A wildcard pattern is just an underscore character, indicating that we do not care about a particular input. Just like a variable pattern, it successfully matches any input; unlike a variable pattern, it does not define a new name.
f(_) = 10
defines the function which always returns
10, no matter what input
it is given. This could also be written
f(n) = 10
n is not used, we can explicitly indicate that we do not
care about it by replacing it with a wildcard pattern