- Haskell-Cafe - Haskell.org

Ambiguous types
by Dušan Kolář 07 Aug '19

07 Aug '19

Dear Café, I'm trying to solve a couple of examples and exercises just for me. I've come to the point, when I'm trying working code manipulating lists rewrite to work on Foldable (etc.). Nevertheless, I must be doing some mistake, overlooking something, as simple code like this: chkDup [] = False chkDup (0:ns) = chkDup ns chkDup (n:ns) = elem n ns || chkDup ns which works fine, type-checks, can be used within other code, I'm trying to replace with more generic, but probably less efficient and wrong code: chkDup ns = fst $ foldr f (False,mempty) ns where f _ res@(True,_) = res f v res@(_,vs) = if v==0 then (False, vs) else (elem v vs, pure v <> vs) which does not even type-check. Nevertheless, the error message is not too helpful, searching the Internet just confirms it's wrong and that adding AllowAmbiguousTypes would not work. The error message is: helper.hs:49:1: error: • Could not deduce (Foldable f0) from the context: (Eq a, Num a, Foldable t, Foldable f, Applicative f, Monoid (f a)) bound by the inferred type for ‘chkDup’: forall a (t :: * -> *) (f :: * -> *). (Eq a, Num a, Foldable t, Foldable f, Applicative f, Monoid (f a)) => t a -> Bool at helper.hs:(49,1)-(53,80) The type variable ‘f0’ is ambiguous • In the ambiguity check for the inferred type for ‘chkDup’ To defer the ambiguity check to use sites, enable AllowAmbiguousTypes When checking the inferred type chkDup :: forall a (t :: * -> *) (f :: * -> *). (Eq a, Num a, Foldable t, Foldable f, Applicative f, Monoid (f a)) => t a -> Bool | 49 | chkDup ns = | ^^^^^^^^^^^... So is there a nicer and working way, how to change my simple example? Is there a way, how to make it compile? Or is it useless to do that, original function is just fine... Best regards, Dušan

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Evaluating an AST with GADTs (and Type Families?)
by Hilco Wijbenga 07 Aug '19

07 Aug '19

Hi all, I'm trying to implement an evaluator with GADTs. This is about as far as I've gotten: https://pastebin.com/XjWBzgw7 . data Value = ValueBool Bool | ValueText String | ValueObject (Map String Value) data Expr t where ExprBool :: Bool -> Expr Bool ExprBoolOr :: Expr Bool -> Expr Bool -> Expr Bool ExprText :: String -> Expr String ExprTextAppend :: Expr String -> Expr String -> Expr String ExprObject :: Map String Value -> String -> Expr Value eval :: Expr t -> t eval (ExprBool value) = value eval (ExprBoolOr lft rgt) = eval lft || eval rgt eval (ExprText value) = value eval (ExprTextAppend lft rgt) = eval lft <> eval rgt eval (ExprObject map fieldName) = map ! fieldName Note that the Value data type was just an attempt and not (necessarily) what I'm looking for. And I'm ignoring all error handling for the moment to keep the example small. This compiles but obviously the object type is completely separate from the Expr Bool and Expr String types. Apparently, Type Familiies might help here? I could not find anything relevant that really explained it. I've been thinking about changing ExprObject to something like ExprObjectBool :: Map String Value -> String -> Expr Bool ExprObjectString :: Map String Value -> String -> Expr String ExprObjectObject :: Map String Value -> String -> Expr ??? but I can't figure out what ??? would be. And this would seem to explode if I add more "primitive" types, especially if I want to support lists and maps as well. (Maps and objects are very similar but not the same.) How would I go about making the object type useful here? Or should I go back to plain "Expr" and just error out when trying to, e.g., "or" 2 Strings? Cheers, Hilco

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cabal: Could not resolve dependencies
by Henk-Jan van Tuyl 07 Aug '19

07 Aug '19

L.S., I am trying to compile wxHaskell with GHC 8.8.0.20190721 and I get the following messages from cabal-install: > cabal new-build all --disable-documentation Resolving dependencies... cabal: Could not resolve dependencies: [__0] trying: samplesContrib-0.93.0.0 (user goal) [__1] trying: base-4.13.0.0/installed-4.1... (dependency of samplesContrib) [__2] trying: wxcore-0.93.0.0 (user goal) [__3] next goal: wxcore:setup.Cabal (dependency of wxcore) [__3] rejecting: wxcore:setup.Cabal-3.0.0.0/installed-3.0... (constraint from maximum version of Cabal used by Setup.hs requires <2.6 [...] What does this mean, how do I solve it? (There is a Setup.hs in package wxcore, but does it have its own Cabal file?) > cabal --version cabal-install version 2.4.1.0 compiled using version 2.4.1.0 of the Cabal library Regards, Henk-Jan van Tuyl -- Message from Stanford University: Folding@home What if you could share your unused computer power to help find a cure? In just 5 minutes you can join the world's biggest networked computer and get us closer sooner. Watch the video. http://foldingathome.stanford.edu/ -- http://members.chello.nl/hjgtuyl/tourdemonad.html Haskell programming --

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instance Monoid a => Monad ((,) a)
by Benjamin Franksen 06 Aug '19

06 Aug '19

I wanted to define a simple Monad instance for (Bool,) as instance Monad ((,) Bool) where return x = (False, x) (True, x) >>= k = (True, snd (k x)) (False, x) >>= k = k x -- or: (b, x) >>= k = let (c, y) = k x in (b||c, y) The idea was to keep track of whether functions change their input value or not. To my astonishment I found that this definition overlaps with an existing one. GHC tells me x.hs:2:10: error: • No instance for (Monoid Bool) arising from the superclasses of an instance declaration • In the instance declaration for ‘Monad ((,) Bool)’ | 2 | instance Monad ((,) Bool) where | ^^^^^^^^^^^^^^^^ x.hs:2:10: error: • Overlapping instances for Monad ((,) Bool) arising from a use of ‘GHC.Base.$dm>>’ Matching instances: instance Monoid a => Monad ((,) a) -- Defined in ‘GHC.Base’ instance Monad ((,) Bool) -- Defined at x.hs:2:10 • In the expression: GHC.Base.$dm>> @((,) Bool) In an equation for ‘>>’: (>>) = GHC.Base.$dm>> @((,) Bool) In the instance declaration for ‘Monad ((,) Bool)’ | 2 | instance Monad ((,) Bool) where | ^^^^^^^^^^^^^^^^ [one more similar overlap error elided] But I could not find the instance Monoid a => Monad ((,) a) documented anywhere in the base package. Should I look harder? Or is this instance accidentally leaking from GHC.Base? Eventually, through some experimenting I found that if I replace Bool with Any, then the existing instance seems to do what I want. Cheers Ben

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Performance best practices
by Todd Wilson 04 Aug '19

04 Aug '19

Dear Cafe: I'm getting to the point in my Haskell learning curve where I'm getting more interested in the fine points of programming for performance and would be grateful for some ideas of the tools and options available for exploring this. To make this concrete, let me give a simple example. Here are two different ways of coding the same list-splitting function: import Data.List (inits,tails) splits1, splits2 :: [a] -> [([a],[a])] splits1 xs = zip (inits xs) (tails xs) splits2 [] = [([],[])] splits2 xs@(x:xs') = ([],xs) : [(x:as,bs) | (as,bs) <- splits2 xs'] For example, splits1 [1,2,3] is [([],[1,2,3]),([1],[2,3]),([1,2],[3]),([1,2,3],[])]. How do these functions compare in their actual time and space usage? It seems that, asymptotically, tails is O(n) while inits is O(n^2) in both time and space (when fully evaluated), and splits2 is also O(n^2), but how would I go about comparing them in actual usage to get a more precise comparison? What profiling or run-time statistics could I gather to make a decision about which one to use in a situation where performance mattered (assuming for the sake of discussion that such a situation existed for this simple example)? And, out of curiosity, is there an even more efficient way to code this function? Thanks, Todd Wilson

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ANN: To Kata Haskellen Evangelion
by Cosmia Fu 02 Aug '19

02 Aug '19

Hi everyone, Though some of you might already knows, I'm pleased to announce a new Haskell book, *To Kata Haskellen Evangelion*. Link: https://cosmius.bitbucket.io/tkhe/ I believe that it does not have to be hard to learn Haskell. I begun to write the book in 2017 December, and... to be honest, I don't know what to write now, though I still think it incomplete. It will probably not be updated for quite a while. So I decide to announce it earlier. I am not a native English speaker, so it might be not fluent or even with a lot of grammar mistakes. It would be very nice of you if you can tell me the mistakes in it, factual, technical or grammar. And also feel free to tell me if you want to read some topic in it. Thank you ---- Cosmia Fu

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Request for Comments
by ibrahim Sagiroglu 01 Aug '19

01 Aug '19

Hello all, I have an attempt to present llvm assembly in [1]. Would you consider taking some time to comment on it? [1] https://github.com/ibrahimsag/rw

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Re: [Haskell-cafe] ANN: To Kata Haskellen Evangelion
by Olaf Klinke 01 Aug '19

01 Aug '19

Isn't the Eq vs Ord problem similar to all sub-classes, e.g. Applicative vs. Monad, Semigroup vs. Monoid, Foldable vs. Traversable? What is the recommended way of coding? The Traversable module for example has default implementations of fmap and foldMap, as Functor and Foldable are superclasses. This suggests it is okay to define the subclass method first and derive the superclass methods from them. But arguments have been put up against this practice in particular cases, e.g. for AMP [1]. Olaf [1] https://mail.haskell.org/pipermail/haskell-cafe/2019-July/131259.html

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RfA: wcwidth
by Jason Dusek 31 Jul '19

31 Jul '19

Hi All, This is a Request for Adoption for WCWidth, a low traffic package that provides the functionality of wcwidth(3) <http://man7.org/linux/man-pages/man3/wcwidth.3.html>: “The wcwidth() function returns the number of columns needed to represent the wide character c.” https://github.com/solidsnack/wcwidth/ Although there have been just 3 issues and 2 PRs in 10 years, that’s still indicative of a need for maintenance and life has drawn me away from these avocations. A contributor suggested I reach out with an RfA. Hopefully, the community can be better served with a new maintainer. Kind Regards, Jason

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Call for Participation: Summer BOB 2019 (August 21, Berlin)
by Stefan Wehr 31 Jul '19

31 Jul '19

Here's a quick reminder on the upcoming BOB conference, happening at the third day of ICFP at the same venue. For those going to ICFP anyway: your ticket also gives you access to all BOB talks. For those not going to ICFP: come to BOB conference and also get access to the ICFP talks of day 3! ======================================================================== Summer BOB 2019 Conference “What happens if we simply use what’s best?” August 21, 2019, Berlin co-located with ICFP 2019 http://bobkonf.de/2019-summer/ Program: http://bobkonf.de/2019-summer/program.html Registration: http://bobkonf.de/2019-summer/registration.html ======================================================================== Are you interested in technologies beyond the mainstream, that are a pleasure to use, and effective at getting the job done? BOB is the forum for developers, architects and builders to explore and discover the best tools available today for building software. Our goal is for all participants to leave the conference with new ideas to improve development back at the ranch. Summer BOB is a one-time-only event, in the spirit of the spectacular Winter BOB. The International Conference on Functional Programming is coming to town, and Summer BOB will be right in the middle of it, on the last day of ICFP proper, prior to all the workshops. Summer BOB will feature two tracks: one from practitioners, and one from researchers, and foster communication and cross-pollination between these communities. BOB features two tracks of seven talk each: One research track with invited talks, and one track by practitioners, designed to cross-pollinate and inspire.http://bobkonf.de/2019-summer/program.html Topics include distributed programming, testing, linear algebra, functional design patterns, type systems, formal methods, and interactive development. We are committed to diversity: We aim at exploring a wide range of tools in a welcoming and friendly crowd of diverse people. To that end, a number of support options for participants from groups under-represented in tech are available.http://bobkonf.de/2019-summer/registration.html

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