Hello,
Where can one find descriptions of Throwable, Error, & Exception? (I mean, how
do you even know they exist?) I could finally guess the constructor must have a
string parameter used for error output.
Also, is it possible to implicitely reuse the superclass's constructor? I had
to write:
class E : Exception {
this (string msg) {
super(msg) ;
}
}
E.this performs nothing new. But without it, I get a compiler error:
trial.d(7): Error: constructor trial.E.this no match for implicit super()
call in constructor
Isn't the constructor inherited like other attributes?
Finally, is it possible to customize the error message construction, using eg
tostring? A big issue is that, currently, an exception's message is computed at
construction time, even if the exception will never be thrown, or more
ccommonly never be output -- because it is caught by a 'catch' clause. In some
cases, constructing the message can be costly; some programming schemes may
throw huge numbers of exceptions, all caught (or nearly all).
Example: in a parsing library, pattern match methods throw an instance of
MatchFailure when matching fails. When there is a pattern choice, there may be
numerous failures for each success. MatchFailure is just a clean way of
signaling this fact (*): each failure exception is caught at a higher level to
allow trying the alternative patterns. Since error messages can be rather
complicated, constructing them uselessly would multiply parsing time by a
rather big factor (in my case, ~ X 30!).
I guess tostring is the right feature for this: it would return the exception's
textual form, ie the message. (For information, this is how Python works.) I
tried to use it, but it seems to be simply ignored. What is the func/method
that constructs the text of an exception, eg what is implicitely called by
"writeln(e);"?
Denis
(*) This is exactly the programming pattern somewhere explained in D docs:
throw an exception instead of returning a fake value used as failure flag.
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