[lifthrasiir]

Of course `do` itself is much more capable, but it has an effect of the conversion for some monads, which was what I wanted to say.

You are correct about in-depth mutations and resulting complications, but that only strengthens my assertion: immutable should be default, but not compulsory (because sometimes you absolutely need them). And mutability doesn't preclude caching or dead-code elimination; you just have to be more careful. Often it's the case that you can convert a mutable code into an immutable form only for the purpose of analysis, which is definitely harder than an immutable code in the first place but not impossible. Scalar compilers have used SSA---an immutable description for mutable programs---for a long time after all.