Pattern match cases I've covered until this point can be composed together in a manner that resembles the terms of a Boolean expression with OR (|) and AND (&) operators. Let me demonstrate this technique by implementing a function that accepts two string arguments that represent keys and validates that both the given values are non-empty, providing a detailed diagnostics.

You should be able to grasp at this point why I should begin the matching with the most specific case when both the keys are empty. The next less specific match is represented by two symmetric cases when either the first or the second key is empty. Here, in order to demonstrate the flexibility provided by F# patterns grouping, I combine these two patterns with Boolean OR and at the same time capture key values into the local context with a variable pattern represented by the tuple (x,y). For the most generic leftover case, I know that both keys are not empty, so just a variable pattern is sufficient here. The sought function definition is as follows (Ch4_4.fsx):

open System 
 
let validate keyA keyB = 
  match (keyA,keyB) with 
  | ("","") -> "both keys are empty" 
  | (x,y) & (("",_) | (_,"")) -> 
    sprintf "one key is empty: keyA = %s; keyB = %s" x y 
  | _ & (x,y) -> 
    sprintf "both keys aren't empty: keyA = %s; keyB = %s" x y 

Although the boolean OR pattern combinator helps reach F# code succinctness by combining some cases that require the same transformation expression, boolean AND is not used that frequently for combining the pattern cases in regular pattern matching practice. However, it gets very relevant when grouping active patterns, which I will be covering in later chapters.