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Re: [Haskell-cafe] Extending Type Classes
by Henning Thielemann 27 Aug '13

27 Aug '13

The problem of refinement of type classes annoys me from time to time when I work on the NumericPrelude. It is an experimental type class hierarchy for mathematical types. Sometimes a new data type T shall be implemented and it turns out that you can implement only a part of all methods of a certain class. Then a natural step is to split the class into two classes A and B: 'A' contains the methods we can implement for T and 'B' contains the remaining methods and 'B' is a sub-class of 'A'. First, this means that all client code has to be rewritten. Second, code for instances becomes very lengthy, because over the time code tends to contain one instances for every method. However the many small instances actually carry information: Every instance has its specialised constraints. E.g. you would certainly try to use only Applicative constraints in an Applicative instance and not Monad constraints. However, if there is a way to define Applicative and Monad instances in one go, the Applicative instance may get Monad constraints.

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Re: [Haskell-cafe] sequence causing stack overflow on pretty small lists
by Niklas Hambüchen 27 Aug '13

27 Aug '13

Maybe an unlimited stack size should be the default? As far as I understand, the only negative effect would be that some programming mistakes would not result in a stack overflow. However, I doubt the usefulness of that: * It already depends a lot on the optimisation level * If you do the same thing in a slightly different way, and you allocate on the heap instead of on the stack you will not get it either

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definition of the term "combinator"
by damodar kulkarni 27 Aug '13

27 Aug '13

Hello, The word "combinator" is used several times in the Haskell community. e.g. parser combinator, combinator library etc. Is it exactly the same term that is used in the "combinatory logic" ? A combinator is a higher-order function that uses *only function application* and earlier defined combinators to define a result from its arguments. [1] It seems, the term combinator as in, say, "parser combinator", doesn't have much to do with the "*only function application*" requirement of the "combinatory logic", per se. If the above observation holds, is the term combinator as used in the Haskell community, properly defined? In other words: Where can I find a formal and precise definition of the term "combinator", as a term used by the Haskell community to describe "something"? Ref: http://en.wikipedia.org/wiki/Combinatory_logic Thanks and regards, -Damodar Kulkarni

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TypeLits & Typeable
by Nicolas Trangez 26 Aug '13

26 Aug '13

Hello Cafe, I was playing around with TypeLits in combination with Typeable (using GHC 7.7.7.20130812 FWIW), but was surprised to find Symbols aren't Typeable, and as such the following doesn't work. Is this intentional, or am I missing something? Thanks, Nicolas {-# LANGUAGE DataKinds, KindSignatures, DeriveFunctor, DeriveDataTypeable #-} module Main where import Data.Typeable import GHC.TypeLits data NoSymbol n a b = NoSymbol a b deriving (Typeable) data WithSymbol (n :: Symbol) a b = WithSymbol a b deriving (Typeable) data Sym deriving (Typeable) main :: IO () main = do print $ typeOf (undefined :: NoSymbol Sym Int Int) let d = undefined :: WithSymbol "sym" Int Int {- print $ typeOf d No instance for (Typeable Symbol "sym") arising from a use of ‛typeOf’ -} return ()

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Proposal: Hackage's packages should be seperated by buildable
by He-chien Tsai 26 Aug '13

26 Aug '13

I'm sick for checking whether package is obsolete or not. I think packages build failed long time ago should be collected and moved to another page until someone fix them, or hackage pages should have a filter for checking obsolete packages.

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xmonad (+ mate) evince problem?
by Johannes Waldmann 26 Aug '13

26 Aug '13

Hi. I am using xmonad + mate (on fedora 19) and evince (PDF viewer) seems unresponsive: it reacts to my (mouse) input only after switching to another screen and back. Any hints? - Thanks, J. W.

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Conduit : is it possible to write this function?
by Erik de Castro Lopo 26 Aug '13

26 Aug '13

Hi all Using the Conduit library is it possible to write the function: eitherSrc :: MonadResource m => Source m a -> Source m b -> Source m (Either a b) which combines two sources into new output source such that data being produced aysnchronously by the original two sources will be returned as either a Left or Right of tne new source? If so, how? Cheers, Erik -- ---------------------------------------------------------------------- Erik de Castro Lopo http://www.mega-nerd.com/

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Renumbered mailing list posts
by Henning Thielemann 26 Aug '13

26 Aug '13

Recently I found that links from Google search results to archive Haskell-Cafe messages are invalid. The messages are still there, but got a different number. E.g. the search result says: http://www.haskell.org/pipermail/haskell-cafe/2011-February/089455.html But the message is at http://www.haskell.org/pipermail/haskell-cafe/2011-February/088146.html Also links from Haskell-Wiki articles to the Haskell-Cafe archive are invalid now. This is very very very bad, since I used tons of such URLs in Wiki articles and it is very hard to find the message I referred to, if the URL does not work anymore.

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reasons why Template Haskell does not propose something similar to Python exec() or eval()
by TP 25 Aug '13

25 Aug '13

Hi everybody, I continue to learn and test Template Haskell (one more time thanks to John Lato for his post at: http://www.mail-archive.com/haskell-cafe@haskell.org/msg106696.html that made me understand a lot of things). I have a question about the way Template Haskell is working. Why Template Haskell does not propose something similar to Python (or bash) exec() or eval(), i.e. does not offer the possibility to take a (quoted) string in input, to make abstract syntax in output (to be executed later in a splice $()). For example, in Python, to make an affectation 'a="a"' programatically, I can simply do (at runtime; even if I am here only concerned with what Template Haskell could do, i.e. at compile time): > def f(s): return '%s = \'%s\'' % (s,s) > exec(f("a")) > a 'a' With Template Haskell, I am compelled to make a function returning the abstract syntax corresponding to variable declaration: ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s) (see complete example in Post Scriptum). This works fine, but it is less direct. I am sure that the Template Haskell approach has many advantages, but I am unable to list them. I think it is important to have the reasons in mind. Could you help me? Thanks in advance, TP PS: the complete Haskell example: ----------------------------------- module MakeVard where import Language.Haskell.TH makeVard :: Monad m => String -> m [Dec] -- Equivalent to "%s = \"%s\"" makeVard s = return [ ValD (VarP $ mkName s) (NormalB $ LitE $ StringL s) [] ] ----------------------------------- tested by ----------------------------------- {-# LANGUAGE TemplateHaskell #-} import MakeVard $(makeVard "a") main = do print a ----------------------------------- resulting in $ runghc -ddump-splices test.hs test_makeVar.hs:1:1: Splicing declarations makeVard "a" ======> test_makeVar.hs:4:3-14 a = "a" "a"

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GHC flags: optghc
by jabolopes＠google.com 24 Aug '13

24 Aug '13

Hi, I am using GHC version 6.12.1. What is optghc ? I can't find that information anywhere... Thanks, Jose -- Jose Antonio Lopes Ganeti Engineering Google Germany GmbH Dienerstr. 12, 80331, München Registergericht und -nummer: Hamburg, HRB 86891 Sitz der Gesellschaft: Hamburg Geschäftsführer: Graham Law, Christine Elizabeth Flores Steuernummer: 48/725/00206 Umsatzsteueridentifikationsnummer: DE813741370

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