Definition of resulting code as _deeply_ imperative was crucial in my phrase.
The only «sine qua non» optimization through opportunistic mutablity is AFAIK tail call optimization. But it is probably too well-known to call it «clever optimization» in 2023.
But, for example, applying a function to a list will produce the code allocating new list in, say, OCaml and Haskell, at least by default. And Roc will produce the code for mutating the list in-place _from the source code with the same semantic_ (see example for the Quicksort algorithm in the video above).
Compile-time lifetime analysis (that probably is not needed at all in functional languages with GC) and widely used unboxed values are way not common in functional language implementations. For example, in OCaml those features are still experimental (and probably never will be used by default, as in Roc).
The only «sine qua non» optimization through opportunistic mutablity is AFAIK tail call optimization. But it is probably too well-known to call it «clever optimization» in 2023.
But, for example, applying a function to a list will produce the code allocating new list in, say, OCaml and Haskell, at least by default. And Roc will produce the code for mutating the list in-place _from the source code with the same semantic_ (see example for the Quicksort algorithm in the video above).
Compile-time lifetime analysis (that probably is not needed at all in functional languages with GC) and widely used unboxed values are way not common in functional language implementations. For example, in OCaml those features are still experimental (and probably never will be used by default, as in Roc).