Pattern matching in a let expression

How do you extract a value from a variable of an unknown constructor?

For instance, I would like to negate the value in an Either if was constructed as a Right:

let Right x = getValue
in Right (negate x)

This code successfully binds Right's value (an Int in this case) to x.

This works, but what if getValue returns a Left instead? Is there a way to determine the type of a variable in a let expression? Or is there a better way to approach this problem?