[stepchowfun]

People often say state is hard in functional programming, but things like the Tardis monad have convinced me that (semantically speaking) functional programming is even better at state than the imperative/OOP paradigms. Imperative programs are essentially restricted to one global monad implicitly hardcoded into the language semantics, whereas with functional programming you have the freedom to choose the most appropriate monad for the task at hand. You want state? Use the state monad. You want continuations? Use that monad. You want time traveling? Yep, there's a monad for that too. IO, randomness, nondeterminism, parallelism, environment variables, failure, backtracking, software transactional memory, you name it. Many problems are best solved with a custom monad, rather than one of the standard ones; for example, using a monad that allows your code to emit constraints to be solved (this is a common technique for implementing type inference in compilers). Satisfyingly, reifying effects with monads preserves referential transparency and is therefore compatible with laziness, unlike traditional side effects which are sensitive to evaluation order.

[mhitza]

State management, for me, is hard in Haskell because monads are a pain to compose, and transformer stacks types give me a headache (anything that's wrapping over (->) tends to confuse me anyway when composed).

I've been reading about effectful systems that would remove the pain points around transformers all the way back to 2016, or earlier. Yet there still doesn't seem to be any to overtake the library ecosystem.

Of course this is where the "write your own custom monad"/free monad interpreter might come into play in practice.