I have a struct NotificationOption and another struct NotificationOption2 as well as an implementation for From<NotificationOption> for NotificationOption2. I'm would like to convert Vec<N...

I declared two functions using the 'forall' quantifier. The first of them has a quantifier with all generic type parameters before the signature. The second has quantifiers in place of the first us...

I have read some articles on polymorphism. But I think I could not quite grasp the meaning of its importance. Most of the articles don't say why it is important and how I can achieve polymorphic be...

I am creating a turn based game. I want to define a datatype that encodes one type out of many possible types. Here is the motivating example: I have defined a Turn type using GADTs, so the type of...

I'm new to Haskell and reading Haskell from first principles. BTW this question is general not related to this book, I have taken following question as example In Chapter 10, Section 10.5, Q 5, par...

AsRef documentation writes Used to do a cheap reference-to-reference conversion. I understand reference-to-reference part what does it mean by cheap? I hope it has nothing to do with complexity t...

I am going to use the wonderful library https://tpolecat.github.io/doobie/ and it is fully functional. I was going through the first example and I have recognized: A Transactor is a data type...

These two rules about Functors are fairly well-known: You can't make a Functor if the type parameter appears contravariantly Any type has at most one valid Functor instance But if you cheat sli...

I found there are many definitions for polymorphism and overloading. Some people said that overloading is one type of polymorphism. While some people said they are not the same. Because only one fu...

In C++ you can create templated classes that use a particular operator on the templated objects and the class from which these objects are instantiated must overload that particular operator for it...

Suppose I have a type consisting of multiple polymorphic variants (covariantly) such as the following: [> `Ok of int | `Error of string] Let's further suppose that I want to factor this defin...

I'm teaching myself elm and see (of course) many references to Html msg-- I understand that this is a 'parameterised type', that is, that (as I understand it), the constructor of the type Html tak...

What is the relationship between polymorphism's rank and (im)predicativity? Can rank-1 polymorphism be either predicative or impredicative? Can rank-k polymorphism with k > 1 be either predica...

Normally in Perl 6, only roles are allowed to be parametric. Here, we'll be attempting to make classes, a kind (referred to from here on out as a metaobject) that isn't normally allowed to be param...

I have a Haskell project that uses fmap on data structures very intensively. In order to avoid traversing the same data structure again and again, while retaining the possibility to use fmap libera...

I'm writing a library for creating monads in Perl 6. Since using just regular parametric roles makes it impossible to extend monads properly (to the best of my knowledge), I decided to create a cus...

Programming in Haskell by Hutton says: A type that contains one or more type variables is called polymorphic. Which is a polymorphic type: a type or a set of types? Is a polymorphic type with...

This was prompted by Resolving the type of `f = f (<*>) pure`, which discusses a more complicated example, but this one works too. The following definition compiles without problem: w :: In...

I have recently been learning about λ-calculus. I understood the difference between untyped and typed λ-calculus. But, I'm not much clear about the distinction between the Hindley-Milner type syste...

In section 23.8 of his book Types and Programming Languages, Benjamin C. Pierce writes the following: Another well-studied restriction of System F is rank-2 polymorphism, introduced by Leivant (...

I'm a Go newcomer, but I have read that Go regulars do not miss parametric polymorphism. Every time I try to learn a new language I use the L99 list of problems to get some practice. Even if I try...

While learning Haskell I found something which puzzled me. I don't understand why this code is valid: Prelude> [y | y <- "a", y <- ["a"]] ["a"] I tried to change to an explicit [Char]...

In case of explicit type annotations Haskell checks whether the inferred type is at least as polymorphic as its signature, or in other words, whether the inferred type is a subtype of the explicit ...

I am pretty sure they are not the same. However, I am bogged down by the common notion that "Rust does not support" higher-kinded types (HKT), but instead offers parametric polymorphism. I tried to...

Coming from Javascript I understand that Haskell's list type enforces homogeneous lists. Now it has surprised me that the following different function types meet this requirement: f :: (a -> a)...