Feature/vector physics

[g-belmonte]

Pull request related to #127

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There are still no tests and documentation. I wanted to check first if I'm going in the right direction :)

[ianmackenzie]

I think for consistency with Vector3d.meters etc., the functions should probably have signatures like

Vector3d.metersPerSecond : Float -> Float -> Float -> Vector3d MetersPerSecond coordinates

I agree that the length-and-direction method is also useful, but being able to write

Vector3d.metersPerSecond 5 Direction3d.z

instead of

Vector3d.withLength (Speed.metersPerSecond 5) Direction3d.z

is not quite as much of a savings as being able to write

Vector3d.metersPerSecond 3 0 4

instead of what you'd currently have to do to get that:

Vector3d.xyz
    (Speed.metersPerSecond 3)
    (Speed.metersPerSecond 0)
    (Speed.metersPerSecond 4)

The three-argument form is also useful for JSON decoding, e.g.

Decode.map3 Vector3d.metersPerSecond
    (Decode.field "x" Decode.float)
    (Decode.field "y" Decode.float)
    (Decode.field "z" Decode.float)

[ianmackenzie]

I'm also not sure if it makes sense to talk about a pressure vector...I think pressure is pretty fundamentally a scalar quantity. Was there a particular application you were considering?

I think area vectors will come up in practice, but also I think those will usually end up being computed with code like

triangleAreaVector =
    Triangle3d.normalDirection triangle
        |> Maybe.map (Vector3d.withLength (Triangle3d.area triangle))
        |> Maybe.withDefault Vector3d.zero

instead of being constructed explicitly from their coordinates, so I don't think it would make sense to add Vector3d.squareMeters or anything like that.

[g-belmonte]

Now I gotcha! I'll do some changes 😁

As for the pressure, yes, you're right. Pressure is always applied perpendicular to the surface. I'll remove those :)

Am I missing any other vector unit?

[g-belmonte]

Changes done :)

About those area vectors, Vector3d.squareMeters sounds a bit weird to me, since area is an scalar 😅
But using the normal of a surface is pretty common.
Wdyt, should I implement it too?

[ianmackenzie]

I'd leave out area for now. Area vectors do come up and are useful mathematically - for example, if you multiply the area vector of a triangle (as computed above) by a scalar pressure value, then the product gives you a force value which is equal to the force vector exerted on that triangle by the given pressure! But again, I don't really see a use case for directly constructing area vectors from their XYZ components, so I would say leave them out.

I think the code is looking great now! I can't think of any other units to add (I did a general review of the units type in elm-units and nothing jumped out at me - and I think speed, acceleration and force will certainly be the main ones when working with elm-physics).

I'm happy to write docs if you'd like, but if you do want to take a crack then I think the best approach would be to have a single documentation paragraph at the top of the 'Physics' section in each of the two modules that describes the general idea and has one or two examples - I certainly don't think you need a separate description/example for each function, they're pretty self-explanatory.

[g-belmonte]

I'm happy to be of assistance! 😀

I was actually going to ask you for help with the docs. I've been playing around with elm-geometry and elm-units for a pair of weeks, so I can't really think of any meaningful examples or describing a good use case at the moment 😅
Could you take care of the docs for me then?

Also, wdyt about testing? Do you prefer to have everything tested or just a few key things? If the latter, then do you see any valuable tests I could write?

[ianmackenzie]

OK just added some simple docs - see what you think!

Honestly not sure what tests would be really meaningful for these - I took a look through to check for any typos and it all looked good, so if you're happy with this then I think it's ready to merge.

[g-belmonte]

The docs looks good :)
I also couldn't think of any meaningful test, so I think it is good to go!