- Haskell-Cafe - Haskell.org

Re: [Haskell-cafe] Wiki account request
by howard.b.golden＠gmail.com 12 Dec '21

12 Dec '21

Hi Hyeon, I have created your Haskell wiki account. You should receive a temporary password by separate email. Please let me know if you have any problems. Regards, Howard On Mon, 2021-12-13 at 06:05 +0900, 김지현 wrote: > Hi folks, > > I'd like to create a new haskell wiki account with a username > "simnalamburt". At first, I tried to send an email to the "wiki- > account-request" mailing list as written in > https://wiki.haskell.org/index.php?title=Special:UserLogin, but I > couldn't find the mailing list. If that mailing list is gone, I think > the notice in the login page should be updated. > > If creating a wiki account takes a long time, can anyone do me a > favor? I just want to change the outdated link "Using haskell- > language-server with Vim or NeoVim." in the > https://wiki.haskell.org/Vim page to point to > https://haskell-language-server.readthedocs.io/en/latest/configuration.html… > . > > Best, > Hyeon Kim

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Priority queue benchmarks
by David Feuer 10 Dec '21

10 Dec '21

I've started working on performance of the venerable but somewhat neglected pqueue package. The changes I've made so far are pretty obviously good, and make a big difference in even the most informal testing. But to make decisions about smaller points, I really need benchmarks, and the package has none. Does anyone know of a benchmark suite for priority queues? One that includes algorithms that involve persistent queue use would be particularly desirable, but I'll take anything.

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Call for Workshop Proposals: ICFP 2022
by Ilya Sergey 08 Dec '21

08 Dec '21

CALL FOR WORKSHOP AND CO-LOCATED EVENT PROPOSALS ICFP 2022 27th ACM SIGPLAN International Conference on Functional Programming September 11 - 16, 2022 Ljubljana, Slovenia https://icfp22.sigplan.org/ The 27th ACM SIGPLAN International Conference on Functional Programming will be held in Ljubljana, Slovenia on September 11 - 16, 2022, with the option of virtual participation. ICFP provides a forum for researchers and developers to hear about the latest work on the design, implementations, principles, and uses of functional programming. Proposals are invited for workshops (and other co-located events, such as symposiums) to be affiliated with ICFP 2022 and sponsored by SIGPLAN. These events should be less formal and more focused than ICFP itself, include sessions that enable interaction among the attendees, and foster the exchange of new ideas. The preference is for one-day events, but other schedules can also be considered. The workshops are scheduled to occur on September 11th (the day before ICFP) and September 15-16th (the two days after ICFP). ---------------------------------------------------------------------- Submission details Deadline for submission: December 24, 2021 Notification of acceptance: January 10, 2022 Prospective organizers of workshops or other co-located events are invited to submit a completed workshop proposal form in plain text format to the ICFP 2022 workshop co-chairs (Arthur Azevedo de Amorim and Zoe Paraskevopoulou) via email to icfp-workshops-2022(a)googlegroups.com by December 24, 2021. (For proposals of co-located events other than workshops, please fill in the workshop proposal form and just leave blank any sections that do not apply.) Please note that this is a firm deadline. Organizers will be notified if their event proposal is accepted by January 10, 2022, and if successful, depending on the event, they will be asked to produce a final report after the event has taken place that is suitable for publication in SIGPLAN Notices. The proposal form is available at: http://www.icfpconference.org/icfp2022-files/icfp22-workshops-form.txt Further information about SIGPLAN sponsorship is available at: http://www.sigplan.org/Resources/Proposals/Sponsored/ ---------------------------------------------------------------------- Selection committee The proposals will be evaluated by a committee comprising the following members of the ICFP 2022 organizing committee, together with the members of the SIGPLAN executive committee. Workshop Co-Chair: Arthur Azevedo de Amorim (Boston University) Workshop Co-Chair: Zoe Paraskevopoulou (Northeastern University) General Chair: Andrej Bauer (University of Ljubljana) Program Chair: Zena Ariola (University of Oregon) ---------------------------------------------------------------------- Further information Any queries should be addressed to the workshop co-chairs (Arthur Azevedo de Amorim and Zoe Paraskevopoulou), via email to icfp-workshops-2022(a)googlegroups.com.

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Logging
by Hilco Wijbenga 08 Dec '21

08 Dec '21

Hi all, I just started using "logging" (https://hackage.haskell.org/package/logging) which is based on "fast-logger" (https://hackage.haskell.org/package/fast-logger) It all works fine except that the timestamp in the log message is output as UTC. I would like it to use my local (i.e. system) TZ. I tried setting the TZ environment variable explicitly (before invoking my main) and by setting it in code (System.Environment.setEnv) but it doesn't make any difference. I did not find anything in either API about this so I'm out of ideas. If this is simply impossible, does anyone have a recommendation for another logging package? Cheers, Hilco

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Re Is the Haskell list comprehension just a Cartesian product machine?
by Douglas McIlroy 08 Dec '21

08 Dec '21

> could I explain Haskell list comprehensions as basically just a > Cartesian/cross product machine? Another deviation from the simple explanation: the pair (2,1) never appears in an enumeration from the Haskell expression [(x,y) | x<-[1..], y<-[1..]] Although the notation is adapted from set theory, it does not necessarily reflect the meaning of its set-theory analog. A list comprehension specifies an enumeration order. In the present case that ordering is not well founded: some elements have an infinite descending chain of predecessors and thus never appear in the enumeration. Doug

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Is the Haskell list comprehension just a Cartesian product machine?
by Galaxy Being 07 Dec '21

07 Dec '21

If I were a teacher of high-schoolers wanting to add some theory to Haskell could I explain Haskell list comprehensions as basically just a Cartesian/cross product machine? Would this be accurate? And then the combinatorics product rule also seems at play here. I would appreciate any theoretical explanation of what a list comprehension is about. ⨽ Lawrence Bottorff Grand Marais, MN, USA borgauf(a)gmail.com

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with -XOverloadedStrings, when is the implied fromString floated out?
by Johannes Waldmann 07 Dec '21

07 Dec '21

Dear Cafe, with OverloadedStrings, the compiler inserts `fromString` applications at String literals. I was wondering - are these floated out? {-# language OverloadedStrings #-} import Data.String import Debug.Trace data Foo = Foo deriving Show instance IsString Foo where fromString "Foo" = trace "**fromString**" Foo main = let { f Foo = "Foo" :: Foo; g _ = "Foo" :: Foo } in print $ map (f . g) [0..1] With ghc -O1, I get **fromString** [Foo,Foo] That's good - not just floated out, but also interned (? - both "Foo" are really the same, just one call) With ghc -00, and in ghci, I get **fromString** **fromString** **fromString** **fromString** [Foo,Foo] no floating here. Is there a way to get this floated in ghci? The background of my question is that overloaded strings, and ghci, are heavily used in Tidal Cycles https://hackage.haskell.org/package/tidal I guess it's mostly fine, because it's rare (in live coding performance) that application code contains function definitions. - J.

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Re: [Haskell-cafe] Endo a, endomorphisms
by Olaf Klinke 06 Dec '21

06 Dec '21

> it might be possible that the intention was > to state that function composition is an endomorphism of the set of endofunctions (Endo a). > So again, given that the context is the Data.Monoid library, it seems much more appropriate to > say that Endo a forms a monoid of endofunctions under composition. As the examples I presented > above show, even stating that function composition is an endomorphism of Endo a (endofunctions) > seems incorrect. > What you write does not even type-check: Composition is an operation (b -> c) -> (a -> b) -> (a -> c) so it is a binary operation. Hence it cannot form an endomorphism, because you can't unify the above type with (x -> x). No ambiguity here. > opening :: String -> String > opening = ("Hello, " ++) > > closing :: String -> String > > closing = (++ "!") > > eo = Endo opening > > ec = Endo closing This is a manifestation of the fact that every monoid has a function a -> Endo a given by \a -> Endo (mappend a) This is indeed a monoid homomorphism (by virtue of associativity). Your 'eo' is of this kind. But then you mixed in another monoid homomorphism, namely \a -> Endo (flip mappend a) where your 'ec' is of this kind. It is not true in general that you can mix these two monoid homomorphisms. Only for commutative monoids the above two homomorphisms are identical. And (String,++) is not commutative. Indeed we could not give a Monoid instance to naked (a -> a) because it overlaps with the Semigroup instance you mention. But historically this is not the reason why the Endo newtype was introduced. One could argue that the Semigroup instance should also be on a newtype. I'd be in favour of such a proposal. Olaf P.S.: Let S be a functor (a signature) and a be an S-algebra, that is, there is a structure map alpha :: S a -> a. Then for any r, the type (r -> a) is an S-algebra, too. Indeed, alpha' :: Functor s => (s a -> a) -> s (r -> a) -> r -> a alpha' alpha sf = \r -> alpha (fmap ($x) sf) This is what makes the ReaderT transformer work. For Semigroup, the signature is S a = (a,a) and alpha = uncurry (<>).

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Endo a, endomorphisms
by Adrian 05 Dec '21

05 Dec '21

In the Data.Monoid library there is a monoid type instance for Endo a that is described in the following manner: > The monoid of endomorphisms under composition. I understand how it is a monoid of endofunctions, but am unclear on how it is a monoid of endomorphisms. In the pattern: "forms a monoid of X under Y", X refers to the set and Y to the associative binary operation. So it seems that X in this case is the set of endofunctions. However, it might be possible that the intention was to state that function composition is an endomorphism of the set of endofunctions (Endo a). However even this statement seems difficult to prove in Haskell: opening :: String -> String opening = ("Hello, " ++) closing :: String -> String closing = (++ "!") eo = Endo opening ec = Endo closing appEndo (Endo $ opening <> closing) "Haskell" -- "Hello, HaskellHaskell!" (appEndo eo <> appEndo ec) "Haskell" -- "Hello, HaskellHaskell!" appEndo (eo <> ec) "Haskell -- "Hello, Haskell!" (opening <> closing) "Haskell" -- "Hello, HaskellHaskell!" (opening . closing) "Haskell" -- "Hello, Haskell!" I believe the reason for this behavior is that the fallback for mappend for the general endofunction with type a -> b is found in the Base library as: -- @since 4.9.0.0 instance Semigroup b => Semigroup (a -> b) where f <> g = \x -> f x <> g x

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Output order of Data.List.permutations?
by Johannes Waldmann 05 Dec '21

05 Dec '21

Dear Cafe - what is the ordering of the output of https://hackage.haskell.org/package/base-4.16.0.0/docs/src/Data.OldList.htm… - and why is it not lexicographic? E.g., *Main> last $ L.permutations [1..8] [5,3,6,2,7,1,8,4] note: [5,_,6,_,7,_,8,_] and [_,3,_,2,_,1,_,4] indeed, an auxiliary function is called "interleave". Data.List.permutations appeared in base-4.0.0.0 (ghc-6.10.1, 2008) https://downloads.haskell.org/~ghc/6.10.1/docs/html/users_guide/release-6-1… What is the source of the algorithm? Is it any of the algorithms in Knuth AOCP 4A(I) 7.2.1.2? Hard to tell - as they are given in an imperative style. The implementation *is* fast, e.g., L.sort . L.permutations seems better than a naive lexicographic enumeration lp1 [] = [[]] lp1 xs = do (ys, z:zs) <- zip (L.inits xs) (L.tails xs) (z:) <$> lp1 (ys <> zs) (un-scientifically measured with :set +s) Here, clearly, L.inits and "<>" are bad. NB - I was looking in to this because of https://www.mathekalender.de/wp/calendar/challenges/challenge-01/ Yes I know that can be solved without enumeration. - J.

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