is perfectly possible to have, for example, a value of type Expr String created by the I constructor: *Main> let expr = I 5 :: Expr String *Main> expr I 5 *Main> :t expr expr :: Expr String So if we define eval the way it is defined in the example, the compiler cannot infer that the type of (I n) is Expr Int, even though it knows that n's type is Int. We could prevent the creation of values with the wrong types by only exporting the smart constructors, but that is no help for the type system. Declaring eval as Expr Int -> Int would make the code compile, but since eval should presumably include patterns for other types, such as eval (B b) = b which would have to be of type Expr Bool -> Bool, we cannot do that. I hope I did not misunderstand your question and this helps. Cheers, Florian

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Re: [Haskell-cafe] [Announce] Compositional Compiler Construction, Oberon0 examples available
by Facundo Domínguez 17 Aug '12

17 Aug '12

Sorry, fixed the subject. On Fri, Aug 17, 2012 at 11:11 AM, Facundo Domínguez <facundominguez(a)gmail.com> wrote: > I have one unbearable doubt, how do you accent CoCoCo? CóCoCo? CoCóCo? CoCoCó? > > Facundo > >> Date: Thu, 16 Aug 2012 21:40:28 +0200 >> From: Doaitse Swierstra <doaitse(a)swierstra.net> >> Subject: [Haskell-cafe] [Announce] Compositional Compiler >> Construction, Oberon0 examples available >> To: "haskell-cafe(a)haskell.org Cafe" <haskell-cafe(a)haskell.org>, >> Haskell List <haskell(a)haskell.org> >> Message-ID: <3B4B9FFC-B1BB-4156-BE80-5E60632EE5CD(a)swierstra.net> >> Content-Type: text/plain; charset=us-ascii >> >> Over the years we have been constructing a collection of Embedded Domain Specific Languages for describing compilers which are assembled from fragments which can be compiled individuallu. In this way one can gradually ``grow a langauge'' in a large number of small steps. The technique replaces things like macro extensions or Template Haskell; it has become feasable to just extend the language at hand by providing extra modules. The nice thing is that existing code does not have to be adapted, nor has to be available nor has to be recompiled. >> >> Recently we have been using (and adapting) the frameworks such that we could create an entry in the ldta11 (http://ldta.info/tool.html) tool challenge, where one has to show how one's tools can be used to create a compiler for the Oberon0 language, which is used a a running example in Wirth's compiler construction book. >> >> We have uploaded our implementation to hackage at: http://hackage.haskell.org/package/oberon0. >> >> More information can be found at the wiki: http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo >> >> You may take a look at the various Gram modules to see how syntax is being defined, and at the various Sem modules to see how we use our first class attribute grammars to implement the static semantics associated with the various tasks of the challenge. >> >> We hope you like it, and comments are welcome, >> >> Marcos Viera >> Doaitse Swierstra

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Re: [Haskell-cafe] Haskell-Cafe Digest, Vol 108, Issue 22
by Facundo Domínguez 17 Aug '12

17 Aug '12

I have one unbearable doubt, how do you accent CoCoCo? CóCoCo? CoCóCo? CoCoCó? Facundo > Date: Thu, 16 Aug 2012 21:40:28 +0200 > From: Doaitse Swierstra <doaitse(a)swierstra.net> > Subject: [Haskell-cafe] [Announce] Compositional Compiler > Construction, Oberon0 examples available > To: "haskell-cafe(a)haskell.org Cafe" <haskell-cafe(a)haskell.org>, > Haskell List <haskell(a)haskell.org> > Message-ID: <3B4B9FFC-B1BB-4156-BE80-5E60632EE5CD(a)swierstra.net> > Content-Type: text/plain; charset=us-ascii > > Over the years we have been constructing a collection of Embedded Domain Specific Languages for describing compilers which are assembled from fragments which can be compiled individuallu. In this way one can gradually ``grow a langauge'' in a large number of small steps. The technique replaces things like macro extensions or Template Haskell; it has become feasable to just extend the language at hand by providing extra modules. The nice thing is that existing code does not have to be adapted, nor has to be available nor has to be recompiled. > > Recently we have been using (and adapting) the frameworks such that we could create an entry in the ldta11 (http://ldta.info/tool.html) tool challenge, where one has to show how one's tools can be used to create a compiler for the Oberon0 language, which is used a a running example in Wirth's compiler construction book. > > We have uploaded our implementation to hackage at: http://hackage.haskell.org/package/oberon0. > > More information can be found at the wiki: http://www.cs.uu.nl/wiki/bin/view/Center/CoCoCo > > You may take a look at the various Gram modules to see how syntax is being defined, and at the various Sem modules to see how we use our first class attribute grammars to implement the static semantics associated with the various tasks of the challenge. > > We hope you like it, and comments are welcome, > > Marcos Viera > Doaitse Swierstra

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How to simplify the code of Maybe within a monad?
by Magicloud Magiclouds 17 Aug '12

17 Aug '12

Hi, Since Maybe is a monad, I could write code like 'maybeA >> maybeB >> maybeC' to check if all these are not Nothing. Or 'liftM foo maybeD' to avoid ugly 'case of'. But how if here maybe[ABC] are like 'IO (Maybe Int)', or foo is type of 'Int -> IO Int'? -- 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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