[floober]

For me, I use the simple stuff (Semigroup, Monoid, Monads, Functors, ..) the most. Often times I'll be reasoning about a problem I'm working on in Haskell and realize it is a monad and I can reuse all of the existing monadic control structures. It is also helpful the other way, where you start working with someone else's code and seeing that it is a e.g. Monad immediately tells you so much concrete info about the structure, where in a less structured language you might need to reed through a bunch of docs to understand how to manipulate some objects. The "killer app" is all of that extra structure shared throughout all of the codebases.

[picardo]

Same here. I drank the cool aid a few years back, and started using fp-ts in my frontend projects, hoping to use algebraic data types regularly. But today all I use is the Monad. I can't find any motivation to write abstract algebra to build UI widgets.

[floober]

> I can't find any motivation to write abstract algebra to build UI widgets

This made me chuckle, because I am at this very moment trying to apply the "tagless final style" described here[0] to a custom GUI in a personal-for-fun-and-learning ocaml project : )

[0] https://okmij.org/ftp/tagless-final/course/optimizations.htm...