Function interface for extracting properties from models

[glwagner]

An important missing component of our user interface is a function-based API for extracting properties from models. For example, we often write

u, v, w = model.velocities

when we really should write something like

u, v, w = velocity_field(model)

A major advantage of a function-based interface is that it is more robust to changes to the underlying model implementation. For example, we will eventually need a VectorField implementation for velocities (eg for the cubed sphere, or non-orthogonal grids). We'll be sorry that we've written u, v, w = model.velocities everywhere when we need to change model.velocities to model.velocity_field. But with a function-based interface we only have to change velocity_field(model) = model.velocities to velocity_field(model) = model.velocity_field.

A few other functions besides velocity_field that we might want are

• tracers(model)
• free_surface_displacement(model)
• kinematic_pressure(model)
• buoyancy(model)

viscosity(model) and diffusivities(model) might be nice too. One challenge there is figuring out what to do when the viscosity or diffusivities are really tensors rather than scalars (eg we need VectorField but we also may need TensorField...)

[francispoulin]

Great idea, but I admit I have no idea how much work this is going to be.

One minor comment is that if we don't have the word field for the 4 other fields, maybe we would just want velocity instead of velocity_field?

[glwagner]

It's very little coding work! Most of the work is designing the interface.

That's true we don't use tracer_fields...

[glwagner]

I'm closing this issue because I'm judging that it's not of current, timely relevance to Oceananigans development. If you would like to make it a higher priority or if you think the issue was closed in error please feel free to re-open.