Chapter 18: Patterns and matching

Patterns come in two forms: refutable and irrefutable. Patterns that will match for any possible value passed are irrefutable. An example would be x in the statement let x = 5; because x matches anything and therefore cannot fail to match. Patterns that can fail to match for some possible value are refutable. An example would be Some(x) in the expression if let Some(x) = a_value because if the value in the a_value variable is None rather than Some, the Some(x) pattern will not match.

Function parameters, let statements, and for loops can only accept irrefutable patterns, because the program cannot do anything meaningful when values don’t match. The if let and while let expressions only accept refutable patterns, because by definition they’re intended to handle possible failure: the functionality of a conditional is in its ability to perform differently depending on success or failure.