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Pros and cons of immutable strings

Some languages ​​(C # or Java) have immutable strings, while others (such as Ruby) have mutable strings. What are the reasons for these desgin options?

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One reason why immutable strings are good is because it makes Unicode support easier. Modern Unicode can no longer effectively integrate into a fixed-size data cell that kills the one-to-one correspondence between the string index and the memory address, which allows them to use mutable strings.

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In the same vein, Clojure has transients that look like elegant pure functions on immutable data structures, but internally use mutable state for efficiency.

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