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Test. Ignore.
by "José Romildo Malaquias" 21 Jul '10

21 Jul '10

Please, ignore this message. Romildo -- _______________________________________________ Surf the Web in a faster, safer and easier way: Download Opera 9 at http://www.opera.com

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Finding zipper for custom tree
by Sergey Mironov 21 Jul '10

21 Jul '10

Hello list! I am trying to understand zipper concept using papers like [1] and [2]. Though main idea looks clear, I still have a problem in applying it for custom data types. Please help me with deriving Zipper-type from > data DTree a = P | D [(a, DTree)] Looking in [1] ('Zippers via Differentiation' chapter) I tried to do the following: 1. Find a DTreeF type which is a pattern functor ([2], chapter 2.1) of my DTree 2. Write DTreeF in 'algebraic' form (using '+' and '*') 3. Find DTreeF' - "derivative" of DTreeF 4. Define my zipper type using list of DTreeF' Step 1 likely ends with > data DTreeF a x = P | D [(a,x)] [2] says that using this pattern functor one could build a fixed-point version of DTree: > data Fix f = In {out :: (f (Fix f))} > data DTreeFP = Fix DTreeF but seems that I have nothing to do with it right now. Step 2 is my main question: In [1] authors did it for binary tree: data Tree a = Leaf a | Bin (Tree a) (Tree a) data TreeF a x = Leaf a | Bin x x and thus TreeF = a + x * x TreeF' = x + x My DTree has inner list of tuples. How should I rewrite it in terms of '+' and '*' ? Any comments are welcome! [1] - http://en.wikibooks.org/wiki/Haskell/Zippers [2] - http://people.cs.uu.nl/andres/Rec/MutualRec.pdf -- Thanks, Sergey

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Merge hsql and HDBC -- there can only be one!
by Christopher Done 21 Jul '10

21 Jul '10

One thing that would be nice is a unification of the general database libraries hsql and HDBC. What is the difference between them? Why are there two, and why are there sets of drivers for both (duplication of effort?)? I've used both in the past but I can't discern a real big difference (I used the hsql-sqlite library and the HDBC-postgresql library, whichever worked...). It seems the best thing to do is either actively merge them together and encourage the community to move from one to the other -- judging from what I've read HDBC is more up to date and newer than hsql -- or have some documentation with damn good reasons to choose one or the other, because currently this is a needless source of confusion and possible duplication of effort for Haskell's database libraries. I wasn't going to post until I'd actually researched the difference myself properly but I didn't get chance to have a look over the weekend, but I thought I'd pose the question. Do people actually care about this?

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Re: Tiger compiler in Haskell: annotating abstract syntax tree
by oleg＠okmij.org 21 Jul '10

21 Jul '10

Jose' Romildo Malaquias wrote: > I am writing here to ask suggestions on how to annotate an ast with > types (or any other information that would be relevant in a compiler > phase) in Haskell. There is also a general way of annotating AST post factum, described in http://okmij.org/ftp/Algorithms.html#tree-annot The method lets one attach arbitrarily many annotations to an already built AST, *without* the need to change the definition of the datatype. One does not even have to anticipate annotations! The method would work with your AST > data Exp > = IntExp Integer > | VarExp Symbol > | AssignExp Symbol Exp as _it is_, without any modifications -- neither to the data type definition, nor to the tree. The method was demonstrated when writing a compiler of sorts: annotating an AST with an inferred type for each node. If the type inference fails, we can still print out the inferred types for the good subexpressions.

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lambda terms in which the variables are positions in lambda terms
by Greg Meredith 21 Jul '10

21 Jul '10

Dear Haskellians, The following code may provide some amusement. i offer a free copy of Barwise's Vicious Circles to the first person to derive deBruijn indices from this presentation. Best wishes, --greg -- -*- mode: Haskell;-*- -- Filename: Term.hs -- Authors: lgm -- Creation: Tue Jul 20 16:37:37 2010 -- Copyright: Not supplied -- Description: -- ------------------------------------------------------------------------ module Generators( Term , MuTerm , DoTerm , ReflectiveTerm , TermLocation , ClosedTermLocation , ClosedReflectiveTerm , unfoldLocation , unfoldTerm , makeMention , makeAbstraction , makeApplication , generateTerms ) where -- M[V,A] = 1 + V + VxA + AxA data Term v a = Divergence | Mention v | Abstraction v a | Application a a deriving (Eq, Show) -- \mu M data MuTerm v = MuTerm (Term v (MuTerm v)) deriving (Eq, Show) -- \partial \mu M data DoTerm v = Hole | DoAbstraction v (DoTerm v) | DoLeftApplication (DoTerm v) (MuTerm v) | DoRightApplication (MuTerm v) (DoTerm v) deriving (Eq, Show) -- first trampoline data ReflectiveTerm v = ReflectiveTerm (MuTerm (DoTerm v, ReflectiveTerm v)) deriving (Eq, Show) -- second trampoline data TermLocation v = TermLocation ((DoTerm v), (ReflectiveTerm v)) deriving (Eq, Show) -- first bounce data ClosedTermLocation = ClosedTermLocation (TermLocation ClosedTermLocation) deriving (Eq, Show) -- second bounce data ClosedReflectiveTerm = ClosedReflectiveTerm (ReflectiveTerm ClosedTermLocation) deriving (Eq, Show) -- the isomorphisms implied by the trampolines unfoldLocation :: ClosedTermLocation -> ((DoTerm ClosedTermLocation), (ReflectiveTerm ClosedTermLocation)) unfoldLocation (ClosedTermLocation (TermLocation (k, t))) = (k, t) unfoldTerm :: ClosedReflectiveTerm -> (MuTerm ((DoTerm ClosedTermLocation), (ReflectiveTerm ClosedTermLocation))) unfoldTerm (ClosedReflectiveTerm (ReflectiveTerm muT)) = muT -- variable mention ctor makeMention :: ClosedTermLocation -> ClosedReflectiveTerm makeMention ctl = (ClosedReflectiveTerm (ReflectiveTerm (MuTerm (Mention (unfoldLocation ctl))))) -- abstraction ctor makeAbstraction :: ClosedTermLocation -> ClosedReflectiveTerm -> ClosedReflectiveTerm makeAbstraction ctl crt = (ClosedReflectiveTerm (ReflectiveTerm (MuTerm (Abstraction (unfoldLocation ctl) (unfoldTerm crt))))) -- application ctor makeApplication :: ClosedReflectiveTerm -> ClosedReflectiveTerm -> ClosedReflectiveTerm makeApplication crtApplicad crtApplicand = (ClosedReflectiveTerm (ReflectiveTerm (MuTerm (Application (unfoldTerm crtApplicad) (unfoldTerm crtApplicand))))) -- a simple test generateTerms :: () -> [ClosedReflectiveTerm] generateTerms () = -- x [(makeMention (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence))))), -- \x -> x (makeAbstraction (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))) (makeMention (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))))), -- ((\x -> x)(\x -> x)) (makeApplication (makeAbstraction (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))) (makeMention (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))))) (makeAbstraction (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))) (makeMention (ClosedTermLocation (TermLocation (Hole, ReflectiveTerm (MuTerm Divergence)))))))] -- L.G. Meredith Managing Partner Biosimilarity LLC 1219 NW 83rd St Seattle, WA 98117 +1 206.650.3740 http://biosimilarity.blogspot.com

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[SOLVED] Default Curl call fails with CurlSSLConnectError
by aditya siram 20 Jul '10

20 Jul '10

Hi all, Just wanted to document this in case others stumble across it. On Ubuntu Lucid "curlGetString "https://<some url>" []" can sometimes fail with (CurlSSLConnectError,""). Setting SSL to version 3 manually fixes the problem: "curlGetString "https://<some-url>" [CurlSSLVersion 3]". This might also be of interest to the library maintainer, and Don Stewart whose download-curl package, because it is a wrapper around the curl package, exhibits the same behavior. thanks ... -deech

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HTTP Redirect With HTTPS Rejected
by aditya siram 20 Jul '10

20 Jul '10

The problem is that any page that that responds with 301.302,303 or 307 and redirects to an "https" is rejected by the library. The code that does the rejection is in "Network.Browser" on line 865. It only accepts "http:". Why was this design decision made? It makes it impossible for me to use this library for my project. -deech On Tue, Jul 20, 2010 at 3:30 PM, aditya siram <aditya.siram(a)gmail.com> wrote: > Hi all, > I am trying to simply download a web page using the HTTP library. It > works fine with "http://www.google.com" but a redirecting page like > "http://gmail.com" creates an error. Code is at > http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27864#a27864. > > Thanks , > -deech >

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Regex multiple matches
by Omari Norman 20 Jul '10

20 Jul '10

Hi, How do I use Text.Regex.PCRE to get information on multiple matches? For instance, in ghci I get this error message: Prelude Text.Regex.PCRE> "foo" =~ "o" :: [(Int,Int)] <interactive>:1:0: No instance for (RegexContext Regex [Char] [(Int, Int)]) arising from a use of `=~' at <interactive>:1:0-11 Possible fix: add an instance declaration for (RegexContext Regex [Char] [(Int, Int)]) In the expression: "foo" =~ "o" :: [(Int, Int)] In the definition of `it': it = "foo" =~ "o" :: [(Int, Int)] "Real World Haskell"'s text suggests this should work, but the ghci output is littered with errors. http://book.realworldhaskell.org/read/efficient-file-processing-regular-exp… Thanks for any help. Omari

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HTTP Redirect Error
by aditya siram 20 Jul '10

20 Jul '10

Hi all, I am trying to simply download a web page using the HTTP library. It works fine with "http://www.google.com" but a redirecting page like "http://gmail.com" creates an error. Code is at http://hpaste.org/fastcgi/hpaste.fcgi/view?id=27864#a27864. Thanks , -deech

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seeking guideline re modules and stacks of State Monad Transformers
by Ali Razavi 20 Jul '10

20 Jul '10

Hi, I am working on a project that features a few modules with inter-dependent states. It is expected that some of the modules import and use some of the stateful functions from other modules, and simultaneously, need to manage the state of their own. I need to make a design choice as to whether I should add the new modules' state as separate layesr by transforming the monad transformer stack in each module, or create a separate module to handle the amalgamation of all states with a single datatype. Both solutions have salient issues. The latter, namely stacking more layers, can quickly become unwieldy, whereas the former breaches encapsulation. Any better ideas? To give it more of a concrete context, imagine the following: module A where type AState = State Int.... someAFunc :: a -> AState a --------------- module B where type BState = StateT Int AState ... someBFunc :: b -> BState b -------------- -- now I want module C to only have access to B's part of BState, but not the A-related part module C where type CState = StateT Int BState Thanks, Ali

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