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This sounds quite similar to pure functional languages like Haskell, where a function call cannot have any side effect.

But those go further in that they don't even have any mutable data. Instead of

    var foo = { a: 1 };
    var bar = foo; // make a copy of foo
    set bar.a = 2; // modify bar (makes a copy)
Haskell has

    foo = Baz { a = 1 }
    bar = foo { a = 2 } // make a modified copy of foo


Personally I think local mutability is quite a useful property, which was part of the inspiration for making this instead of just another pure functional language:

- All functions are still referentially transparent, which means we get all the local reasoning benefits of pure functions. - We can mutate local variables inside loops (instead of just shadowing bindings), which makes certain things a lot easier to write (especially for beginners). - Mutating nested fields is super easy: `set foo.bar[0].baz = 1;` (compare this to the equivalent Haskell).




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