custom exception type

[spir]

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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