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Mutable objects are there so that you don't have to create new instances whenever a field changes. The danger of shared mutable objects is that the ground can change beneath your feet.

Immutable objects enable safe sharing, however, if you have an immutable object and want to change it, you have to copy it and make the change during construction of the new object.

I guess the 'copy penalty' in either case depends on the size of your objects.

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If your compiler is smart enough (i.e. you are programming in clean, or use Haskell's GHC) the `copy penalty' can be zero for objects that only have "one future", i.e. you only ever keep one copy of your structure. (And that's still cleaner and safer than mutable structures.)

The `copy penalty' for multiple uses of your objects, say a = update1 (x), b = update2 (x), does not depend so much on the size of your object as one the amount of sharing you can do. Immutable objects allow sharing. For mutable ones you have to work much harder.

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