I don't think anybody (even us Haskell weirdos) are clamoring for full immutability of anything. For data structures it's AMAZING unless you really cannot live

I don't think anybody (even us Haskell weirdos) are clamoring for full immutability of anything. For data structures it's AMAZING unless you really cannot live with a log factor, but that's neither here nor there. Anyway, for me it's all about controlling and sequestering off (shared) mutability... and -- at a slightly higher level of abstraction -- capabilities given to random bits of code. Being able to conclusively say that a function "foo" can only perform operations on a "User repository" (or whatever) and not randomly go prodding the database directly is amazing. (Think of "mut" as a capability and you'll probably get the gist of what I mean. It's just a bit more general.)