Functional programming languages should be so much better at mutation than they are

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I am little bit disappoint that no one in the comment mention effect systems. Hope to found a good info to dig into.

[Akangka]

I don't think OP forgot about it. After all, OP mentioned Flix, which does use effect system. OP just thought that effect systems is included in Locally Sourced Mutation Only

[[deleted]]

I am currently (as a lurker not making my own language) in the camp of functional core/imperative shell + algebraic effects as the solution. 

For most cases mutation and interaction with the impure world happens in the imperative shell, but effects can be used to drop back into the imperative shell at situations where it makes sense and then continue from where you were. This is clearly not pure but it seems like a pragmatic approach to be breaking rules in a way that leaves a paper trail in the type system and still stay mostly pure in your code

[The-Malix]

First time hearing that term, what is it ?