Optional¶

Represents a pattern with an optional default value.

Optional[p, default] is the explicit form of p : default:

$ wo 'f[a] /. f[x_, Optional[y_, 2]] -> {x, y}'
{a, 2}

The arguments that are present fill the slots from the left, so the optionals falling back on their defaults are the rightmost ones:

$ wo 'g[1] /. g[x_ : 0, y_ : 0] -> {x, y}'
{1, 0}