On Thu, Sep 12, 2013 at 08:13:37PM +0200, Joseph Rushton Wakeling wrote:
> Hi all,
> 
> Suppose that I want to insert some variable values into an assert error message.  The easy way to do this is something like:
> 
>     assert(someCondition, text("x = ", x, ", cannot blah blah blah"));
> 
> However, the use of text() prevents me from applying @safe and other
> such attributes to the function containing this assert, even though
> text() will only be called in the event of an assertion failure.
> 
> Is there any reliable way to include variable values in the error message that doesn't interfere with @safe, pure, etc.?  Using to!string has a similar problem.
[...]

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));

Don't think it will work on 2.063.2, though, since it was a relatively recent change. I suspect this will only work starting from 2.064.


T

-- 
MASM = Mana Ada Sistem, Man!

On 12/09/13 20:49, H. S. Teoh wrote:
> 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));
>
> Don't think it will work on 2.063.2, though, since it was a relatively
> recent change. I suspect this will only work starting from 2.064.

Ahh, good to know, thanks.  I guess I'll withhold tweaking such things until after the next release ... :-)

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

Bye,
bearophile

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.


T

-- 
Food and laptops don't mix.

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.

- Jonathan M Davis

On 12/09/13 22:07, Jonathan M Davis wrote:
> 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.

The annoyance of this is that it means that any otherwise @safe pure nothrow function that has an assert() in it can be blocked from being nothrow if it needs a formatted string for the assert failure message.

Note that this restriction still applies even if the code is being compiled with -release and the assertion therefore stripped out.

Or am I missing a trick as to how to deal with assert() and enforce() ... ? :-)

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

On Thursday, September 12, 2013 22:17:30 Joseph Rushton Wakeling wrote:
> On 12/09/13 22:07, Jonathan M Davis wrote:
> > 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.
> 
> The annoyance of this is that it means that any otherwise @safe pure nothrow function that has an assert() in it can be blocked from being nothrow if it needs a formatted string for the assert failure message.
> 
> Note that this restriction still applies even if the code is being compiled with -release and the assertion therefore stripped out.
> 
> Or am I missing a trick as to how to deal with assert() and enforce() ... ?
> :-)

You can put the try-catch in a version(assert) block to get around that problem, but it is true that it's not exactly ideal. However, there really isn't any way around that with a function that takes a format string unless you want it to ignore bad arguments. You'd need a function like text which just appends all of its arguments together in order to get around that problem.

- Jonathan M Davis

On Thu, Sep 12, 2013 at 10:17:30PM +0200, Joseph Rushton Wakeling wrote:
> On 12/09/13 22:07, Jonathan M Davis wrote:
> >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.
> 
> The annoyance of this is that it means that any otherwise @safe pure nothrow function that has an assert() in it can be blocked from being nothrow if it needs a formatted string for the assert failure message.
> 
> Note that this restriction still applies even if the code is being compiled with -release and the assertion therefore stripped out.
> 
> Or am I missing a trick as to how to deal with assert() and enforce()
> ... ? :-)

Using assumeWontThrow, as defined in my previous post, you can write this:

	auto myFunc(int x) nothrow {
		// Look, ma! This line doesn't violate nothrow!
		assert(isValid(x), assumeWontThrow!format(
			"Invalid args (%s), dude!", x));

		... // do work here
	}

:-)


T

-- 
"Uhh, I'm still not here." -- KD, while "away" on ICQ.

On Thursday, September 12, 2013 13:21:26 H. S. Teoh wrote:
> 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);
> }

Sounds like a good suggestion, though an actual implementation should have an error message in the assert(0). You could also implement it as a lazy parameter like enforce and assertNotThrown do (which I think is more user- friendly), but that would likely be less efficient due to how lazy doesn't get optimized very well (or at least, it's my understanding that it doesn't).

- Jonathan M Davis