- Haskell-Cafe - Haskell.org

Haskell on windows 7 without chocolatey?
by Jim Blorg 06 Dec '20

06 Dec '20

Is there a way to install haskell on windows 7 manually via zip without all the requirements for scripting?

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Re: [Haskell-cafe] [Ann] group-theory
by Emily Pillmore 06 Dec '20

06 Dec '20

Hi Marcin, > > Nice work; I also implemented free groups in the free-algebra package > > Wonderful - I'll take a look. Those were a bit rough, and it'd be good to see how you encoded them in `free-algebra`. Maybe we can collaborate on expanding the knowledgebase and utility of free group calculations in our respective packages :) > > However 'FG' is the free group in the class of all groups. > > I'm glad you noted this as well! The `DList` encoding is interesting - I'll need to study that a bit :) Someone has also linked me to Sjoerd's `free-functors` library, which seems is also very interesting: https://github.com/sjoerdvisscher/free-functors/blob/master/src/Data/Functo… > > There's an interesting difference between 'FreeGroup' (using notation of > your package) and 'FG'. First one is not a free group in the class of all > groups, but only free in the class of groups for which multiplication is > strict in the left argument > > Correct. Regarding motivation, I provided both because I know some people do like to operate in the fast-and-loose strict subset of Haskell. It's no skin off my back. But I was very unsatisfied with strict (vis. "moral") Free constructions with respect to bottoms, and, inspired by Dan Doel's article ( http://comonad.com/reader/2015/free-monoids-in-haskell/ ), attempted to find a free construction for groups that was more in line with Haskell's domain-like types. I note this article in the documentation for FG. Maybe this deserves a broader blog post? > > I identified that `Foldable` can only be defined for algebraic structures > for which `Endo b` has the same algebra type; this fails for groups: only > automorphisms form a group. Having a seprate `GroupFoldable` class makes > sense. > > That class ended up being good for two things: * Great puns. * Very convenient word evaluation If you'd like to add to it, I'd be stoked. Cheers, Emily On Sat, Dec 05, 2020 at 2:53 PM, < coot(a)coot.me > wrote: > > Hi Emily, > > Nice work; I also implemented free groups in the free-algebra package. > > > There's an interesting difference between 'FreeGroup' (using notation of > your package) and 'FG'. First one is not a free group in the class of all > groups, but only free in the class of groups for which multiplication is > strict in the left argument, i.e. ones that satisfy the equation `⊥ <> a > == ⊥`. That's because `++` is strict in the left argument. For the same > reason `[]` is free monoid in the class of left strict monoids. > > > > However 'FG' is the free group in the class of all groups. Another > isomorphic construction of a free group (in the class of all groups) can > be based on `DList` - the free monoid. You might want to check out https:/ > / hackage. haskell. org/ package/ free-algebras-0. 1. 0. 0/ docs/ Data-Algebra-Free. > html ( > https://hackage.haskell.org/package/free-algebras-0.1.0.0/docs/Data-Algebra… > ) which gives a uniform interface for all free algebras (not just groups). > You'll recover there `Monad` instance of `FG` (and `FreeGroup`), and a bit > more. > > > I identified that `Foldable` can only be defined for algebraic structures > for which `Endo b` has the same algebra type; this fails for groups: only > automorphisms form a group. Having a seprate `GroupFoldable` class makes > sense. > > > > Cheers, > > Marcin Szamotulski > > > > > > > > ‐‐‐‐‐‐‐ Original Message ‐‐‐‐‐‐‐ > > On Saturday, December 5th, 2020 at 19:52, Emily Pillmore < emilypi@ cohomolo. > gy ( emilypi(a)cohomolo.gy ) > wrote: > > > > >> Hello all, >> >> >> >> I am pleased to announce the release of the group-theory ( >> https://hackage.haskell.org/package/group-theory ) package: a package aimed >> at implementing the theory of finite groups in a reasonably ergonomic, >> well-documented, and featureful way. It's hardly finished, but my >> co-maintainer, Reed Mullanix (@totbwf) and I thought this was a good mvp >> with which we could announce. >> >> >> >> Contributions are very welcome, and I hope everyone enjoys. >> >> >> >> Cheers, >> >> Emily Pillmore >> >> > >

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STM rollback?
by amindfv＠mailbox.org 05 Dec '20

05 Dec '20

I'd like to be able to give up on an STM transaction: roll back and don't retry. I've cooked up something with exceptions but it feels a bit icky to use exceptions for something like this - is there a better way?: data Rollback = Rollback deriving (Show) instance Exception Rollback rollback :: STM x rollback = throwSTM Rollback atomicallyWithRollback :: STM x -> IO (Maybe x) atomicallyWithRollback a = (Just <$> atomically a) `catch` (\Rollback -> pure Nothing) The alternative I've found is something like: otherWay :: STM x -> IO (Maybe x) otherWay a = atomically $ (Just <$> a) `orElse` pure Nothing But this turns any "retry" in "a" into a rollback, and I'd like to have the option to do either (retry or rollback). Thanks, Tom

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untyped printf
by Ben Franksen 05 Dec '20

05 Dec '20

I am writing an interpreter for a very simple untyped language and I want to provide a built-in function to format a list of values in a printf like fashion. There is the beautiful Text.Printf module with its multi-variadic printf function, and I would like to use that to do all the heavy lifting. I can easily implement an instance PrintfArg Value for my Value type, but what I need in addition to that is an "untyped" version of printf i.e. something like format :: String -> [Value] -> String with the property that format fmt [] = printf fmt format fmt [x] = printf fmt x format fmt [x,y] = printf fmt x y ... I am pretty sure this is possible using a bit of class hackery and perhaps GADTs or continuation passing thrown in, but I can't seem to get it working. Any hint that helps to untangle the knots in my brain would be appreciated! Cheers Ben

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2020 Haskell.org Call for Nominations
by Jasper Van der Jeugt 04 Dec '20

04 Dec '20

Dear Haskellers, It is time to put out a call for new nominations (typically but not necessarily self-nominations) to the haskell.org committee. Three members of the committee are at the ends of their current terms: Ryan Trinkle, George Wilson and Tikhon Jelvis. The Haskell Foundation has also recently put out a call for nominations. How does Haskell.org differ from the new Haskell Foundation? Why should you nominate yourself for a role? While the Haskell.org committee shares many goals with the Haskell Foundation, it remains an independent organization to operate the haskell.org website, provide resources and infrastructure, and run programs such as GSoC, like it has been doing for years. The Haskell.org committee has voted to affiliate itself with the Haskell Foundation and several of its members were involved in the founding of the Haskell Foundation. We believe that Haskell.org can represent the community as a stakeholder in the Haskell Foundation and help drive it forward. This makes it an exciting time to nominate yourself, as we'll be able to serve the community better, working closely with other groups in the Haskell community. To nominate yourself, please send an email to committee at haskell.org by the 7th of January, 2020. Current members are eligible to re-nominate themselves. Please feel free to include any information about yourself that you think will help us to make a decision. Being a member of the committee does not necessarily require a significant amount of time, but committee members should aim to be responsive during discussions when the committee is called upon to make a decision. Committee members do not need to be deep technical Haskell experts. Instead we look for enthusiasm towards improving the Haskell community. We aim to represent the different facets of the community. We aim to be diverse in terms of industry or research, and in terms of gender, race and location. The role is about setting policy, providing direction/guidance for Haskell.org infrastructure, planning for the long term, and being fiscally responsible with the Haskell.org funds and donations. As overseers for policy regarding the open source side of Haskell, committee members must also be able to set aside personal or business related bias and make decisions with the good of the open source Haskell community in mind. More details about the committee's roles and responsibilities are on https://www.haskell.org/haskell-org-committee/ If you have any questions about the process, please feel free to e-mail us at committee at haskell.org, or contact one of us individually. Warm regards Jasper

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Question on Coerce
by Brent Walker 03 Dec '20

03 Dec '20

In the following code, function fmap does not compile because variable 'y' on line marked <***> has type (Expr a) where an (Expr b) is expected. The code can be fixed simply by returning (Val x) on the rhs of the function but then we are allocating a new object for something we could potentially reuse since (Val n) has the same runtime representation in (Expr a) and (Expr b) (it has no dependence on the type variable). I was reading about coerce (Data.Coerce) recently and thought this could be a place it could be used but simply replacing the 'y' on the rhs with 'coerce y' does not compile (error message below). Is it possible to use coerce in this context? Thanks for any help, Brent ======================================= data Expr a = Var a | Val Int | Add (Expr a) (Expr a) deriving Show instance Functor Expr where fmap :: (a -> b) -> Expr a -> Expr b fmap f (Var a) = Var (f a) fmap f (Add e0 e1) = Add (fmap f e0) (fmap f e1) fmap _ y@(Val x) = y -- <***> ======================================= /dev/proj/src/Ex12.hs:69:22: error: • Couldn't match representation of type ‘a’ with that of ‘b’ arising from a use of ‘coerce’ ‘a’ is a rigid type variable bound by the type signature for: fmap :: forall a b. (a -> b) -> Expr a -> Expr b at /dev/proj/src/Ex12.hs:66:11-38 ‘b’ is a rigid type variable bound by the type signature for: fmap :: forall a b. (a -> b) -> Expr a -> Expr b at /dev/proj/src/Ex12.hs:66:11-38 • In the expression: coerce y In an equation for ‘fmap’: fmap _ y@(Val _) = coerce y In the instance declaration for ‘Functor Expr’ • Relevant bindings include y :: Expr a (bound at /dev/proj/src/Ex12.hs:69:10) fmap :: (a -> b) -> Expr a -> Expr b (bound at /dev/proj/src/Ex12.hs:67:3) | 69 | fmap _ y@(Val _) = coerce y | ^^^^^^^^ =======================================

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Textbook example of instance Foldable ((,) a)
by Johannes Waldmann 30 Nov '20

30 Nov '20

Dear Cafe - you might find this entertaining: an error in my code that cost me some hours (most Pictures are equal?) until I finally realized instance Eq a => Eq (Picture a) where p == q = dim p == dim q - && all (\ i -> contents p i == contents q i) (bounds p) + && all (\ i -> contents p i == contents q i) (A.range $ bounds p) Henning already called this "Matlab league" behaviour, https://mail.haskell.org/pipermail/libraries/2016-February/026678.html Well it's just a fun observation now, I am not implying anything. Enjoy! - J. Link to full source: https://gitlab.imn.htwk-leipzig.de/autotool/all0/-/issues/660#note_24567

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Amazonka, conduit and sockets not closing
by Magnus Therning 29 Nov '20

29 Nov '20

I've run into a problem with running out of filedescriptors. The following snippet is a trimmed down version of what I'm doing: #+begin_src haskell main :: IO () main = do awsEnv <- newEnv Discover runAWSCond awsEnv $ sqsSource queueUrl .| C.mapC snd .| sqsDeleteSink queueUrl where runAWSCond awsEnv = runResourceT . runAWS awsEnv . within Frankfurt . C.runConduit sqsSource :: MonadAWS m => T.Text -> C.ConduitT () (T.Text, T.Text) m () sqsSource queueUrl = do (_, msgs) <- C.lift $ recvSQS queueUrl C.yieldMany msgs sqsSource queueUrl sqsDeleteSink :: MonadAWS m => T.Text -> C.ConduitT T.Text o m () sqsDeleteSink queueUrl = do C.await >>= \case Nothing -> pure () Just receiptHandle -> do void $ C.lift $ delSQS queueUrl receiptHandle sqsDeleteSink queueUrl recvSQS queueUrl = do let rm = receiveMessage queueUrl & rmMaxNumberOfMessages ?~ 10 rmrs <- send rm let status = rmrs ^. rmrsResponseStatus msgs = rmrs ^. rmrsMessages & traversed %~ extract pure (status, catMaybes msgs) where extract msg = do body <- msg ^. mBody rh <- msg ^. mReceiptHandle pure (body, rh) delSQS queueUrl receiptHandle = do let dm = deleteMessage queueUrl receiptHandle send dm #+end_src This works fine for a while, but given a queue with enough messages it will fail with something like #+begin_example TransportError (HttpExceptionRequest Request { host = "sqs.eu-central-1.amazonaws.com" port = 443 secure = True requestHeaders = [("Host","sqs.eu-central-1.amazonaws.com"),("X-Amz-Date","20201126T101659Z"),("X-Amz-Content-SHA256","2e4bdf20a857a1416f218b1218670cf019ff53268d0adb34fe06402a62f3271d"),("Content-Type","application/x-www-form-urlencoded; charset=utf-8"),("Authorization","<REDACTED>")] path = "/" queryString = "" method = "POST" proxy = Nothing rawBody = False redirectCount = 0 responseTimeout = ResponseTimeoutMicro 70000000 requestVersion = HTTP/1.1 } (ConnectionFailure Network.Socket.getAddrInfo (called with preferred socket type/protocol: AddrInfo {addrFlags = [AI_ADDRCONFIG], addrFamily = AF_UNSPEC, addrSocketType = Stream, addrProtocol = 0, addrAddress = <assumed to be undefined>, addrCanonName = <assumed to be undefined>}, host name: Just "sqs.eu-central-1.amazonaws.com", service name: Just "443"): does not exist (System error))) #+end_example After some detours I found out that it's actually not a network issue, but rather that the process runs out of filedescriptors. Using =lsof= I can see that it doesn't seem to close /any/ sockets at all, instead they get stuck in a =CLOSE_WAIT= state: #+begin_example COMMAND PID USER FD TYPE DEVICE SIZE/OFF NODE NAME wd-stats 88674 magnus 23u IPv4 815196 0t0 TCP ip-192-168-0-9.eu-central-1.compute.internal:60624->52.119.188.213:https (CLOSE_WAIT) wd-stats 88674 magnus 24u IPv4 811362 0t0 TCP ip-192-168-0-9.eu-central-1.compute.internal:43482->52.119.189.184:https (CLOSE_WAIT) wd-stats 88674 magnus 25u IPv4 811386 0t0 TCP ip-192-168-0-9.eu-central-1.compute.internal:60628->52.119.188.213:https (CLOSE_WAIT) wd-stats 88674 magnus 26u IPv4 813527 0t0 TCP ip-192-168-0-9.eu-central-1.compute.internal:43486->52.119.189.184:https (CLOSE_WAIT) ... #+end_example Am I using Amazonka and/or Conduit in a way that results in this? How do I should I use them? Or, is it an issue somewhere "below" my code? What can I do address that? Thanks for any insights or help /M -- Magnus Therning OpenPGP: 0x927912051716CE39 email: magnus(a)therning.org twitter: magthe http://magnus.therning.org/ Action is the foundational key to all success. — Pablo Picasso

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Re: [Haskell-cafe] Textbook example of instance Foldable ((,)a)
by Barak A. Pearlmutter 27 Nov '20

27 Nov '20

On Tue, 24 Nov 2020, Henning Thielemann wrote: > If people would use custom pair types we would not need Foldable on pairs, > at all. To be fair, if people would use custom pair types we would not need *pairs* at all. --Barak Pearlmutter

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network v3.1.2.1
by Kazu Yamamoto 26 Nov '20

26 Nov '20

Hi cafe, network v3.1.2.1 has been released. This version is bug-fixes only: * Increasing base lower bound to 4.9. [#473](https://github.com/haskell/network/pull/473) * Suppressing errors from removeFile in UNIX bind. [#478](https://github.com/haskell/network/pull/478) * Restoring UNIX stub functions on Windows. [#489](https://github.com/haskell/network/pull/489) --Kazu

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