Safety/purity and assert/enforce error messages

[H. S. Teoh]

On Thu, Sep 12, 2013 at 04:07:24PM -0400, Jonathan M Davis wrote:
> On Thursday, September 12, 2013 12:42:34 H. S. Teoh wrote:
> > On Thu, Sep 12, 2013 at 09:12:18PM +0200, bearophile wrote:
> > > H. S. Teoh:
> > > >In phobos git HEAD, std.format has been made pure @safe nothrow
> > > >(and CTFE-able), so you should be able to write your assert as:
> > > >assert(condition, format("x = %s, blahblah", x));
> > > 
> > > With the latest DMD from updated GIT head:
> > > 
> > > 
> > > import std.string: format;
> > > void main() pure nothrow {
> > > 
> > > string x = "hello";
> > > bool condition = true;
> > > assert(condition, format("x = %s, blahblah", x));
> > > 
> > > }
> > > 
> > > 
> > > It gives:
> > > 
> > > test.d(5): Error: 'std.string.format!(char, string).format' is not nothrow
> > > test.d(2): Error: function 'D main' is nothrow yet may throw
> > 
> > [...]
> > 
> > Oops. Apparently I wrote nothrow but forgot to test it. I did test
> > pure and @safe, though, so at least those two should work.
> 
> format can't be nothrow, because it throws when you screw up the
> format specifiers. You have to wrap it in a try-catch block and
> assert(0) in the catch block if you want to put it in a nothrow
> function. std.datetime does this in at least a few places.
[...]

Right. Makes me wanna suggest adding this template to std.exception:

	auto assumeWontThrow(alias fun, T...)(T args) nothrow {
		try {
			return fun(args);
		} catch(Exception) {
			// You broke your promise, I die.
			assert(0);
		}
	}

Then you can write things like:

	int mayThrow(int arg) {
		if (arg < 0) throw new Exception(...);
		return BBN(arg);
	}

	int iWontThrow() nothrow { // <-- N.B. this function is nothrow
		static assert(123 > 0);
		return assumeWontThrow!mayThrow(123);
	}


T

-- 
It won't be covered in the book. The source code has to be useful for
something, after all. -- Larry Wall