typeclass Questions
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For the sake of exploring libraries with complex types and typeclass instances, I want to check from the REPL whether a typeclass constraint is satisfied or not (provided all the relevant dataypes ...
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I have a typeclass A with a non injective associated type Context
class A a where
type Context a
so many instances of A can have the same Context.
Some instances of A are also instances of the cl...
Intoxicated asked 20/12, 2023 at 12:19
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Data.Dynamic has the following implementation:
data Dynamic where
Dynamic :: TypeRep a -> a -> Dynamic
It has occurred to me that the following definition would be equivalent (at least I th...
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I've got situation where I had a product type (say T), and an instance of say C derived using anyclass deriving, which works because each member of T is an instance of C.
I changed one member of T ...
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Is there a way to persuade GHCi to tell me what typeclass instances some arbitrary type expression implements?
Obviously, you can use the :info command to find all the instances for a type constru...
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Here is a short example in Scala 3:
type Ext[S <: Seq[_]] = S match {
case Seq[t] => t
}
trait XX[A, B <: Seq[A]]
trait XX1[B <: Seq[_]] extends XX[Ext[B], B]
So far it appears t...
Zanazander asked 24/3, 2023 at 20:9
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Suppose a scenario where you have an application config, the structure of which has changed a few times. To provide ease of use for users, you wish to allow automatic migration from each version to...
6
I like to put type signatures for all top-level definitions in my code. However, type signatures in instance declarations don't seem to be allowed, and if I put one I get a "Misplaced type signatur...
Betthezul asked 3/12, 2011 at 11:48
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I have a scenario where I would like to call sum on a sequence of (Double, Double) tuples.
Ideally I would like to do something like the following:
implicit def toTupleNumeric[T](num: Numeric[T]) =...
Nikaniki asked 15/2, 2023 at 11:14
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I suspect I have a fundamental misunderstanding to be corrected, so will start with the general concept and then zoom in on the particular instance that lead me to think this way.
Generally speakin...
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I have this typeclass:
class Monad m => Convertible m a b where
convert :: a -> m b
For many pairs of types, the conversion can be done purely, without requiring a monadic effect.
Now, I kn...
Villager asked 25/11, 2022 at 20:1
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given:
sealed trait Data
final case class Foo() extends Data
final case class Bar() extends Data
final case class TimestampedData[A <: Data](data: A, timestamp: Long)
Is there a succint way to...
Vitiated asked 14/11, 2022 at 10:27
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tl;dr: How do I do something like the made up code below:
def notFunctor[M[_] : Not[Functor]](m: M[_]) = s"$m is not a functor"
The 'Not[Functor]', being the made up part here.
I want it to su...
Stanzel asked 12/4, 2013 at 3:51
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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...
Brassbound asked 6/9, 2022 at 16:27
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Agda's mixiture of records and the instance keyword give us behaviour similar to that of Haskell's typeclasses. Moreover, ignoring the instance keyword, we can have more than one instance for the s...
Retharethink asked 22/4, 2016 at 20:52
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Below is code that will happily compile (once adding the constraints package). Foo1 and Foo2 are two alternate definitions of Foo, which I can write f1 and f2 sensibly for.
However, I think Foo3 is...
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I have a type-class instance like this:
instance {-# OVERLAPPABLE #-} (TypeError ( 'Text "Some error")) => SomeClass x where
someMethod = undefined
This instance exists at the end of...
Prebend asked 27/6, 2022 at 23:13
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The following code successfully compiles, but gets a warning when using GHC 9.2.3 with -Wredundant-constraints:
{-# LANGUAGE UndecidableInstances, FlexibleInstances #-}
class Functor f => C f w...
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It kind of seems like in Haskell one can use instance declarations similarly to superclass constraints. I'll give an example using Semigroup and Monoid from Prelude. Here's a simplified definition ...
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Returning the current type questions are often asked on StackOverflow. Here is one such example. The usual answers seem to be either F-bounded polymorphism or typeclass pattern solution. Odersky su...
Catamite asked 19/1, 2020 at 18:54
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In object-oriented languages (e.g; Java and Python) we can make objects/instances from classes.
In Haskell we can make instances from type-classes, ex:
data ShirtSize = S | M | L -- Here ShirtSize ...
Jaehne asked 3/7, 2017 at 13:20
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What are some ways that I can accomplish what Haskell's typeclasses do in OCaml? Basically, I want to write a polymorphic function without writing too much code. The typical way to do polymorphism ...
Justino asked 18/2, 2013 at 10:35
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According to What does comparable mean in Elm? comparable is built-in type constraint that can be used to limit type variables to those built-in types that are, well, comparable. The following ques...
Oversubtlety asked 29/6, 2016 at 12:8
2
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Is there a clean way to access the default values of a case class fields when performing type class derivation in Scala 3 using Mirrors? For example:
case class Foo(s: String = "bar", i: ...
Law asked 17/7, 2021 at 13:42
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