- Haskell-Cafe - Haskell.org

Tweaking the garbage collector for realtime usage
by Peter Verswyvelen 19 Mar '09

19 Mar '09

The GHC documentation lists a lot of tweaks that can be done to the garbage collector. However, Haskell spin-offs like Timber <http://www.timber-lang.org/> implement their own incremental garbage collector that is better suitable for real-time usage. Did someone already fiddle with GHC's gc flags so it works better for real-time applications, like games and simulations?

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Crash in GHCI - what is the correct behavior here?
by Zachary Turner 19 Mar '09

19 Mar '09

Basically just learning haskell, I would have posted this in the beginners list but since it involves a segfault of GHCI, I figured it might be worth posting here. I was trying to get a good understanding of local variable scoping issues, so I tried the following: f :: (Num a) => a -> a f x = let p = x*x in let p = x*p in p I have some background in ML, which led me to believe that what should happen here is that the function would return x^3. Instead, GHCI just completely terminates, I guess with a segfault. What's the "correct" behavior here? Should it even compile? I understand that you can't redefine the same symbol twice in the same scope, so I tried this specifically to see what would happen if you defined the same variable again in a nested scope. I thought it would just shadow the original declaration, while still using the original p to calculate the value of the new p. I don't think the problem is the re-declaration of the same symbol in a nested scope (although if someone could clarify that would be nice), but rather the fact that I've attempted to use the previous declaration of p in defining the new declaration of p. Would it be a safe assumption that a bug report should be submitted over this?

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Type equality proof
by Martijn van Steenbergen 19 Mar '09

19 Mar '09

Olá café, With the recent generic programming work and influences from type-dependent languages such as Agda, the following data type seems to come up often: > data (a :=: a') where > Refl :: a :=: a Everyone who needs it writes their own version; I'd like to compile a package with this data type and related utilities, if such a package doesn't exist already (I couldn't find one). Below is what I have so far; I'd much appreciate it if you added your ideas of what else the package should contain. > {-# LANGUAGE GADTs #-} > {-# LANGUAGE TypeOperators #-} > > module Eq where > > data (a :=: a') where > Refl :: a :=: a > > class Eq1 f where > eq1 :: f a -> f a' -> Maybe (a :=: a') > > class Eq2 f where > eq2 :: f a b -> f a' b' -> Maybe (a :=: a', b :=: b') > > class Eq3 f where > eq3 :: f a b c -> f a' b' c' -> Maybe (a :=: a', b :=: b', c :=: c') Thank you, Martijn.

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Fw: hw do i solve this problems:
by THANDO NTUANE 19 Mar '09

19 Mar '09

----- Forwarded Message ---- From: THANDO NTUANE <thandodarl85(a)yahoo.com> To: Haskell-Cafe(a)haskell.org Sent: Thursday, March 19, 2009 10:22:40 AM Subject: hw do i solve this problems: kindly help me.. im stack but managed to solve exercise 1 and 2.

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Memoizing partially evaluated computations.
by Sebastiaan Visser 19 Mar '09

19 Mar '09

Suppose I have a list of IO computations that depends on a few very time consuming pure operations. The pure operations are not dependent on the real world: > computation :: [IO Int] > computation = [ > smallIOfunc timeConsumingPureOperation0 > , smallIOfunc timeConsumingPureOperation1 > , smallIOfunc timeConsumingPureOperation2 > , smallIOfunc timeConsumingPureOperation3 > ] > where smallIOfunc a = print a >> return a In my main function I would like to repeatedly print the values > main = forever $ > sequence_ (map (>>=print) computation) When I do this, all the time consuming operations will be reevaluated every run of the main loop. Is there a any (simple or smart) way to prevent the garbage collector from cleaning up the fully evaluated thunks inside my computation? As if it were something like this: > computation :: [IO Int] > computation = [smallIOfunc 42, smallIOfunc 34385, smallIOfunc 3, smallIOfunc 55] Of course I could plugin some kind of Int memoizer inside my computation, but I do not really have the control to change things `deep' inside the code. I want to have some form of snapshot of a list of partially evaluated IO computations... Any suggestions? Tanks, Sebastiaan

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Haskell Style - Pattern Matching with case vs. function declarations
by Tom.Amundsen 19 Mar '09

19 Mar '09

Hi All, I am new to Haskell. I just started reading "Real World Haskell" a few days ago, so I apologize for being such a noob. But, I am curious why I see a lot of code where people do pattern matching via multiple function declarations instead of using the case ... of ... construct? For example: [code] foldl' _ zero [] = zero foldl' step zero (x:xs) = let new = step zero x in new `seq` foldl' step new xs [/code] instead of this, which I prefer: [code] foldl' f acc xs = case xs of [] -> acc (x:xs) -> let new = f acc x in new `seq` foldl' f new xs [/code] -- View this message in context: http://www.nabble.com/Haskell-Style---Pattern-Matching-with-case-vs.-functi… Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.

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hw do i solve this problems:
by THANDO NTUANE 19 Mar '09

19 Mar '09

kindly help me.. im stack but managed to solve exercise 1 and 2.

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Any way to catch segmentation fault?
by Nick Rudnick 18 Mar '09

18 Mar '09

Hi all, doing some work with a regex library I stumbled over some segmentation faults due to illegal byte combinatations. Looking for a way to get this caught some way, I failed in finding any place at GHC (6.10.1) or Hackage libraries where this is covered -- a quick check with HUnit revealed it to be crashing by this phenomenon. So I would like to ask about the state of this issue, o Is there principally no way to (generally) catch segmentation faults?? (This would be hard to believe, as the described kind of noise is to be expected at production systems, especially with user generated content.) o Are segmentation faults «prohibited» in Haskell so the advice is just to change the used library and forget the whole stuff?? o Did I oversee the generic mechanism for catching of segmentation faults?? (If yes, do you please give me a hint??) o If no, is there a workaround which might be practicable?? Thanks a lot in advance, Dorinho

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Google Summer of Code
by Malcolm Wallace 18 Mar '09

18 Mar '09

I am pleased to announce that haskell.org has once again been accepted as a mentoring organisation, for the 2009 Google Summer of Code. Student applications open on Monday (23rd March) at 1900 UTC, for a period of 12 days (until Fri 3rd April, also at 1900 UTC). Students applicants are encouraged to interact with the community via mailing lists, prior, during, and after the submission of their ideas for projects. In general terms, project ideas that benefit the whole community (e.g. infrastructure like ghc, cabal, libraries) will be preferred over things of more marginal interest (e.g. games). Unless you make a _really_ good case, of course! Because (sadly) the darcs community did not get accepted as a separate organisation this year, haskell.org will be willing to accept proposals relating to darcs (as it has done in previous years), but of course they will be competing in the general pool, alongside all the other ideas. Also, we are still accepting mentors. Regards, Malcolm

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.hi inconsistency bug.
by Joe Fredette 18 Mar '09

18 Mar '09

List, I've got this project, source on patch-tag here[1] It's a nice little project, I've got the whole thing roughly working, it compiles okay, everything seems to work, until I try to run it, specifically when I run it in ghci, or when I run the main executable (which uses hint), and look at any type involving my "Email" type, it gives me the following error: Type syonym HackMail.Data.MainTypes.Filter: Can't find interface-file declaration for type constructor or class HackMail.Data.ParseEmail.Email Probable cause: bug in .hi-boot file, or inconsistent .hi file Use -ddump-if-trace to get an idea of which file caused the error As far as I understand, it wants to find the interface-file declaration for a specific type (Email) exported by the ParseEmail module, all of the exports (I think) are in order. I've tried mucking around with it a bit, but I don't fully understand what the error even means, much less how to fix it. Other relevant info, Email is exported in a roundabout way, namely by importing a module MainTypes, which exports a module Email, which exports a the ParseEmail Module, which exports the datatype Email. The "Filter" delcaration it _actually_ complains about (it's just the first place the email type is invoked) is: type Filter a = ReaderT (Config, Email) IO a nothing particularly special. Any help fixing this is greatly appreciated, I did find this bug report[2] which seems like it might be relevant. But trying to unregister -> cabal clean -> cabal install doesn't fix it. I've also tried manually removing the dist/ folder, and also unregistering the package. Thanks again. /Joe [1] http://patch-tag.com/repo/Hackmail/browse [2] http://hackage.haskell.org/trac/ghc/ticket/2057

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