I have written a library that allows for all of the following as native syntax:

A + B - D = C;
A - B - D = C;
A - D - B = C;
(B - A) - D = C;
A - D - B - D = C;
A - B - (D + D) = C;
C = A + B - D;
C = B * A;
C = B * A + B * D;
A / B  + D / B = C;

In all of these cases the assignment is to B.

The library is works by using term re-writing to reverse evaluate the expression. The fun part is that the term re-writing is done with the type system and template expressions. This results in there being little inherent runtime cost.
I hope to expand it to work with as wide a variety of equation forma as I can. I wold love to be able to get it to even solve system of equations like

a*B + c*D = e;
f*B + g*D = h;

where B and D are both unknown.

Now, full disclosure:
-it's incomplete, there a lots of cases it won't handle (but I plan on improving that)
-it is not thread safe, this is because the templates need alias to variables to work correctly and the only variables that can be aliased are globals.*, #
-It's unproven, I can formally prove all the "single sided" rules, but haven't figured out how to prove the "double sided" rules, yet like the last 2 examples, %
-No, I didn't bang out 400 lines of static if's, the "business logic" is generated using a lisp program that is harder to read than what it produces ^

Now for the challenge. Is anyone willing to try and generate a C++ equivalent? I think it would be really cool to have a side by side comparison of the two libs. I think the difference in readability say a lot.

hosted at:
  http://www.dsource.org/projects/scrapple/browser/trunk/backmath

* Walter can we please get this fixed??
# I have a way in mind to fix this but it won't look near as good.
% I'm using ACL2 to do the proofs. http://www.cs.utexas.edu/users/moore/acl2/
^ I'll clean it up and post it as well soon. (finals week is coming soon so don't hold your breath or anything)

"BCS" <ao@pathlink.com> wrote in message news:ce0a3343268928ca04931959c536@news.digitalmars.com...

> the only variables that can be aliased are globals.

Template alias parameters can be locals.

BCS wrote:
> I have written a library that allows for all of the following as native syntax:
> 
> A + B - D = C;
> A - B - D = C;
> A - D - B = C;
> (B - A) - D = C;
> A - D - B - D = C;
> A - B - (D + D) = C;
> C = A + B - D;
> C = B * A;
> C = B * A + B * D;
> A / B  + D / B = C;
> 
> In all of these cases the assignment is to B.
> 
> The library is works by using term re-writing to reverse evaluate the expression. The fun part is that the term re-writing is done with the type system and template expressions. This results in there being little inherent runtime cost.
> I hope to expand it to work with as wide a variety of equation forma as I can. I wold love to be able to get it to even solve system of equations like
> 
> a*B + c*D = e;
> f*B + g*D = h;
> 
> where B and D are both unknown.
> 
> Now, full disclosure:
> -it's incomplete, there a lots of cases it won't handle (but I plan on improving that)
> -it is not thread safe, this is because the templates need alias to variables to work correctly and the only variables that can be aliased are globals.*, #
> -It's unproven, I can formally prove all the "single sided" rules, but haven't figured out how to prove the "double sided" rules, yet like the last 2 examples, %
> -No, I didn't bang out 400 lines of static if's, the "business logic" is generated using a lisp program that is harder to read than what it produces ^
> 
> Now for the challenge. Is anyone willing to try and generate a C++ equivalent? I think it would be really cool to have a side by side comparison of the two libs. I think the difference in readability say a lot.
> 
> hosted at:
>   http://www.dsource.org/projects/scrapple/browser/trunk/backmath
> 
> * Walter can we please get this fixed??
> # I have a way in mind to fix this but it won't look near as good.
> % I'm using ACL2 to do the proofs. http://www.cs.utexas.edu/users/moore/acl2/
> ^ I'll clean it up and post it as well soon. (finals week is coming soon so don't hold your breath or anything)
> 
> 

Where was this lib when I was doing Algebra I in middle school :-)?

Very cool stuff; I'm always amazed at what D can do with metaprogramming. FWIW, I think local variables can be template alias parameters, I'm using just that fact to get the names of the functions a particular template is instantiated in. I'm not sure if this is what you meant, though.

Reply to Robert,

> Where was this lib when I was doing Algebra I in middle school :-)?
> 
> Very cool stuff; I'm always amazed at what D can do with
> metaprogramming.

thanks :)

> FWIW, I think local variables can be template alias
> parameters, I'm using just that fact to get the names of the functions
> a particular template is instantiated in. I'm not sure if this is what
> you meant, though.
> 


what I need is something that would work like this:

struct S(alias r)
{
  real Get(){return r;}
}

void Do()
{
  real a;
  S!(a) s;
  real b = s.Get();
}

the issue is that the struct doesn't have access to the function. There is no reason this should be impossible, but the semantics of how to make it work would be a major project in and of it's self.

If anyone can find a workaround I'm open to suggestions.

"BCS" <ao@pathlink.com> wrote in message news:ce0a33432689c8ca049e1da7fc2e@news.digitalmars.com...
>
> what I need is something that would work like this:
>
> struct S(alias r)
> {
>   real Get(){return r;}
> }
>
> void Do()
> {
>   real a;
>   S!(a) s;
>   real b = s.Get();
> }
>
> the issue is that the struct doesn't have access to the function. There is no reason this should be impossible, but the semantics of how to make it work would be a major project in and of it's self.
>
> If anyone can find a workaround I'm open to suggestions.

Ooh, I see.  Actually I think templated _functions_ with alias parameters can be called with aliases to local variables -- in which case, IIRC, the templated function will actually be instantiated as a nested function within that function, giving it access to the calling function's stack frame.  But you're right, this is incredibly difficult to do in the general case.