[injuly]

> which is as performant than using mutable array.

I get what you're trying to say, but that is provably false. As great as the OCaml compiler is, it currently is not capable of the aggressive optimizations that GHC can do with lists.

More often than not, the compiler mostly won't have enough static assertions to reliably generate machine code like that in a real world application (unless explicit mutation is used, of course).

> Functional programmers just trust that their compiler will properly optimize their code.

Precisely. This is why having safe local mutation as a language level feature can give more control to the programmer. We no longer have to rely on the compiler to correctly guess whether a routine is better expressed as an array or a cons list.

> The whole article is secretly about Haskell.

and ML, Koka, Clean, Mercury. The article is about allowing local mutation without breaking referential transparency at the language level.

"Stop using haskell" is a very shallow conclusion, IMO.