Could support the use of function factories like `scales::pal_hue()` and `scales::pal_viridis()`? about ggplot2 HOT 1 OPEN

This has been on my mind as well to make the interface between scales and palettes somewhat smoother.
Your first example can be replaced with discrete_scale("colour", palette = scales::pal_hue()), but this is not so much the case for the continuous examples.

For this to happen, I think {scales} needs to start describing palette functions more so that some properties are easily queried.
Useful properties to know about palettes are:

• Is it continuous, i.e. does it accept values in the (0, 1) range.
• Is it discrete, i.e. does it accept n values.
• If discrete, what is the maximum n it will accept?
• Do they have built-in NA handlers (e.g. via scales::colour_ramp())?

If we know this, we can automatically expand discrete palettes to continuous ones by using e.g. colour_ramp(discrete_pal()(n_max)), or discretise continuous palettes by using continuous_pal()(seq(0, 1, length.out = n).

from ggplot2.