On 20-nov-10, at 09:07, Andrei Alexandrescu wrote:
TDPL has an example that can be reduced as follows:
void main() {
uint[string] map;
foreach (line; stdin.byLine()) {
++map[line];
}
}
byLine reuses its buffer so it exposes it as char[]. Therefore,
attempting to use map[line] will fail. The program compiled and did
the wrong thing because of a bug.
The challenge is devising a means to make the program compile and
work as expected. Looking up an uint[string] with a char[] should
work, and if the char[] is to be inserted, a copy should be made
(via .idup or to!string). The rule needs to be well defined and
reasonably general.
I think that you basically stated the important rules: lookup, update
should use const(char[]), set (i.e. potentially add a new key)
immutable(char[]).
The effect is something like this:
void main() {
uint[string] map;
foreach (line; stdin.byLine()) {
auto p = line in map;
if (p) ++*p;
else map[line.idup] = 1;
}
}
Ideally the programmer would write the simple code (first snippet)
and achieve the semantics of the more complex code (second snippet).
Any ideas?
I would consider the first program as written invalid (at runtime)
because the initial value is not set, so you cannot update it with ++.
Also potential hidden idup should not be added IMHO.
I don't find the second program so bad...
Fawzi
