May 2020 - Haskell-Cafe

cost of Text.Parsec.Expr
by Johannes Waldmann 02 May '20

02 May '20

Dear Cafe, I use `buildExpressionParser` https://hackage.haskell.org/package/parsec-3.1.14.0/docs/Text-Parsec-Expr.h… with a table similar to https://hackage.haskell.org/package/haskell-src-exts-1.23.0/docs/Language-H… and I find it's somewhat expensive. I think the algorithm makes me pay for each operator that is in the table even when just a few of them are actually in the input. (I removed from the table those that don't appear in my test cases, and it got faster.) Certainly this is known? What could be done about it? What does Megaparsec do? It seems that Text.Megaparsec.Expr vanished from version 6 to version 7. A classical shift/reduce algorithm would help (?) but not with the given type of table, which does not allow to parse an operator just once, and then look up its precedence. - J.W.

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ICFP 2020 will be held ONLINE Aug 23-28
by Sam Tobin-Hochstadt 01 May '20

01 May '20

The ICFP 2020 organizers would like to announce that the conference and co-located events, originally scheduled for August 23-28, in Jersey City, New Jersey, will now be held online during the same dates. Further information for presenters, authors, attendees, sponsors, and the ICFP community will be provided as it becomes available. The ICFP Organizing Committee

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[Haskell-Cafe] How to deal with looped references inside instances?
by Daneel Yaitskov 01 May '20

01 May '20

{- Hi, I am trying to translate a Java sample with looped references below to Haskell. I've found 2 solutions, but both of them look ugly, because they require a helper class. So I am looking for an advice for best practices on working with looped references in polymorphic types. interface Engine { String showEng(); } class Car<E extends Engine> { int id; E eng; String show() { return "Car with " + eng.showEng(); } int getId() { return id; } } class Steam implements Engine { Car<Steam> car; Stream(Car<Steam> car) { this.car = car; } String showEng() { return "id " + car.getId(); } } Car<Steam> car = new Car<>(); car.id = 42; car.eng = new Stream(car); car.show(); -} {-# LANGUAGE UndecidableInstances #-} -- Fix2 shortcoming class Car c where getId :: c -> Int class Engine e where showEng :: e -> String data SteamEng c = SteamEng c instance (Car c) => Engine (SteamEng c) where showEng (SteamEng c) = "id " ++ show (getId c) {- First solution is via ShowEng class. I don't know how to tell type checker that the type which is got after application of type function is instantiating Engine class?? instance (Engine eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ showEng eng -} data Car1 eng = Car1 Int (eng (Car1 eng)) class ShowEng f where showEng2 :: (Car a) => f a -> String instance ShowEng SteamEng where showEng2 = showEng {- A side note. Intuitive alternative for ShowEng which is not working! showEng2 = showEng means showEng2 gets Engine, but class HaveEngine f where cast :: (Car a) => f a -> f a instance HaveEngine SteamEng where cast = id ... instance (HaveEngine eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ (showEng (cast eng)) nor instance (HaveEngine eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ (showEng eng) -} instance Car (Car1 e) where getId (Car1 id _) = id instance (ShowEng eng) => Show (Car1 eng) where show (Car1 _ eng) = "Car1 with " ++ (showEng2 eng) c1 = Car1 42 (SteamEng c1) {- Second solution is removing argument from engine type parameter. -} data Car0 eng = Car0 Int eng instance Car (Car0 e) where getId (Car0 id _) = id instance (Engine eng) => Show (Car0 eng) where show (Car0 _ eng) = "Car0 with " ++ showEng eng {- in this case I have to avoid infinite type and introducing an extra wrapper and instantiating business logic classes for him. λ c0 = Car0 42 (SteamEng c0) <interactive>:30:6: error: • Occurs check: cannot construct the infinite type: car ~ Car0 (SteamEng car) • In the expression: Car0 (SteamEng c0) In an equation for ‘c0’: c0 = Car0 42 (SteamEng c0) • Relevant bindings include c0 :: car (bound at <interactive>:30:1) -} newtype Fix2 f g = Fix2 (f (g (Fix2 f g))) instance (Show (f (g (Fix2 f g)))) => Show (Fix2 f g) where show (Fix2 a) = show a {- λ c0 = Car0 42 (SteamEng (Fix2 c0)) <interactive>:62:1: error: • No instance for (Car (Fix2 Car0 SteamEng)) arising from a use of ‘print’ • In a stmt of an interactive GHCi command: print it -} instance (Car (f (g (Fix2 f g)))) => Car (Fix2 f g) where getId (Fix2 a) = getId a c0 = Car0 42 (SteamEng (Fix2 c0)) {- Thanks, Daniil -}

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