Summary: Generic programming should be based on representational equality. We can obtain this by reformulating “data” in terms of “newtype”: manually or (tentatively) with compiler support

Background

In modern Haskell we often seek to derive instances, meaning to have the compiler generate them for us, as opposed to writing them by hand. Deriving instances is not just more convenient but also safer and encourages deduplication and code reuse.

Currently GHC provides three different strategies for deriving instances: stock, anyclass and newtype. The stock strategy is the original approach to deriving in Haskell and is “magic”: it is hardcoded in the compiler and only works on a predefined set of classes, such as Eq, Show, Typeable, Functor and so on.

The anyclass strategy is just syntactic sugar for writing empty instance declarations and we will not be focusing on it here. The most...

Most Haskell programmers have come across the term “free”, often used in conjuction with “monad”, “monoid”, “algebra”. Though I never had a problem understanding these concepts in isolation, it was not entirely clear to me how they related to their definition in category theory.

Given a forgeful functor U : C -> D an object x ∈ D, we define a free C-object on x as an object y ∈ C and a morphism η : x -> U y in D such that for any z ∈ C and f : x -> U z, there exists a unique g : y -> z such that U g ∘ η = f.

Normally we will pick our C to be a category of some algebraic structure such as monoids, groups etc, and D to be Set (or as we like to call it in Haskell: Type). Intuitively U is forgetful because it removes struture from a set. For example, consider a forgetful functor from Monoid -> Type. A free monoid on a consists of a monoidal type FreeMonoid a together with a morphism...

GHC 7.10 introduced a new language construct called static pointers. The goal is to allow safe and efficient sharing of morally top-level definitions across processes boundaries. This is most commonly used for sending functions (not serializable on their own in Haskell). In this post we will look at how static pointers are currently unsafe (as of GHC 8.4.2), look at a proposed specification to make them safer, and how we can achieve the same guarantees with the current GHC version.

Static pointers are often mixed up with Cloud Haskell and various high-level libraries that make use of them, but in this post we will concentrate on the minimal API provided by GHC, see how to make it safe, and how powerful features can be built on top of it.

The main use of static pointers is sharing functions, but GHC generalizes the concept to work for arbitrary expressions. The basic idea is simple: we...