Is there any way to write a type family that sometimes evaluates to a constrained type such as C a => T? This question arose when I was writing the following type family: type family Function (c...

If I have an arbitrary type union in Scala 3, is it possible to write a method that "removes" one type from the union? Similar to shapeless.ops.coproduct.Remove, but for native Scala 3. F...

How can I impletement type-level operations (addition in this case) in Scala 3 ? Here is what I would like to do (this does not compile) : case class foo[S <: Int & Singleton](value: Double)...

Wanted to implement type safe matrix multiplication in Haskell. Defined the following: {-# LANGUAGE TypeFamilies, DataKinds, GADTs #-} module Vector where data Nat = Succ Nat | Zero data Vector (...

Given: abstract class Quantity[A <: Quantity[A]] sealed trait UnitOfMeasure[A] class Time extends Quantity[Time] object Minutes extends UnitOfMeasure[Time] class PowerRamp extends Quantity[Pow...

How would I go about deriving the function getField :: (Generic a, HasDatatypeInfo a) => Proxy (name :: Symbol) -> a -> b to project a field from an arbitrary record using a type-level st...

I would like to get GHC to infer a constraint past a GADT pattern match. For instance, suppose I have two expressions, each with an inferred constraint: f :: _ => a g :: _ => a (In my use ca...

I have a trait Foo[T, U] and a type-level algorithm that given an L <: HList and a target type U, tells me whether there exists T in L such that there is an implicit Foo[T, U] in scope. This is ...

type a = [1,2,3] type Invert<T extends any[] & {'0': any}> = ??? type b = Invert<a> // should yield [3,2,1] I am stucked to figure out the definition of Invert type of a tuple, al...

I'd like to do something similar to below with labelled tuples and wondering it this posible in TS4 yet type stringProperties<T extends {}> = {[k in keyof T]: string} This would mean that I ...

I create a lot of data processing pipelines using mass spectrometry data, where data from the instrument gets cleaned up, transformed, scaled, inspected, and finally analyzed. I tend to use a recur...

I'm trying to implement Conway's surreal numbers in Scala. A surreal number is defined recursively – as a pair of sets of surreal numbers, called left and right, such that no element in the right s...

A nice true fact about concatenation is that if I know any two variables in the equation: a ++ b = c Then I know the third. I would like to capture this idea in my own concat so I use a functio...

I'm trying to understand singleton types in shapeless and faced misunderstanding about singleton types compile-time type. Here is an example: val x: Witness.`120`.T = 120.narrow It works fine, b...

My issue I have the following type family that splits arguments off of a function: type family SeparateArgs ( a :: Type ) :: ( Type, [Type] ) where SeparateArgs (a -> b) = SndCons2 a (Se...

I have a class instance that has some relatively unintuitive constraints. This results in unreadable error messages if this constraint is violated. What I would like to do is provide a custom type ...

Considering https://speakerdeck.com/folone/theres-a-prolog-in-your-scala, I would like to "abuse" the Scala type system to find all instances of e.g. CanBuildFrom that match a given criteria. Prolo...

Consider following definition of a HList: infixr 5 :> data HList (types :: [*]) where HNil :: HList '[] (:>) :: a -> HList l -> HList (a:l) And a type family Map to map over typele...

I have a typeclass Search, which has an instance Search[A] if we have a TypeClass1[A] or a TypeClass2[A] instance. With preference given to the 1 instance. The following compiles: trait TypeClass...

To motivate this question, let's first recall a bog-standard Hoare-Dijkstra-style indexed monad, and the example instance of an indexed writer monad. For an indexed monad, we just require type ali...

I've got a type family defined as follows: type family Vec a (n :: Nat) where Vec a Z = a Vec a (S n) = (a, Vec a n) I'd like to assert that the result of applying this type family always fulf...

I have a type class that looks a bit like the following: class Foo a b | a -> b where f :: a -> Bool g :: b -> Bool h :: a -> b -> Bool Or at least these are the bits that are ...

I recently read the very interesting paper Monotonicity Types in which a new HM-language is described that keeps track of monotonicity across operations, so that the programmer does not have to do ...

I'm trying to define some structure like this case class Transformer[From, To]( name: String, get: PaymentEvent => From, to: From => To I want to filter elements with names that are par...

Consider this code: trait TypeOr[E, F] { type T } implicit def noneq2[E, F](implicit ev: E =!= F): TypeOr[E, F] = new TypeOr[E, F] { type T = (E, F) } sealed trait Error[+E, +A] case cl...