On Sat, 20 Nov 2010 15:22:37 +0100
Tomek Sowiński <j...@ask.me> wrote:
> > I find this proposal really necessary. But aren't there two issues here?
> > * Comparison (for lookup) by value equality should not care about
> > qualifiers (ie compare raw content, here plain array memory areas).
> > * Assignment should perform "qualification conversion" automatically, eg
> > char[] chars = "abc";
> > string s = chars;
> > This involves no implicit magic here, as target qualification is
> > explicit. So, why not?
>
> It's busting the whole const system to smithereens.
>
> char[] chars = "abc";
> char[] backdoor = chars;
> string s = chars;
> assert (s == "abc");
> backdoor.front = 'k';
> assert (s == "abc"); // fails. not so immutable, huh?
???
backdoor and s do not denote the same element. One is a mutable array, the
other is immutable. Why should changing backdoor affect s? Whether backdoor and
chars denote the same array depends on whether "=" copies or not dyn arrays.
But from immutable string to mutable array, there must be a copy (read: dup).
Anyway, the most annoying issue is not about assignments inside a given scope,
but parameter passing (including implicit ones like in Andrei's example of
foreach).
void f (char[] chars) {}
void g (string str) {}
...
string str = "abc";
char[] chars = "abc".dup;
f(str);
g(chars);
__trials__.d(30): Error: function __trials__.f (char[] chars) is not callable
using argument types (string)
__trials__.d(31): Error: function __trials__.g (string str) is not callable
using argument types (char[])
...
f(str.dup); // ok
g(chars.idup); // ditto
> > There is a repetitive programming pattern in D:
> > * play around with *string's in general
> > * as soon as text processing is needed, convert to *char[]
> > * when finished, convert back to *string
>
> Meh, immutable strings make life easier.
For sure, I'm 100% for immutable strings by default, esp that literals produce
immutable elements. But one cannot perform text processing on immutable
thingies:
s[3] = 'x';
s.replace("abc","ABC");
So that we constantly have to juggle between mutable and immutable versions of,
conceptually, the _same value_.
By the way, why isn't the definition of string immutable(char[]), instead of
immutable(char)[]?
Denis
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