August 2012 - Haskell-Cafe

phantom types
by TP 21 Aug '12

21 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? Thanks, TP

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Re: [Haskell-cafe] [Haskell] Compositional Compiler Construction, Oberon0 examples available
by S. Doaitse Swierstra 21 Aug '12

21 Aug '12

On Aug 21, 2012, at 13:46 , Heinrich Apfelmus <apfelmus(a)quantentunnel.de> wrote: > Doaitse Swierstra wrote: >> Heinrich Apfelmus wrote: >>> 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? >> 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. > > Ah, I see. So the custom syntax offered by UUAGC is still appreciated, but you now intend to compile it to first-class attribute grammars instead of "bare metal Haskell". Makes sense. Thanks! It is not much that it is our intention, but it is an easy way to make an existing compiler extensible. The main (fixed) part of the compiler is constructed in the "old" way from an UUAGC description, and those attributes are grouped (and quite a bit more efficient). On top of this you can define extra attributes and computations, which plug in to the old system. Notice that there is a main difference between the two approaches is that the uuagc route gives you fast compilers, because we can analyse the grammar, and generate efficient tree walking evaluators, whereas the first-class approach gives you great flexibility and the possibility to abstract from common patterns for which others prefer to get lost in stacks of monads, or find out that monads do not work at all since they cannot feed back information into a computation easily. Doaitse > > > Best regards, > Heinrich Apfelmus > > -- > http://apfelmus.nfshost.com > > > _______________________________________________ > Haskell mailing list > Haskell(a)haskell.org > http://www.haskell.org/mailman/listinfo/haskell

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Lazy decoding of a list with Data.Binary
by Niklas Hambüchen 21 Aug '12

21 Aug '12

I just encountered the following problem: http://stackoverflow.com/questions/11695373/lazy-decoding-of-a-list-with-da… If I get that correctly, there once was a discussion about adding a Data.Binary.Lazy to binary: http://www.haskell.org/pipermail/haskell-cafe/2007-November/034758.html What happened to this? And if binary is strict by default now, shouldn't the binary-strict package be deprecated? Is there an alternative these days how to lazily write/read binary? Thanks Niklas

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Wanted: Haskell binding for libbdd (buddy)
by Johannes Waldmann 21 Aug '12

21 Aug '12

Are there any Haskell bindings for BDD libraries (reduced ordered binary decision diagrams)? E.g., it seems "buddy" is commonly used http://packages.debian.org/squeeze/libbdd-dev and it has an Ocaml binding. Yes, there is http://hackage.haskell.org/package/obdd but I need better performance (with the same API, ideally). Thanks - J.W. PS: I wonder where performance goes out the window ... I suspect "Map (Int,Int) whatever" should really be a hashtable but I don't like it in IO, it should be in ST?

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Data structure containing elements which are instances of the same type class
by oleg＠okmij.org 20 Aug '12

20 Aug '12

> data A = A deriving Show > data B = B deriving Show > data C = C deriving Show > > data Foo = forall a. Show a => MkFoo a (Int -> Bool) > > instance Show Foo where > show (MkFoo a f) = show a I'd like to point out that the only operation we can do on the first argument of MkFoo is to show to it. This is all we can ever do: we have no idea of its type but we know we can show it and get a String. Why not to apply show to start with (it won't be evaluated until required anyway)? Therefore, the data type Foo above is in all respects equivalent to > data Foo = MkFoo String (Int -> Bool) and no existentials are ever needed. The following article explains elimination of existentials in more detail, touching upon the original problem, of bringing different types into union. http://okmij.org/ftp/Computation/Existentials.html

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Flipping type constructors
by Tony Morris 20 Aug '12

20 Aug '12

I have a data-type that is similar to EitherT, however, I have ordered the type variables like so: data EitherT (f :: * -> *) (a :: *) (b :: *) = ... This allows me to declare some desirable instances: instance Functor f => Bifunctor (EitherT f) instance Foldable f => Bifoldable (EitherT f) instance Traversable f => Bitraversable (EitherT f) However, I am unable to declare a MonadTrans instance: instance MonadTrans (EitherT a) -- kind error I looked at Control.Compose.Flip to resolve this, but it does not appear to be kind-polymorphic. http://hackage.haskell.org/packages/archive/TypeCompose/0.9.1/doc/html/src/… I was wondering if there are any well-developed techniques to deal with this? Of course, I could just write my own Flip with the appropriate kinds and be done with it. Maybe there is a more suitable way? -- Tony Morris http://tmorris.net/

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Haskell master thesis project
by Francesco Mazzoli 20 Aug '12

20 Aug '12

Hi list(s), I've been hooked on Haskell for a while now (some of you might know me as bitonic on #haskell), and I find myself to decide on a project for my masters thesis. Inspired by the David Terei's master thesis (he wrote the LLVM backend), I was wondering if there were any projects requiring similar effort that will benefit the Haskell community. -- Francesco * Often in error, never in doubt

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Haskell position in Israel
by Michael Snoyman 20 Aug '12

20 Aug '12

Hi all, Just passing on this job opportunity for another company. SQream is looking for Haskellers located in Israel. They are working on high performance solutions for large databases using Haskell. If you're interested, please contact me off-list, and I'll pass your information along. Thanks, Michael

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Compositional Compiler Construction, Oberon0 examples available
by Doaitse Swierstra 20 Aug '12

20 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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Network.Curl cookie jar madness
by Michael Orlitzky 19 Aug '12

19 Aug '12

I'm one bug away from a working program and need some help. I wrote a little utility that logs into LWN.net, retrieves an article, and creates an epub out of it. Full code here: git clone http://michael.orlitzky.com/git/lwn-epub.git This is the code that gets the login cookie: cj <- make_cookie_jar li_result <- log_in cj uname pword case li_result of Left err -> do let msg = "Failed to log in. " ++ err hPutStrLn stderr msg Right response_body -> do hPutStrLn stderr response_body return $ cfg { C.cookie_jar = Just cj } Curl is making the request, but if I remove the (hPutStrLn stderr response_body), it doesn't work! What's even more insane is, this works: hPutStrLn stderr response_body and this doesn't: hPutStrLn stdout response_body whaaaaaaatttttttt? I really don't want to dump the response body to stderr, but I can't even begin to imagine what's going on here. Has anyone got Network.Curl working with a cookie jar? For anyone with an LWN account: you can set, username = foo password = bar in ~/.lwn-epub/lwn-epub.conf and attempt to run with, lwn-epub -o current.epub current/bigpage

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