November 2008 - Haskell-Cafe

Instances that shouldn't overlap
by oleg＠okmij.org 27 Nov '08

27 Nov '08

Paul Johnson wrote: > class (Eq a) => AppEq f a where > instance (Applicative f, Eq a) => AppEq f a where > instance (Ord a) => AppEq Interval a where In Haskell, instances are selected based solely on the types in the head. Constraints like `Applicative f' are not consulted when the instance is being selected. Rather, constraints are checked after the instance has been selected. The above two instances are overlapping since Interval is a particular case of `f'. That said, it is possible to select an instance based on constraints. The approach is described at http://haskell.org/haskellwiki/GHC/AdvancedOverlap

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Finalisers
by Andrew Coppin 26 Nov '08

26 Nov '08

Here's an interesting question... Is it possible to attach finalisers to a value? (That is, have some Haskell code executed when the item in question is reclaimed by the GC.) I'm interested in knowing whether a particular data structure is shared (i.e., whether it's safe to mutate it or whether it must be copied first), and a simple reference-counting scheme looks like the easiest option.

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build tools and Cabal
by John Lato 26 Nov '08

26 Nov '08

Hello, Cabal allows specifying arguments for tools it recognizes on the command line, e.g. runhaskell Setup.hs configure --c2hs-option=some_option Unfortunately, I can't find a way to make this work with .cabal (or .buildinfo) files, except for the specific cases of ghc, hugs, and nhc98 options. Am I missing something? If not, could this be added easily? It would be very helpful as I'm trying to work around the broken cpp Apple provides. Thanks, John

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Re: [Haskell] Wait for *either* MVar to be set
by Isaac Dupree 26 Nov '08

26 Nov '08

Peter Verswyvelen wrote: > ... by spawning and killing two threads (which might be an expensive operation, I'm not sure) pretty cheap in GHC -- they're not system threads > Am I wrong in this? If so, is this something that might be considered as a future enhancement in the GHC libraries and runtime? Also, look at STM (e.g. TVars), which is designed to do what you want more directly, I think. (timeouts might still use the GHC-thread thing. Don't worry about its performance unless you're measurably suffering from it...) -Isaac

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monads with take-out options
by Greg Meredith 26 Nov '08

26 Nov '08

Haskellians, Some monads come with take-out options, e.g. - List - Set In the sense that if unit : A -> List A is given by unit a = [a], then taking the head of a list can be used to retrieve values from inside the monad. Some monads do not come with take-out options, IO being a notorious example. Some monads, like Maybe, sit on the fence about take-out. They'll provide it when it's available. Now, are there references for a theory of monads and take-out options? For example, it seems that all sensible notions of containers have take-out. Can we make the leap and define a container as a monad with a notion of take-out? Has this been done? Are there reasons for not doing? Can we say what conditions are necessary to ensure a notion of take-out? Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 806 55th St NE Seattle, WA 98105 +1 206.650.3740 http://biosimilarity.blogspot.com

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template haskell overly conservative during splicing?
by Nicolas Frisby 26 Nov '08

26 Nov '08

When using template haskell (via Derive) to generate this (exact) instance: instance Foldable ((->) Int) => Foldable Data.Derivable.InterpreterLib.Test.List where foldMap f (Cons x0 x1) = (const mempty Cons `mappend` foldMap f x0) `mappend` foldMap f x1 foldMap f (Nil) = const mempty Nil I realize the context is insatisfiable. My issue, is that I can't even reach that "challenge". I get this error instead: Malformed type AppT ArrowT (ConT GHC.Base.Int) When splicing generated code into the program I couldn't find an existing ticket or discussion for this issue relying on the phrase "malformed type". I couldn't even find the source that generates that string in haskell-src, template-haskell, or ghc-6.8.2. Can anybody help? Thanks for your time.

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IRC question
by Galchin, Vasili 26 Nov '08

26 Nov '08

Hello, I am using Ubuntu Linux and I want to get the Haskell IRC feed. What IRC client can I use and how to configure? Thanks, Vasili

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Password hashing
by Bit Connor 25 Nov '08

25 Nov '08

Hello, What library can be used to securely hash passwords? From what I understand, the "bcrypt" algorithm is what the experts recommend. It is described in the paper: http://www.openbsd.org/papers/bcrypt-paper.ps I couldn't find a haskell library for this. There is a BSD licensed C implementation that looks very simple here: http://www.mindrot.org/projects/py-bcrypt/ A translation to haskell should be straight forward. Alternatively, a haskell bcrypt library could directly use this implementation and provide a very light FFI wrapper. Any thoughts?

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Interactive
by Andrew Coppin 25 Nov '08

25 Nov '08

I have a small question... Given that interactivity is Really Hard to do in Haskell, and that mutable state is to be strongly avoided, how come Frag exists? (I.e., how did they successfully solve these problems?)

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Hackage/Cabal/Haddock question
by Robert Greayer 25 Nov '08

25 Nov '08

How does Hackage run 'haddock' on uploaded packages? I had assumed it directly runs the cabal 'haddock' target, e.g. runhaskell Setup.hs haddock but it appears to perhaps be more complex than that. Some backrgound -- haddock doesn't seem to like quasiquotation - running haddock on a source tree that includes quasiquotations eventually results in: haddock: internal Haddock or GHC error: Maybe.fromJust: Nothing (eliminating the code that contains [$xxx|....] constructs gets rid of the error.) so "runhaskell Setup.hs haddock" ends up not generating any documentation. I worked around this problem by using a 'UserHook' and adding in an extra path element to the source path prior to running haddock via Cabal: > main = defaultMainWithHooks (simpleUserHooks { > preHaddock = \_ _ -> return (Just $ emptyBuildInfo { hsSourceDirs = ["noqqsrc"]},[]) }) The additional directory contains an alternate version of modules that don't contain quasiquotation (just types and stubs), which seems to hide the versions that do. This works fine locally, but not on hackage (still get the same behavior in the build failure log). Of course, I'd prefer not to have to do this at all, so if anyone knows a way around the problem (or if its purely my problem -- I'm doing something wrong), I'd appreciate hearing about it. I'm using GHC 6.10.1, and have tried setup haddock with both the shipped-with-ghc version of haddock and the latest version. Thanks, rcg

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