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From: Damien Doligez <damien.doligez@inria.fr>
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Date: Wed, 28 Sep 2011 17:30:33 +0200
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To: Thomas Fischbacher <t.fischbacher@soton.ac.uk>
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Subject: Re: [Caml-list] Weird GC behaviour

Dear Thomas,

Others have mostly answered your questions, but to be complete I want
to add the following:

> How come (Ftag "funny") is regarded as constant while
> (Rtag (ref "funny")) is not?

The difference here is that (ref "funny") is a function call.  The
compiler doesn't know that the "ref" function just allocates a record.

> After all, strings are mutable in OCaml,
> so there really is not that much of a conceptual difference between a
> string and a string ref in that respect:

You should rather look at the analogy between strings and arrays, but
as Gabriel said, string literals are very rarely mutated in practice,
so it makes more sense to have "funny" return the same string instead
of allocating.

> But the problem I think I have with OCaml is: there just seems to be no
> way to properly express the conceptual difference between '(1 2 3 4 5)
> and (list 1 2 3 4 5): All I can say above is: Ftag "Hello".

You should say (Ftag (String.copy "Hello")) if you want a fresh mutable
string.  I wouldn't recommend appending "" or using Obj.dup (yuck!)

I agree it was a mistake to make strings mutable, but we have to live
with it for the time being.  If you want to be perfectly safe, you can
wrap all string literals with String.copy in your program.

-- Damien