On Wed, 1 Dec 2010 03:25:24 -0800
Jonathan M Davis <jmdavisp...@gmx.com> wrote:
> On Wednesday 01 December 2010 03:17:48 spir wrote:
> > Well, nearly everything is in the title... I would intuitively opt for the
> > second type if I had to define an immutable string type.
> >
> > Answers from various points of view (semantic, praticality, efficiency...)
> > would be great if ever they may differ.
>
> string str = "hello";
> str ~ = " world";
>
> That's wouldn't be possible if string were fully immutable.
This, I do not understand. I thought immutable applies to the value, not to the
variable. At least, this is what I know from other languages using immutable;
but it seems, then, that this term does not mean the same in D. Immutable, for
me, mean no reference to the value can change it. Thus, for instance:
immutable(char[]) s = "abc";
immutable(char[])* p = &s;
*p = "def"; // Error: *p is not mutable
EDIT: All right, got it! I was caught once more by the fact that, for most
types, in languages of the traditional imperative kind, a variable maps to a
memory cell. Thus, immutability of the value also means "unchange-ability" of
the variable. In other words:
s = "def"; // Error: variable __trials__.main.s cannot modify immutable
because there is no way to change or replace s's value without mutating the
original one. If I change s, then *p will see it. Is this reasoning correct?
(I was mislead because, in functional and dynamic languages from which I learnt
the notion of immutability, elements are referenced, so that one can change a
variable denoting an immutable value: it just silently points to another place
in memory, but the original value never mutates. In such a language, changing s
would not affect any other reference to the original value. Eg, python strings
are immutable and referenced:
s = "abc" ; t = s
s = "def"
print s,t # "def abc"
)
> And since the fact
> that the elements of a string are immutable, passing a string to a function
> is
> guaranteed not to mess with the original, even if they append to the one
> passed
> in. So, making it fully immutable wouldn't really buy you anything anyway.
Yop.
> - Jonathan M Davis
Thank you, Jonathan.
Denis
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vit esse estrany ☣
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