- Haskell-Cafe - Haskell.org

High level overview of GHCi?
by Maarten Faddegon 13 Jan '14

13 Jan '14

Dear Café, I was reading http://www.aosabook.org/en/ghc.html. Figure 5.2 gives a high level overview of the compiler passes when compiling Haskell with GHC. Is anyone aware of a similar figure that gives an overview of the passes when Haskell code is interpreted with GHCi? Thank you! Maarten Faddegon

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ANN: tree-view-0.1
by Emil Axelsson 13 Jan '14

13 Jan '14

tree-view is a package for rendering trees as foldable HTML and Unicode art. http://hackage.haskell.org/package/tree-view Example: *Data.Tree.View> drawTree $ Node "Add" [Node "Sub" [Node "3" [], Node "Mul" [Node "1" [], Node "2" []]], Node "4" []] Add ├╴Sub │ ├╴3 │ └╴Mul │ ├╴1 │ └╴2 └╴4 / Emil

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access programs own documentation
by Corentin Dupont 13 Jan '14

13 Jan '14

Hi guys, I'd like to access, from within my program, the program own haddock documentation. >From the cabal autogen path file, I can access the location of the program's data file, binaries etc. (getBinDir, getLibDir, getDataDir) but not the location of the doc. How to do that? For example on my machine: datadir = "/home/kau/.cabal/share/i386-linux-ghc-7.6.3/Nomyx-0.4.1" The documentation is generated in "/home/kau/.cabal/share/doc/i386-linux-ghc-7.6.3/Nomyx-0.4.1", but it depends on the configuration. Thanks, Corentin

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Haddock - How to write formulas ?
by Peter Caspers 13 Jan '14

13 Jan '14

Hi, I am still very new to Haskell, trying to start my very first project. For its documentation I want to use Haddock and suitable comments in the source code. I notice that (e.g. different from doxygen) there is no direct way of writing formulas, say in TeX style. Looking into some projects on Hackage, formulas there seem to be written in "pseudo-code" more or less like TeX but not following any strict standard. As far as I can see. What would be your recommendations concerning this ? Is there some guideline on how to include formulas ? I understand that there is "literal programming" where you can e.g. write a TeX article with embedded code blocks that can be extracted for the compiler. However, I do not want to follow this path, also the result is a bit different from what is produced in the "traditional" approach, isn't it. Thanks a lot Peter

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Re: [Haskell-cafe] Licenses and dependencies
by Mike Meyer 13 Jan '14

13 Jan '14

On Sat, Jan 11, 2014 at 2:17 PM, Mateusz Kowalczyk <fuuzetsu(a)fuuzetsu.co.uk> wrote: > Hey, > Perhaps you should have CC'd the list as well for this reply. Oops. I've moved it back, since you suggested it. > On 11/01/14 20:04, Mike Meyer wrote: > >> On 11/01/14 18:44, Ben Foppa wrote: > >> Once you distribute the binary, you have probably pulled in and used the > >> licensed libraries (unless replaced) so you're subject to the strongest > >> license used. > > > > Not "strongest", but all. Well, the GPL licenses sort of "stack", > > since they explicitly allow you to use newer (and usually stronger) > > versions of themselves, but other license don't. > > > > The GNU project provides a description of many of the available > > licenses at: http://www.gnu.org/licenses/license-list.html, including > > compatibility notes. > Right, all, but what this usually entices is that the strongest one > prevails. For example, code with BSD3 and GPLv3 sources will have to > be released under GPLv3 as a whole (although each part is still what > it was licensed under individually). I worked under the assumption > that all dependencies are have compatible licenses (and their > dependencies too &c). No, it's not a contest. You *have* to abide by them all. If you're dealing with the GPL, it may look like the "strongest" prevails, but that's a property of the GPL. It specifically requires any derived work be distributed under the GPL, with no added restrictions. So a compatible license will have no extra restrictions, and appear "weaker". A "stronger" license with extra restrictions (like the 4-clause BSD license) will be incompatible with the GPL, so you can't distributed a derived work at all. If you take the GPL out of the mix, and wanted to distributed a work derived from something covered by (for instance) BSD4 and the CPL - well, that would be fine, because I don't think either prevents the restrictions in the other. But the derived work would be covered by both, not just the stronger of the two (whichever that is). Your usual case may well be GPL'ed code. I happen to work with BSD-licensed code bases more often than not. > >> I think you'll be fine just choosing the PublicDomain license option. [...] > >> Things you can't do with such an option: > >> > >> * bundle other people's code which isn't under public domain > > > > This is vague. If your bundle is a single work (like a static binary), > > then it has to obey all the applicable licenses, and probably can't be > > in the public domain. If it's a collection of works, then it can > > include anything you want, so long as each element in the collection > > obeys it's license. > > I meant bundle as in the repository/source tarball/however you publish > your source, _not_ the binary (I explicitly mention binary somewhere else). In that case, you're perfectly free to include sources with other licenses in the tarball. You just need to make sure you follow their license for their entry in the collection. It's well-established that a simple collection of works (a tarball, etc.) is not a derived work. > >> * somehow enforce that only certain libraries will run with your > >> program: the user should be able to replace the BSD3 licensed > >> dependencies with their own if they want to > > Is there any license that enforces that? That seems more like a > > technical issue than a legal one. Though if you're dynamically linking > > the libraries, things get incredibly confusing. > LGPL, does it not? It even outlines what constitutes as a prevention > of other libraries. I believe the LGPL does just the opposite - it *permits* linking with non-free code without creating a derived work. I think I may not be understanding what you're saying, though. I think you're implying that it's possible to force users to use specific libraries. That is indeed the case - you could write a modified version of the GPL that says "You may only redistribute works derived from my project if they use my libraries". You are correct in that you can't do that if you place the work in the public domain. On the other hand, I don't know of any commonly used open source library that does that. <mike

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different behaviours with or without putStrLn
by Corentin Dupont 12 Jan '14

12 Jan '14

Hi guys, I'm experimenting different behaviours with or without a "putStrLn"! :( Basically, with the following code, I want the evaluation to really happen on the "evaluate". I found out that it doesn't: it is evaluated elsewhere (I don't know where). If I put a putStrLn (commented below), the evaluation really happens there. *execCommand :: (TVar MyState) -> StateT MyState IO () -> IO ()execCommand ts sm = do s <- atomically $ readTVar ts s' <- execStateT sm s s'' <- evaluate s' --evaluation should happen here, but it doesn't --putStrLn $ displayMulti $ _multi s'' atomically $ writeTVar ts s''* To give you more context, I have a state that, when evaluated, might not terminate. So I added a watchdog (like in mueval), that will kill the thread in case the evaluation doesn't terminate. That's why I need to be sure of where the evaluation takes place. Thanks! Corentin

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Parse Error ( Parsec )
by mukesh tiwari 12 Jan '14

12 Jan '14

Hello Cafe, I am trying to write a parser for propositional logic[1]. It's working fine for every input except equivalence ( <=> ). *Main> calculator "a=>b" Imp (Lit 'a') (Lit 'b') *Main> calculator "a<=b" Red (Lit 'a') (Lit 'b') *Main> calculator "a<=>b" *** Exception: failed to parse I think, the reason is parser taking equivalence ( <=> ) as reduction ( <= ) and next character is '>' so it is parse error . If I remove both implication and reduction then equivalence is working fine. *Main> calculator "a<=>b" Eqi (Lit 'a') (Lit 'b') Could some please tell me how to solve this problem. I also tried fixity declaration but got this error LogicPraser.hs:12:10: The fixity signature for `<=>' lacks an accompanying binding -Mukesh Tiwari [1] http://logic.stanford.edu/classes/cs157/2010/notes/chap02.html {-# LANGUAGE NoMonomorphismRestriction #-} import Text.Parsec.Token import Text.Parsec.Prim import Text.Parsec.Char import Text.Parsec.Expr import Text.Parsec.Combinator import Text.Parsec.Language import Control.Applicative hiding ( ( <|> ) , many ) import Data.Maybe ( fromJust ) --infixl 9 <=> data LExpr = Lit Char | Not LExpr | And LExpr LExpr | Or LExpr LExpr | Imp LExpr LExpr -- (=>) | Red LExpr LExpr -- ( <= ) | Eqi LExpr LExpr -- ( <=> ) deriving Show exprCal = buildExpressionParser table atom table = [ [ Prefix ( Not <$ string "~" ) ] , [ Infix ( And <$ string "&" ) AssocLeft ] , [ Infix ( Or <$ string "|" ) AssocLeft ] , [ Infix ( Imp <$ string "=>" ) AssocLeft , Infix ( Red <$ string "<=" ) AssocLeft , Infix ( Eqi <$ string "<=>" ) AssocLeft ] ] atom = char '(' *> exprCal <* char ')' <|> ( Lit <$> letter ) calculator :: String -> LExpr calculator expr = case parse exprCal "" expr of Left msg -> error "failed to parse" Right ( val ) -> val ~

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TypeLits and ScopedTypeVariables
by Ben Foppa 12 Jan '14

12 Jan '14

I've been playing with TypeLits recently, trying to create a vector with type-fixed size. I've hit a stumbling block trying to compile this (simplified) code: {-# LANGUAGE DataKinds #-} {-# LANGUAGE KindSignatures #-} import Control.Exception (assert) import GHC.TypeLits data Foo (n :: Nat) a = Foo a size :: SingI n => Foo n a -> Int size = fromInteger . fromSing . sizeSing where sizeSing :: Foo n a -> Sing n sizeSing _ = sing The error is: Could not deduce (SingI Nat n1) arising from a use of `sing' from the context (SingI Nat n) bound by the type signature for size :: SingI Nat n => Foo n a -> Int at /home/ben/test.hs:10:9-33 Possible fix: add an instance declaration for (SingI Nat n1) In the expression: sing In an equation for `sizeSing': sizeSing _ = sing In an equation for `size': size = fromInteger . fromSing . sizeSing where sizeSing :: Foo n a -> Sing n sizeSing _ = sing Can anybody shed light on this? It seems like the kind of thing that ScopedTypeVariables should solve, but it doesn't make a difference.

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Licenses and dependencies
by Ben Foppa 11 Jan '14

11 Jan '14

Many permissive licenses (I'm specifically looking at MIT and BSD3) include a clause that the license file must be included in distributions of the source or the binary. It seems to me that if I have a project on Github, that qualifies as redistribution of the source. My questions are thus: Do I need to include the license files from all my dependencies in my Github repository? Does having them listed as dependencies in my .cabal file "implicitly" include them? Does making a project "PublicDomain" only refer to the code contained in that project, or would it (unintentionally and unlawfully) give distributors of that source code the freedom to not include the BSD3/MIT license files from dependencies? Essentially my goal is to waive all intellectual property rights to most of my Haskell projects, to the extent that, were I to unintentionally sign away my intellectual property, my open-source contributions would be safe - what's the easiest way to do this? Thanks, Ben Foppa

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ANN: xxHash, fast, high quality, non-cryptographic checksums.
by christian＠ponies.io 10 Jan '14

10 Jan '14

I've just finished implementing a haskell version of the xxHash algorithm, which is documented briefly here (mostly in code): https://code.google.com/p/xxhash/ The haskell version is *almost* as fast as C and allows the generation of a better quality checksum than adler32, in less time. Benchmarks are here: http://ponies.io/posts/2013-01-10-xxhash.html Code is here: https://github.com/christian-marie/xxhash Critique welcome!

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