The solution to "Error handling"...

[H. S. Teoh]

On Thu, Jul 09, 2026 at 04:26:03PM -0700, Walter Bright via Digitalmars-d wrote:
> On 7/9/2026 12:32 AM, Robert Collins wrote:
> > Instead of only catching exceptions, we started testing common
> > failure scenarios such as invalid input, missing resources, API
> > timeouts, and permission issues. We also separated user-friendly
> > messages from detailed logs, so users received clear guidance while
> > developers still had enough information to troubleshoot the root
> > cause.
> 
> One testing technique that works well is to have input objects,
> processing code, and output objects. One creates "mock" input objects,
> which can be used to generate specific bad input. Then write "mock"
> output objects that, instead of reporting the error, compare the error
> against what the expected error would be.
> 
> You can see this in action in the lexer unittests in the dmd compiler.

Mock objects are awesome for unittests.  I've taken advantage of D
templates to generate instantiations of functions that use mock objects
instead of real OS syscalls, in order to ensure the logic is sound. For
example:

	auto myFunc(File = std.stdio.File)(File input, File output) {
		...
	}

	unittest {
		struct MockFile {
			// insert stuff to inject errors & various
			// conditions
		}

		MockFile mockInput = ...;
		MockFile mockOutput = ...;
		auto ret = myFunc(mockInput, mockOutput);
		assert(ret == expectedReturn);
		... // check of MockFile state is as expected
	}

When compiling with -unittest, the mock version of myFunc is
instantiated and gets called by the unittest.  When not compiling with
-unittest, only the non-mock version of myFunc is generated, so there's
no template bloat in the release build.

This technique can be used not just for files; with suitable template
parameters I've managed to write unittests for functions that manipulate
the filesystem -- by creating a mock filesystem (with minimal
functionality, just enough to run the desired tests).


T

-- 
May you live all the days of your life. -- Jonathan Swift