I understand that many of the names in Haskell are inspired by category theory terminology, and I'm trying to understand exactly where the analogy begins and ends. The Category Hask I already know ...

I find the concept of PhantomData in Rust quite confusing. I use it extensively to constrain object lifetimes in my FFI-based code, and I am still not sure whether I do it correctly or not. Here's...

Suppose I have a parameter that exists only for the benefit of the type system, for example as in this small program: {-# LANGUAGE GADTs #-} module Main where import Data.Proxy import Data.List d...

I was going through some rust source code and I found a data type called PhantomData. I was going through the rust documentation and searched through the internet a lot. However, I couldn't underst...

I'm trying to implement a CurrencyQty type that acts like a number tagged at compile time: data Currency = Usd | Eur | Gbp data CurrencyQty (a :: Currency) = CurrencyQty Double deriving (Num) A...

I have a structure that needs to be generic over a type, yet the type is not actually contained in the structure: it's used in methods of this structure, not in the structure itself. And so, the st...

I'm currently going through the Type-Driven Development with Idris book. I have two questions relating to the design of the example data store in Chapter 6. The data store is a command line applic...

I'm trying to learn more about Phantom Types. I'm trying to read through Fun with Phantom Types by Ralf Hinze. He's using a keyword with I haven't seen before and that I can't get to compile. I get...

I'm learning about existential quantification, phantom types, and GADTs at the moment. How do I go about creating a heterogeneous list of a data type with a phantom variable? For example: {-# LANG...

Don Stewart's Haskell in the Large's presentation mentioned Phantom Types: data Ratio n = Ratio Double 1.234 :: Ratio D3 data Ask ccy = Ask Double Ask 1.5123 :: Ask GBP I read over his bullet p...

Is it possible to write something like an identity function with phantom types, with the purpose of converting the type? For example, given the following type definitions data Nucleotide a = A |...

Suppose I have: class Bounded[A] { type apply[C <: A] = C } This compiles: implicitly[Bounded[Any]#apply[String] =:= String] This fails: type Str = Bounded[Any]#apply[String] ...with: ...

So I'm trying to make a type for variable-length tuples, basically as a prettier version of Either a (Either (a,b) (Either (a,b,c) ...)) and Either (Either (Either ... (x,y,z)) (y,z)) z. {-# LANGU...