Wildcard patterns

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.

For example,

f(_) = 10

defines the function which always returns 10, no matter what input it is given. This could also be written

f(n) = 10

but since n is not used, we can explicitly indicate that we do not care about it by replacing it with a wildcard pattern _.