Program logic bugs vs input/environmental errors

[Marco Leise via Digitalmars-d]

Am Sat, 04 Oct 2014 01:43:40 -0700
schrieb Walter Bright <newshound2 at digitalmars.com>:

> Would you agree that every time DMD reports a syntax error in user code, it 
> should also include a stack trace of the DMD source code to where in DMD it 
> reported the error?

Of course not. DMD is a compiler and it is part of its normal
operation to have sophisticated code to report syntax errors.
I distinguish between "expected" exceptions and "unexpected"
exceptions.

Expected exceptions are those that occur due to validating
user input or as part of handling documented error codes at
the interface layer with external APIs.

Unexpected exceptions are those we don't handle explicitly
because we don't expect them. Maybe because we think that
code wont throw, since the input had been validated or a bug
in an external library or laziness.

A FileNotFoundException can be the expected result of an "open
file" dialog with the user typing the name of a non-existent
file or the unexpected result of loading a static asset with
an index that is off-by-one (e.g. icon<idx>.bmp).
In the case of DMD, syntax errors are *expected* as part of
validating user input. And that's why it prints a single line
that can be parsed by IDEs to jump the the source.
Now to make things interesting, we also see *unexpected*
exceptions in DMD: internal compiler errors.

While expected exceptions are commonly handled with a simple
message, unexpected exceptions are handled depending on
application. In non-interactive applications like DMD they can
all be AssertErrors that terminate the program. In interactive
software like a video editor, someone might just have spent
hours on editing and the user should be allowed to reason
about the severity of the fault and decide to ignore the
exception or quit the program. There are other solutions like
logging and emailing the error to someone.

The example above, about the off-by-one when reading an icon
is a typical case of an exception that you would want to
investigate with an exception stack trace, but keep the program
running.

-- 
Marco