Using: {-# LANGUAGE GADTs #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE DeriveDataTypeable #-} And given the following datatype: data Event a where PureE :: a -> Event a MapE ...

Is there a way in Scala 3 to use derives keyword in combination with opaque type aliases? It would be nice to have a boilerplate-free way to provide a typeclass instance to a given opaque type alia...

I'm trying to deriving Show, Eq, Ord etc in Idris, but none of the following trials works: trail #1: data Expr = Lit Int | Neg Expr | Add Expr Expr deriving (Show) got: deriving.idr:5:15-1...

Let us say we have data D = X Int | Y Int Int | Z String I wish to have a function getDConst getDConst :: D -> String that returns either "X", "Y", or "Z", according to the data constructo...

Suppose we have some class Foo such that an instance of Foo f gives us everything necessary to implement Functor f, Foldable f and Traversable f. To avoid overlapping instances, can witness this re...

GHC has several useful language extensions for mechanically deriving various common Haskell typeclasses (-XDeriveFunctor, -XDeriveFoldable, -XDeriveTraversable). It seems that Applicative is anothe...

For a simple example, say I want a type to represent tic-tac-toe marks: data Mark = Nought | Cross Which is the same as Bool Prelude> :info Bool data Bool = False | True -- Defined in ‘GHC.Type...

I'm trying to build a streaming library using the abstractions described in the paper "Faster coroutine pipelines". I've modified the code so that it correctly handles pipeline exiting (i...

From a blog post I read -- | Newtype for disabling logging newtype NoLoggingT m a = NoLoggingT { runNoLoggingT :: m a } deriving newtype (Functor, Applicative, Monad) deriving (MonadTrans) via I...

I am trying to define an instance: newtype Join a = Join { getJoin :: a -> Bool } deriving Generic instance Monoid (Join a) where f <> g = ??? mempty = ??? The goal is that the functi...

Related question - Is it safe to derive MonadThrow, MonadCatch, MonadBaseControl, MonadUnliftIO, etc? - where I had enabled, both - DeriveAnyClass and GeneralizedNewtypeDeriving to get the code to...

I am trying to use this blogpost's approach to higher-kinded data without dangling Identity functors for the trival case together with quantified-constraint deriving: {-# LANGUAGE TypeFamilies #-}...

I'm experimenting with the Foldable typeclass in Haskell, using the following data type as an example: data Tree a = Empty | Node (Tree a) a (Tree a) If I use the DeriveFoldable GHC extension, ...

I have the following code: sealed trait Animal case class Cat(name: String) extends Animal case class Dog(name: String) extends Animal trait Show[A] { def show(a: A): String } class Processor[A...

I'm trying to create my own data type, which will be part of Monad class, but newtype Container a = Container a deriving Monad gives me this error: * Can't make a derived instance of `Monad Co...

I have a dozen of newtypes like this: newtype MyBool = MyBool Bool newtype MyInt = MyInt Int I want to reuse existing instances: instance MArray IOUArray Int IO where ... instance MArray (STUAr...

I'm refactoring some old code, which is in a polymorphic, but type-class constrained, monad: class ( MonadIO m , MonadLogger m , MonadLoggerIO m , MonadThrow m , MonadCatch m , MonadMask m ,...

My question seems to be closely related to this one. My code parses a yaml file, rearanges the objects and writes a new yaml file. It works perfectly well, but there is a particularly ugly part i...

So, in Haskell, it's really easy to do this: data Foo = Bar | Baz deriving (Read, Show) This is great, but I'd like to be able to pass some data as a string from Haskell to the Elm language. Th...

I'm not really sure what I'm doing wrong here: data Vector2D u = Vector2D { _x :: u, _y :: u } deriving stock (Show, Eq, Functor, Foldable, Traversable) {-# INLINE addVector2 #-} addVector2 ...

In Haskell, you can derive Functor, Foldable and Traversable automatically using deriving. There is no way to derive Applicative, though. Considering there is one obvious way to define an Applicati...

I'm trying to use DerivingVia to cut the boilerplate on instance definitions for a multi parameter type class with functional dependencies. I have these types and class: {-# LANGUAGE FunctionalDe...

I want to have an annotated AST, so I defined those recursive data structures using Fix: data Term a = Abstraction Name a | Application a a | Variable Name deriving (Read,Show,Eq,Functor,Fold...

I would like to use derived instance like this: data Test3D = forall a. (Show a, Eq a, Typeable a, Generic a) => Test3D { testDt :: String , testPrm :: a } deriving (Show, Eq, Typeable, Gen...

Is there a way to write the following: {-# LANGUAGE DeriveDataTypeable #-} {-# LANGUAGE DeriveAnyClass #-} data X = A | B | C deriving (Eq, Ord, Show, Read, Data, SymWord, HasKind, SMTValue) ...