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Take over maintainership binary search
by Jappie Klooster 28 Jan '21

28 Jan '21

Hi, I wish to take over the maintainership of the binary search package. The company I work for (riskbook) currently uses binary search. However the current version doesn't build from hackage because ghc changed how setting files are used, so I made a PR simply deleting that: https://github.com/nushio3/binary-search/pull/3 That has been several months ago and now I'm running against the same issue. There hasn't been a from the author (nushio3), and the email address CC'ed in here are full according to gmail. With regards, Jappie Klooster Riskbook | Software engineer e jappie(a)riskbook.com p +31644237437 Riskbook is a Limited Company (Company No. 10475714) registered at 1 Adamson Drive, Horsehay, Telford, TF4 3UJ, United Kingdom. All contents of this email should be treated as private and confidential unless otherwise stated.

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explicit type for "local" functions
by Jean-Marc Alliot 27 Jan '21

27 Jan '21

Just a short and probably simple question. Let's consider the following function: collatz n = let coll (n,cpt) | n==1 = (1,cpt) | even n = coll (div n 2,cpt+1) | odd n = coll (div (n*3+1) 2,cpt+1) in snd (coll (n,0)) It is easy to set explicitly the type of "collatz" by writing before the function definition: collatz :: Integer->Int But can I set explicitly the type of the "local" function "coll" ? Thanks

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Re: [Haskell-cafe] A quick question about distribution of finite maps.
by Olaf Klinke 26 Jan '21

26 Jan '21

> I find the idea that finite sets and functions may be > represented in Haskell very attractive in general Okay, so the Map was a red herring. It is only one possible implementation of the mathematical concept you are after. To be more clear, let's use Map with capital M whenever we mean the data structure and "map" or "function" whenever we mean the mathematical concept. What you really want is functions between finite sets. Maps do not, in general, represent functions in the Control.Category sense. That got me hooked onto the Kleisli thing. Classical case of nerd-sniping. But you want to deviate from Control.Category and that's perfectly fine because we know the math works. > This establishes a Cartesian closed category of somewhat idealized > entities. Can > the realization of this category in Haskell be perfected to exclude > pathologies > by construction? I think you need dependent types, but let's try. import Data.Map (Map) import qualified Data.Map.Strict as Map import Data.Set (Set) import qualified Data.Set as Set -- Note: One could also define -- FiniteFunction a b = (Set a, a -> b) -- hence the Map in your OP was misleading. newtype FiniteFunction a b = FMap {asMap :: Map a b} domain :: FiniteFunction a b -> Set a domain = Map.keysSet . asMap codomain :: Ord b => FiniteFunction a b -> Set b codomain = Set.fromList . Map.elems . asMap unsafeRunFiniteFunction :: Ord a => FiniteFunction a b -> a -> b unsafeRunFiniteFunction = (assertDomain.) . flip Map.lookup . asMap where assertDomain (Just b) = b assertDomain Nothing = error "argument outside of domain" -- | @(unsafeRunFiniteFunction f) `restrictedTo` (domain f) = f@ restrictedTo :: (a -> b) -> Set a -> FiniteFunction a b f `restrictedTo` dom = FMap (Map.fromSet f dom) -- | (.) of the category. -- 'id' on dom is @id `restrictedTo` dom@ compose :: (Ord a, Ord b, Ord c) => FiniteFunction b c -> FiniteFunction a b -> FiniteFunction a c compose (FMap g) (FMap f) = FMap (fmap g' f) where g' b = case Map.lookup b g of Nothing -> error "type mismatch between domain and codomain" Just c -> c {-- Now to the CCC structure. At this point we run into a problem: Categorically, if f :: A -> (C^B) and we uncurry it, then the domain of (uncurry f) is a square, the cartesian product of A and B. Trying to curry a function whose domain is a proper subset of the cartesian product of A and B should be a type error in my opinion, but Haskell's type system can not express this. I imagine failure of this property may lead to unexpected results when used in conjunction with 'compose'. But maybe currying functions with non-square domain is a desirable feature. If so, remove assertUncurried below. --} -- | @'isSquare' setOfPairs = True@ iff @setOfPairs@ -- is of the form -- @Set.fromList [(a,b) | a <- setA, b <- setB]@. isSquare :: (Ord a, Ord b) => Set (a,b) -> Bool isSquare setOfPairs = let fstProj = Set.map fst setOfPairs sndProj = Set.map snd setOfPairs in all (\a -> all (\b -> (a,b) `Set.member` setOfPairs) sndProj) fstProj isUncurried :: (Ord a, Ord b) => FiniteFunction (a,b) c -> Bool isUncurried = isSquare . domain -- | Beware: we can not express in the type system that -- for all keys @a@ the 'domain' :: Set b of the value under @a@ -- is the same Set, which is a prerequisite for the CCC operation. -- Hence we can not guarantee that the resulting 'domain' 'isSquare'. uncurryFiniteFunction :: (Ord a, Ord b) => FiniteFunction a (FiniteFunction b c) -> FiniteFunction (a,b) c uncurryFiniteFunction = FMap . Map.foldMapWithKey unc . asMap where unc a (FMap bc) = Map.mapKeysMonotonic ((,) a) bc -- | likewise, if the 'domain' of the uncurried function -- fails 'isSquare' then the result is not a well-defined -- higher-order 'FiniteFunction'. curryFiniteFunction :: (Ord a, Ord b) => FiniteFunction (a,b) c -> FiniteFunction a (FiniteFunction b c) curryFiniteFunction f@(FMap ab_c) = assertUncurried (FMap a_bc) where filterFst a = Map.foldMapWithKey (\ab c -> if fst ab == a then Map.singleton (snd ab) c else Map.empty) ab_c as = Map.foldMapWithKey (\(a,_) _ -> Set.singleton a) ab_c a_bc = Map.fromSet (FMap . filterFst) as assertUncurried = if isUncurried f then id else error "not an uncurried function"

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Puzzling "Reduction stack overflow" error message
by CASANOVA Juan 25 Jan '21

25 Jan '21

Hello Haskell Cafe, I almost feel guilty for posting this since I almost never read messages from this list. But I guess others do and do so because they want to so it's an asymmetry that is fine, and which probably won't be like that forever (I'll get to reading them again at a different stage of my life I'm feeling better). I also have very bad experience with asking "complex" Haskell questions in Stack Overflow, with people either not replying or replying stuff that indicates they did not understand the question. Digression aside, I have encountered this error before and it is a nasty one, but this time I am just wildly puzzled by what it's trying to tell me. This is part of a very large program (and I think the huge stack of types and instances is what is hiding the problem here), but I'll try to condense it into a small bit. The following function I have is the one that throws the error: resolve_to_constraints_metacnf :: SOMetaSignature -> SOMetaCNF -> Computation (Maybe SOMetaUnifSystem) resolve_to_constraints_metacnf sig cnf = result where f1 = (ADDirect <$>) :: SOMetaliteral -> SOMetaUnifLiteral; f2 = (f1 <$>) :: [SOMetaliteral] -> [SOMetaUnifLiteral]; f3 = (f2 <$>) :: [[SOMetaliteral]] -> [[SOMetaUnifLiteral]]; ucnf = f3 cnf :: [[SOMetaUnifLiteral]]; resolved = res_computeresolve SOResGreedyFactorH ucnf :: StateT uv Computation (Maybe SOMetaUnifSystem); runstated = runStateT resolved (UnifVar 0); result = fst <$> runstated The error starts as follows: Reduction stack overflow; size = 201 When simplifying the following type: Eq OFunction The rest is the standard for this type of errors. OFunction does not appear explicitly on my function signature, but if we look at the definitions of SOMetaSignature, SOMetaCNF, SOMetaLiteral, SOMetaUnifLiteral and SOMetaUnifSystem, we see they are type aliases for parameterised types, all of which include OFunction as one of their parameters. Here's an example: type SOMetaUnifLiteral = Literal (AtomDependant CAtomPF CTermF LambdaCNF SOPredicate OPredicate OFunction OVariable SOAMVariable SOMVariable UnifVariable) (Yes, I know how tedious it is to have so many type parameters, tell me about it). And perhaps also relevantly, this is the signature of res_computeresolve (which type checks): res_computeresolve :: (ResLiteral l r mt, Heuristics h [Literal l] () (ResStep l) m) => h -> [[Literal l]] -> mt m (Maybe r) (note, might be relevant, that ResLiteral requires mt to be a MonadTrans and MFunctor, which StateT, the type it gets instantiated to in the calling function, is. m gets instantiated to Computation which is a Monad I implemented myself (and works fine).) Now, the natural thing to think is to track the Constraints to find which one is asking for an Eq OFunction and how that works. It definitely is a constraint that I use all over the place. But, here is the puzzling thing. Eq OFunction is clearly implemented. OFunction is a really simple type. Here is the basic definition: data OFunction = OFun Int Int deriving (Eq, Ord) So, why is it having such a hard time proving Eq OFunction, if it is a really simple type with a derived instance of Eq (which has worked thus far without any issues)? My suspicion is that the message is bugged, and the stack overflow is not on that specific instance, but rather on some type variable on a larger instance that it can never entirely instantiate no matter how many constraints it resolves. This is why I added those inline type signatures to the triple fmap, to try to help GHC know what the types should look like. But I am still getting the error, and I really don't know what else to do now. Sorry for the long message, but since I don't know where the error is coming from, I wanted to give some context, and my entire program is tens of thousands of lines of codes by now... I hope people are able to skim and find the relevant bit that tells them what's likely the stupid mistake I am making. Thanks in advance, Juan. The University of Edinburgh is a charitable body, registered in Scotland, with registration number SC005336.

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Re: [Haskell-cafe] stack: duplicate modules in library
by Chris Reade 22 Jan '21

22 Jan '21

Solved - thanks to suggestions from Sebastiaan Joosten. A syntax error (missing hyphens) in package.yaml file was creating duplicates in the .cabal file. So nothing to do with cached binary at all. Chris > On 21 Jan 2021, at 13:57, Chris Reade <chrisreade(a)mac.com> wrote: > > I have come across a problem with haskell stack putting duplicate modules in a library. > > On "stack build" I get > > "Cabal-simple_mPHDZzAJ_3.0.1.0_ghc-8.8.4: Duplicate modules in library:..” > > This may have originally been caused when I changed capitalisation of a filename, but changing back did not resolve the problem. > > My question is how do I get rid of the library so I can rebuild? > I tried "stack purge” and also removing ~/.stack with no luck > (the latter causes a new ghc download on stack build, but with the same message afterwards) > > I know the source modules are fine because they all load in "stack ghci” > Is there some other directory that stack/cabal uses for binary cache that I can safely delete? > > Chris > > >

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[Lambda Days + TFP + TFPIE 2021] call for participation
by Peter Achten 22 Jan '21

22 Jan '21

------------------------------------------------------------------------------- C A L L F O R P A R T I C I P A T I O N 8th Lambda Days + 10th International Workshop on Trends in Functional Programming in Education + 22nd Symposium on Trends in Functional Programming 16 - 19 February 2021 https://www.lambdadays.org/lambdadays2021 ------------------------------------------------------------------------------- The programmes for Lambda Days, TFPIE, and TFP are online: - Lambda Days overall program at: https://www.lambdadays.org/lambdadays2021 - TFPIE specific program at: https://wiki.tfpie.science.ru.nl/TFPIE2021 - TFP specific program at: http://tfp2021.org/ (all times are in Central European Time Zone: 12.00:18.00 (CEST)) Lambda Days is a place where academia meets industry, where research and practical application collide. Once again Lambda Days joins forces with Trends in Functional Programming (TFP) and Trends in Functional Programming in Education (TFPiE) so that for four days you can be at the centre of the functional programming world. The program has a lot of interesting talks, possibilities for interaction, and a host of keynote presentations: Lambda Days keynote speakers: ----------------------------- Perdita Stevens, Pat Hannahan, Simon Peyton Jones, Andy Gordon, Philip Wadler. Trends in Functional Programming in Education keynote speakers: --------------------------------------------------------------- Francesco Cesarini, Simon Thompson, and Bartosz Milewski. Trends in Functional Programming keynote speaker: ------------------------------------------------- Zhenjiang Hu. Thanks to a generous sponsor, TFP has a limited number of free tickets for PhD, Msc, Bsc students with an interest in functional programming. To apply, send a mail to the program chair of TFP, Viktória Zsók (zsv(a)elte.hu) with your name, university, 2-3 lines of motivation, and a valid student card photo to get the registration code. Tickets will be delivered in the order of application.

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Munich Virtual Haskell Meeting, 2021-01-25 @ 19:30
by Leah Neukirchen 22 Jan '21

22 Jan '21

Dear all, Next week, our monthly Munich Haskell Meeting will take place again on Monday, January 25 on Google Meet. For details see here: **Due to meetup limitations in Bavaria, this meeting will take place online!** For details see here: http://muenchen.haskell.bayern/dates.html A Google Meet link to join the room is provided on the page. Everybody is welcome, especially the Haskellers from Bavaria that do not usually come to our Munich meetings due to travel distance! cu, -- Leah Neukirchen <leah(a)vuxu.org> https://leahneukirchen.org/

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stack: duplicate modules in library
by Chris Reade 22 Jan '21

22 Jan '21

I have come across a problem with haskell stack putting duplicate modules in a library. On "stack build" I get "Cabal-simple_mPHDZzAJ_3.0.1.0_ghc-8.8.4: Duplicate modules in library:..” This may have originally been caused when I changed capitalisation of a filename, but changing back did not resolve the problem. My question is how do I get rid of the library so I can rebuild? I tried "stack purge” and also removing ~/.stack with no luck (the latter causes a new ghc download on stack build, but with the same message afterwards) I know the source modules are fine because they all load in "stack ghci” Is there some other directory that stack/cabal uses for binary cache that I can safely delete? Chris

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Bind the GAP barely legible?
by Viktor Dukhovni 20 Jan '21

20 Jan '21

I tried to read "Bind the GAP": https://bindthegap.news/issues.html but the colours, fonts, and layout are so disorienting that I'm afraid it is just too much work to make much sense of it. Am I just too old-fashioned in my expectations, or do others alos find that the festive artwork completely overwhelms the content in this publication? If there's anyone on this list who's involved in producing the publication, please consider drastically dialing back the decorations. -- Viktor.

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Time to vote on possible haskell-language-server logos
by Alan & Kim Zimmerman 20 Jan '21

20 Jan '21

As we announced in December last year, we are looking for a logo for the haskell language server. We now have a selection of candidates, and the voting is open [1] You can vote for as many options as you like. We are closing the poll on 26 Jan (a week from now). Make your preference known, you will hopefully be seeing it a *lot*. Alan [1] https://github.com/haskell/haskell-language-server/issues/694#issuecomment-…

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