Add multiply-by-positive-constant primitive hmulpos.

[cheater]

Following your tips I have rewritten the "concatenation" function a little and it seems I have found a simple primitive that should provide stronger monotonicity.

varsAsVect xs = vOut
  -- | Take a list of scalar variables and output them as a single vector Expr
  where
    vOut = foldl' (+~) (head vs) (tail vs)
    xsIndices = zip ([0..] :: [Int]) xs
    vs = f <$> xsIndices
    f (ind, x) = v *~+ x
      where
        v = EConst $ orthonormal ind

    orthonormal i = assoc (n, 1) 0.0 [((i, 0), 1.0)]
    n = length xs

here *~+ is exactly the same as *~ except you can only have non-negative constants on the left, or it will throw an exception. Is my understanding correct that this requirement on the left argument will preserve monotonicity of the right argument?

[chrisnc]

I'm still skeptical about adding more exports. Also it seems unrelated to the other changes you're making.

[chrisnc]

Can you collapse this to just (*~+) = hmulpos? While you're here might as well to the same for (*~). I did that for the other operator aliases but I guess I missed one.

[cheater]

Sure, I just did it that way because you had (*~) and hmul.

[chrisnc]

"left-positive" seems redundant. You already mentioned you're referring to the left argument. Same for the next line.

[cheater]

Ok so the issue i'm trying to solve here is that when a non-positive entry is entered, we see an error like this:

> solve 1
([("x1",(1><1)
 [ 10.0 ]),("x2",(1><1)
 [ 200.0 ]),("x3",(1><1)
 [ 400.0 ]),("lineVar",(1><1)
 [ 10.0 ])],*** Exception: The left argument of a left-positive multiply must have non-negative entries
CallStack (from HasCallStack):
  error, called at src/HVX/Primitives.hs:78:17 in hvx-0.3.1.0-HnMwpnJtEwx2GYI4IYHzYJ:HVX.Primitives

so we need to identify the function throwing the error, otherwise the user won't know why the error is happening and will be unable to debug it. We also identify the reason the function is throwing the error, so that the user doesn't have to look at the hmulpos source code to figure that out.

How would you do both without being redundant but maintaining a clear description of what went wrong and where?

[chrisnc]

Sorry, I just meant that you could change left-positive to positive. This still distinguishes it from the generic multiply-by-constant function.

[cheater]

I see! I'm worried "positive multiply" might imply that the right argument has to be positive too.

[chrisnc]

But the message as a whole doesn't imply that. I think it's fine.

[cheater]

I'm confused now :) ok can we just leave it as is? I think no one will find it offensive and we'll have ticked off this pr :)

[chrisnc]

Still change it from "left-positive" to "positive". After that I think it's unambiguous.

[cheater]

Sure, i just pushed