Thanks a lot for your quick response, I'm glad somebody actually read through the whole thing and that I got some feedback. I'll try and do my best to answer your questions.<br><br><div class="gmail_quote">On 28 March 2011 19:15, spir <span dir="ltr"><<a href="mailto:denis.spir@gmail.com">denis.spir@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div class="im">On 03/28/2011 05:43 PM, Cristi Cobzarenco wrote:<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Thus, the requirements for the unit system would be:<br>
1. One line definition of new units.<br>
2. Simple, yet safe and explicit conversion between units.<br>
3. Zero runtime overhead.<br>
4. Minimal extra coding effort to use units.<br>
</blockquote>
<br></div>
I like your study very much. Measurement units are, for me, kinds of conceptual types --as opposed to primitive types. I think they would be welcome, possibly for Phobos.<br>
<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
[...]<div class="im"><br>
I propose using types for base units and mixins to define derived units.<br>
One can use the typenames of the units in arithmetic operations this way:<br>
<br>
struct metre {}<br>
struct second {}<br>
<br>
void f() {<br>
Quantity!("metre") dist1 = quantity!(3.0, "metre");<br>
Quantity!("metre^2") area = dist1 * dist1;<br>
Quantity!("metre/second") speed = dist1 / quantity!(2.0, "second");<br>
}<br>
</div></blockquote>
<br>
I think (not sure) this could be even simpler:<br>
1. Thank to D's very used auto feature, avoiding double specification of the type; you could get rif of quantity, if it's only a convenience factory func.<br>
2. I guess it's not needed for the user to give a quantity type to /constants/ (here 2.0). It is /meant/ to be a a number of seconds. This, as opposed to measure /variables/.<br>
Thus, we possibly could have:<br></blockquote><div><br></div><div>I know about the 'auto' feature, and you're right, it would make more sense use it in this situation, I just wanted to make the type of the variables clearer. If one wants to have a data member with a measurement unit, one needs to know the type of the object. But, you're right, that is definitely how you would do it.</div>
<div>Regarding, the factory function, I was thinking that for convenience (not write two pairs of parenthesis) we could have the function. Maybe using the constructor is a better idea, we could provide both.</div><div>The system should definitely be able to figure out the type of dist1 / Quantity!("second")(2.0) or dist1*dist1 automatically, you're right.</div>
<div>About constants. How would the system know that 2.0 is supposed to be seconds? Imagine the following situation:</div><div><br></div><div>auto distance = Quantity!("metre")(3.0);</div><div>auto mass = Quantity!("kilogram")(2.0);</div>
<div>auto speed = Quantity!("metres/second")(mass/2.0); // oops, the programmer meant distance/2.0</div><div><br></div><div>Now the system would automatically infer that the unit for 2.0 is "metre/(second*kilogram)" to get metres/second and the whole point of units would be defeated. My idea is to only allow adimensional initializers (floats, doubles etc.) for the quantites, and specify the unit for the constants. So there would be no need to do Quantity!("meters/second"). One could simply say:</div>
<div><br></div><div>auto speed = distance / Quantity!("second")(2.0);</div><div><br></div><div>One final note about 'auto'. If a programmer abuses auto, then the part of the reason to have units is lost. In the above example, if one simply used:</div>
<div><br></div><div>auto speed = mass / Quantity!("second")(2.0);</div><div><br></div><div>Then this would not fail here (it might fail later, or not at all), since there is no unit checking performed. I think that ideally one should avoid using auto when initializing from a result of an operation between two other variables.</div>
<div><br></div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<br>
auto dist1 = Quantity!("meter")(3.0);<br>
auto area = Quantity!("metre^2")(dist1 * dist1);<br>
auto speed = Quantity!("metre/second")(dist1 / 2.0);<br>
<br>
(Unsure whether your system could automagically infer the unit of area. ?)<br>
<br>
I'm not fan of using strings as unit identifiers. Why don't you want to use use the corresponding types (structs or classes) themselves?<br>
<br>
auto dist1 = Quantity!Meter(3.0);<br>
<br>
Thus requiring each unit to be identified by a (possibly empty) struct or class. This also gives the opportunity for the user to actually implement some useful stuff in there; or to *reuse* existing types for things-to-be-counted (in the latter case, measures would actually be integer /counts/); see your Widget vs Gadget example below).<div class="im">
<br></div></blockquote><div><br></div><div>I agree that using strings as unit identifiers is a bad idea. I'm not sure if I was clear when I said I wanted to use mixins for the units. The strings would be converted (using string mixins) to actual type names, therefore you wouldn't be able to use any string for a unit - it has to be the name of a type. The reason to use strings is to be able to write derived units easily. "Quantity!(Meter/Second)(3.0)" is not valid D if Meter and Second are types, right? So, if you wanted to use the type names you would have to do "Quantity!( UnitDivison!(Meter,Second) )(3.0)". An idea would be to allow both. When there is a single type involved, you could write Quantity!Meter(3), but when one wants to get a derived unit, one could use a string "Meter*Second^-1" - this would behind the scenes call a function like UnitDivision, of course. What do you think?</div>
<div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div class="im">
<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Conversion between units can be done specifying a single factor with a<br>
proper unit:<br>
<br>
template conversion( alias unit : "kilometer/meter" ) {<br>
immutable Quantity!(unit) conversion = quantity!(123.0,unit);<br>
}<br>
</blockquote>
<br></div>
Unsure of the right approch here. All we need is a registered conversion factor for every needed (U1,U2) pair. It could be a kind of 2D associative array with unit keys and float values. Then, would it be possible to have a single convert template, like:<br>
<br>
Quantity!U2 convert (string U1, string U2) (Quantity!U1 measure) {<br>
auto factor = conversionFactors[U1][U2]; // throws if undefined<br>
return Quantity!U2(measure * factor);<div class="im"><br>
}<br>
<br>
?<br>
<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
void f() {<br>
Quantity!("metre") d1 = quantity!(123.0,"metre");<br>
// convert calls conversion! with the right argument<br>
Quantity!("kilometre") d2 = convert!(d1,"kilometre");<br>
}<br>
</blockquote>
<br></div>
auto d1 = Quantity!("metre")(123.0);<br>
auto d2 = convert!(d1,"kilometre");<br>
or using my approach:<br>
auto d2 = convert!("metre","kilometre")(d1);<br>
<br>
Here the type for d2 is really unnecessary.</blockquote><div><br></div><div><div>This would not be static any more, since the lookup would have to be done at runtime, right? If the conversion template function is used, then the code simply gets replaced by the Quantity!(measure*factor,"kilometre") at compile time.</div>
<div>There is another argument for using the conversion template function. One could only define conversion for single units (e.g. kilometres->metres, minutes->seconds), and then conversion should be able to automagically determine the conversion between "kilometres/minute" to "meters/second", the factor being a multiplication of two other factors.. Again, all of this at compile time. Does it make sense? Or am I missing something?</div>
</div><div><br></div><div> </div><blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><div class="im"><br>
<br>
<blockquote class="gmail_quote" style="margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
Also, notice this approach imposes no restriction to the types that define<br>
units, therefore our Widget/Gadget counters could be defined without any<br>
extra work:<br>
<br>
class Widget { /* complicated class definition */ }<br>
class Gadget { /* complicated class definition */ }<br>
<br>
Quantity!("Widget",int) nWidgets;<br>
Quantity!("Gadget",int) nGadgets;<br>
</blockquote>
<br></div>
See note about identifying units above.<br>
<br>
Denis<br><font color="#888888">
-- <br>
_________________<br>
vita es estrany<br>
<a href="http://spir.wikidot.com" target="_blank">spir.wikidot.com</a><br>
<br>
</font></blockquote></div><div><br></div>Thanks again and I'm looking forward to hearing back from you.<br><br clear="all"><br>-- <br>(Cristi Cobzarenco)<br>Pofile: <a href="http://www.google.com/profiles/cristi.cobzarenco">http://www.google.com/profiles/cristi.cobzarenco</a><br>