Question

It’s the reverse of this question: Why can't strings be mutable in Java and .NET?

Was this choice made in Ruby only because operations (appends and such) are efficient on mutable strings, or was there some other reason?

(If it’s only efficiency, that would seem peculiar, since the design of Ruby seems otherwise to not put a high premium on faciliating efficient implementation.)

Answer 1

This is in line with Ruby’s design, as you note. Immutable strings are more efficient than mutable strings – less copying, as strings are re-used – but make work harder for the programmer. It is intuitive to see strings as mutable – you can concatenate them together. To deal with this, Java silently translates concatenation (via +) of two strings into the use of a StringBuffer object, and I’m sure there are other such hacks. Ruby chooses instead to make strings mutable by default at the expense of performance.

Ruby also has a number of destructive methods such as String#upcase! that rely on strings being mutable.

Another possible reason is that Ruby is inspired by Perl, and Perl happens to use mutable strings.

Ruby has Symbols and frozen Strings, both are immutable. As an added bonus, symbols are guaranteed to be unique per possible string value.