Harrison, John R writes:
 > The point is not that I will mutate a string by accident.

I once discovered a bug in  the Coq proof assistant that was precisely
due  to  a  string (an  identifier)  mutated  by  accident (may  be  I
shouldn't say  it :-) A name  was capitalized in-place  somewhere in a
piece  of code  unrelated with  the Coq  kernel but  of course  it had
consequences all over the system (including the kernel).

So I'm definitely in favor of immutable strings, for the exact reasons
mentioned by John.

But I  think an abstract data type  is not really an  issue, since one
does little pattern-matching on  strings in practice.  And having your
own  abstract data type  for immutable  strings has  other advantages,
such as  the ability  to share equal  strings (using  hash-consing) to
speedup  names comparisons. Even  printing is  not painful  provided a
suitable formatter-based printing function and %a.

-- 
Jean-Christophe Filliâtre (http://www.lri.fr/~filliatr)