Immutability by default is not a requirement for functional programming (I mean... if it were, Haskell would be obviously not FP since the 'default' entrypoint is very mutable).

Neither are monads. There are entire FP languages without monads for effects (obviously, you can write a monadic interface in them, but it's not part of the idiomatic core). For example, clean uses linear types to control effects and purescript / Idris use a custom effect-system. So no, monads are not a requirement, and even if they are, modern c++ fully supports them, as does rust, javascript, etc. It's very common in javascript to use Array.map() and Array.flat().

> Immutability by default is not a requirement for functional programming (I mean... if it were, Haskell would be obviously not FP since the 'default' entrypoint is very mutable).

I mean the bindings and collections. For example, when you make an array in JS, the default syntax is a mutable list with a mutable binding:

    let xs = [ 1, 2, 3 ]

> Neither are monads. There are entire FP languages without monads for effects (obviously, you can write a monadic interface in them, but it's not part of the idiomatic core).

I should be more precise - there needs to be some way to mange effects. Monads with syntax extensions is simply the most common (Haskell, Scala, F#, Closure macros)

> and even if they are, modern c++ fully supports them, as does rust, javascript, etc. It's very common in javascript to use Array.map() and Array.flat().

JavaScript does not have good monad support. This is why async/await was added as a new language feature. Yeah, I know you can hack together something with generator functions, but it’s hardly idiomatic.

But we’re getting into the weeds here. My point is: I don’t consider a language to support FP when writing functional code in that language leads to lots of friction compared to what is idiomatic.

Have you ever tried FP in Java? It works for some toy thing but then you hit the lack of TCO, or the ridiculously long type names (not inferred) or the pyramid of doom…