The only time I hear about a mainstream language being immutable by default is from Rust people telling me that Rust is a mainstream language now.
A Haskell program covered in piles of monads to abstract all state away still follows the functional core imperative shell pattern, because the runtime is the imperative shell. This doesn't make such a thing desirable for other languages.
The practical lesson I learned from dabbling with functional programming in academia is that functional programming is often impractical and does not offer any of the purported benefits of better code.
> The only time I hear about a mainstream language being immutable by default is from Rust people telling me that Rust is a mainstream language now.
Most mainstream languages are old, they can't be immutable my default without breaking backwards compatibility. I'm talking about the design direction of languages that are already based on mutability. For example, Java added Records which are immutable, as well as overhauling the date/time api with an immutable version.
> This doesn't make such a thing desirable for other languages.
I've used this pattern in other languages, none of this is a silver bullet that makes software engineering easy; but I do find it makes some code much easier to test and reuse.
Monads are just one design pattern for handling state, and not the only way. It's no different than having an OOP using dependency injection everywhere - it's a design decision that you use only when it makes architectural sense.
A shell is not more imperative than a processor - both are Turing machines, a completely functional mathematical object. Seeing them as imperative is a semantic interpretation of the people watching it, not an inherent property.
Functional languages have either converged on either just allowing mutable values or relying on Monads as the ways of handling state. I'm not aware of a third solution.
A Haskell program covered in piles of monads to abstract all state away still follows the functional core imperative shell pattern, because the runtime is the imperative shell. This doesn't make such a thing desirable for other languages.
The practical lesson I learned from dabbling with functional programming in academia is that functional programming is often impractical and does not offer any of the purported benefits of better code.