More descriptive error for environment variable not found

[tkelman]

When ERROR: key not found: "foo" happens deep in some internal function, it's not always obvious that it's referring to an environment variable. Bikeshedding on the error message welcome (or if this needs to be done differently), but I think this is a special enough case that it should get a specific message.

[tknopp]

While the error message is cleaner, its now not possible anymore to catch KeyErrors (or are they mapped to the same exception?). IMHO error should be used in a lot less situations. More specific exceptions make the exception system a lot more useful.

[tkelman]

Thanks, that's the kind of thing I meant by "if this needs to be done differently." I don't know enough about exception handling to be sure how to do this properly. Would throw(KeyError("environment variable not found: $k")) work here?

[ivarne]

That will look funny when it prints like

ERROR: key not found: "environment variable not found: foo"

[tknopp]

Actually I am not an expert on Julia exceptions either but have worked a lot with exceptions in C++ and C#. And from this I know that it is pretty important to
a) have an exceptions hierarchie
b) and actually also use it
c) and don't (!!!) allow to throw integers ;-)

[tknopp]

@ivarne: Are the exceptions defined in the C part of Julia? Maybe KeyError should only print key not found when no string is passed as constructor argument.

[ivarne]

Some exceptions is defined in C because of bootstrapping issues. The rest is defined in base/boot.jl. The printing of the exceptions is defined in base/repl.jl as methods on the showerror function.

[tknopp]

Okay thanks. I find it a little bit confusing that the naming Exception and Error seems to be a little mixed. In most cases one cannot know if it is an Error or an Exception at the point where it is thrown. In the case of the KeyError it can be part of the regular workflow to catch it if an environement variable is not defined.

[tkelman]

That will look funny when it prints like

ERROR: key not found: "environment variable not found: foo"

Agree that it looks funny. Maybe remove the duplication of "not found," just say throw(KeyError("environment variable $k"))?

[JeffBezanson]

I think the argument to KeyError should be the key itself in all cases.

[tkelman]

I think the argument to KeyError should be the key itself in all cases.

That's perfectly reasonable. What about adding a subtype of KeyError called EnvError or something, for which the showerror method says "environment variable not found" instead of "key not found"?

[ivarne]

If we had inheritance from concrete types, that would be the right approach. Currently the Julia exception system is not designed for the inheritance hierarchy, we know (and love) from Java and Python.

[tkelman]

I'll close this PR for now, until I figure out a better way to do what I'm going for.

It seems like making an almost identical copy of KeyError specifically for environment variables that only differs in the error message might be the solution, but I don't know how many places that would need to touch.

_{^{@ivarne I don't particularly like anything about either of those languages but that's beside the point}}