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#Step 1: Template Haskell splices The [d| ... |] is a syntax of Template Haskell declaration quote. For example, [d| answer = 42 |] would produce a value of type Q Dec, which is an abstract syntax representation of it. Relatedly, the $(...) is splice syntax. As it appears on the top level, it splices declarations (of the type Q [Dec], or expressions of the type Q Exp depending on the context) into

As you are reading this post, you probably know that there is the lens library by Edward Kmett et al. which is de facto optics library for Haskell. It's famous also for its type errors. the optics library by Adam Gundry, Andres Löh, Andrzej Rybczak and myself uses different representation for optics (note: slanted optics is a concept, monospace optics is a library name). I recommend reading throug

This posts complements two other recent blogs: Code smell: Boolean blindness by Thomas Tuegel and Ceci n'est pas un default by Guillaume Allais. Thomas writes The popular term “boolean blindness” refers to the information lost by functions that operate on Bool when richer structures are available and indeed, in one project I wrote a function with a very horrible type: I could add comments, but tho

I define free Monad and free Applicative using single type, parametrised over a tensor. It's possible, because both Monad and Applicative are monoids in the category of endofunctors. There is little, if any, practical applications I can foresee for these definitions, but it's fun to see how things are connected. I won't prove laws hold, or constructions are free (= are left adjoint of a forgetful

It's good time to write tongue-in-cheek posts on Christmas Holidays. You have probably have heard about Profunctor Optics, there is a paper by Matthew Pickering et. al, and my Glassery post. Functor optics are similar and simple. With profunctors you can define type-changing optics, with bare Functor you can define only so called simple optics. As this is a literal Haskell file, first a moderate p