Yep, and moreover, the combination of most library design and general culture around the language reinforces the dynamic of using mutability only when it's need

Yep, and moreover, the combination of most library design and general culture around the language reinforces the dynamic of using mutability only when it's needed or the most straightforward way to implement something, and contain that with immutable interfaces wherever possible. I like to think we helped with this several years ago when making official guidance: https://learn.microsoft.com/en-us/dotnet/fsharp/style-guide/conventions#immutability-and-mutation https://learn.microsoft.com/en-us/dotnet/fsharp/style-guide/...