On 2015-12-06 00:09, Chris Wright wrote:
But there are problems with saying that the builtin assert function
should show the entire expression with operand values, nicely formatted.
assert has to serve both unittesting and contract programming. When
dealing with contract programming and failed contracts, you risk objects
being in invalid states. Trying to call methods on such objects in order
to provide descriptive error messages is risky. A helpful stacktrace
might be transformed into a segmentation fault, for instance. Or an
assert error might be raised while attempting to report an assert error.
assert is a builtin function. It's part of the runtime. That puts rather
strict constraints on how much it can do. The runtime can't depend on the
standard library, for instance, so if you want assert() to include the
values that were problematic, the runtime has to include that formatting
code. That doesn't seem like a lot on its own, but std.format is probably
a couple thousand lines of code. (About 3,000 semicolons, including
unittests.)
I would like these nicely formatted messages. I don't think it's
reasonably practical to add them to assert. I'll spend some thought on
how to implement them outside the runtime, for a testing framework,
though I'm not optimistic on a nice API. Catch does it with macros and by
parsing C++, and the nearest equivalent in D is string mixins, which are
syntactically more complex. Spock does it with a compiler plugin. I know
I can do it with strings and string mixins, but that's not exactly going
to be a clean API.
Another good use case for AST macros.
--
/Jacob Carlborg
