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Questions tagged [deriving]

In Haskell, a derived instance is an instance declaration that is generated automatically in conjunction with a data or newtype declaration. The body of a derived instance declaration is derived syntactically from the definition of the associated type.

In Haskell, a derived instance is an instance declaration that is generated automatically in conjunction with a data or newtype declaration. The body of a derived instance declaration is derived syntactically from the definition of the associated type.

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Haskell DerivingVia on multi param type classes with fun deps

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 FunctionalDependencies #-} {-# LANGUAGE StandaloneDeriving #-} {-#…
Guillaum
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How to derive instances for records with type-families

Here's what I'm trying but it doesn't compile: {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE StandaloneDeriving #-} {-# LANGUAGE FlexibleInstances #-} import Data.Text as T import Data.Int (Int64) type family Incoming validationResult baseType type…
Saurabh Nanda
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Haskell Deriving Show Instance

I am playing with a Red-Black tree: -- Taken from Okasaki 1999 module RedBlackTree where --node coloring data --a node is R (red) or B (black) data Color = R | B --tree constructor --a RBT can be E (empty) or be T (a non empty tree) data RBT e = E…
Joe
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How to derive Eq for a GADT with a non-* kinded phantom type parameter

For example, trying to compile the following code {-# LANGUAGE StandaloneDeriving, KindSignatures, DataKinds, GADTs#-} data ExprTag = Tag1 | Tag2 data Expr (tag :: ExprTag) where Con1 :: Int -> Expr tag Con2 :: Expr tag -> Expr tag Con3 ::…
Alex R
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Any way to customize one or two methods of GeneralizedNewtypeDeriving instances?

Haskell's GeneralizedNewtypeDeriving mechanism is great; for those who haven't seen it, writing something like newtype SkewOptT α = SkewOptT (StateT Bool α) deriving (Applicative, Functor, Monad, MonadTrans) will automatically generate…
gatoatigrado
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Derive typeclass instances for opaque types in Scala 3

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 alias by automatically rely on the instance of the same…
alessandro candolini
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Can I use DerivingVia to derive instances for data types isomorphic to tuples

Given the following data type data Both a b = Both { left :: a, right :: b } I can write instances for Applicative etc. like so (omitting Functor here as we can use DeriveFunctor): instance Monoid a => Applicative (Both a) where pure x = Both…
l7r7
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Alternative of Show that only uses name

Is there something like Show (deriving Show) that only uses an algebraic datatype's constructors? (please don't mind that I'm using the word constructor, I don't know the right name...) The reason for this question is that with many of my algebraic…
John Smith
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Is there syntax for creating [Maybe a] analog, but of kind (* -> *)

I was trying to create an fmap that works on [Maybe a]. However, Maybe a has kind *, and fmap demands kind * -> *. This leads to the following unfortunate solution: newtype Unfortunate a = Unfortunate ([Maybe a]) deriving Show instance Functor…
josinalvo
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Why can you create a value with "Just (+)"?

Currently I'm learning Haskell and are stuck with the instantiation of types to typeclasses. I actually don't understand, why it's possible to create a value of the Maybe a type with Just (+). The problem why this behaves strange to me is, that the…
Daniel
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Can't derive Generic for this type?

Compiling this short snippet on GHC 8.6.2: {-# LANGUAGE DeriveGeneric, PolyKinds #-} import GHC.Generics data Foo f = FA | FB (f (Foo f)) deriving (Generic, Generic1) Results in this error: Can't make a derived instance of ‘Generic1…
yairchu
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Why can't we have Random class instances derived for enumerations in Haskell?

I wrote this today: data Door = A | B | C deriving (Eq,Bounded,Enum) instance Random Door where randomR (lo,hi) g = (toEnum i, g') where (i,g') = randomR (fromEnum lo, fromEnum hi) g random = randomR (minBound,maxBound) And I figured this is…
hipogumafe
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Haskell instance read for newtype that is just an int

I am new to Haskell and I want to be able to a newtype so I can tell what is what, but I also have to read it from a string. I have newtype SpecialId Int deriving (Eq, Ord, Show) I want to be able to read "5" :: SpecialId if I derive Read in…
Ross MacArthur
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Type class instance not being used when deriving containing data structure

I've been exploring using more newtype wrappers in my code to make more distinct types. I also do a lot of cheap serialization using Read/Show, particularly as a simple form of strongly-typed config file. I ran into this today: The example starts…
dino
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Deriving Read in Haskell: Why do I have to use the contructor's parameter names?

I've been experimenting with deriving and had some hard time to understand how deriving (Read) works. Let's have a look at the following data definition: data Data = D Int deriving (Eq, Read, Show) data DataWithName = DWN { val :: Int }…
Markus Weninger
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