F# Pattern Matching

Pattern matching is logically similar to a switch statement in C#, Java, C++, etc; but itz much more powerful. A pattern match is a series of rules that will execute if the pattern matches the input. The pattern match expression then returns the result of the rule that was matched. So, all rules in a pattern match must return the same type.

In F#, you use match and with keywords with a series of pattern rules, followed by an arrow -> for pattern matching. Herez a sample F# pattern match code for Odd validation.

> let isOdd x = (x % 2 = 1) let describeNumber x = match isOdd x with | true -> printfn “number is odd” | false -> printfn “ number is even”;; > describeNumber 4;; number is even val it : unit = ()

Truth Table
Letz take AND truth table example using type inference on pattern matches. Herez the code:

> let testAND x y = match x, y with | true, true -> true | true, false -> false | false, true -> false | false, false -> true > testAND true false;; val it: bool = true

Match Failure
If no match is found during a pattern matching, an exception of type Microsoft.Fsharp.Core.MatchFailureException is raised. You can avoid this by making sure that all possible cases are coverd. Fortunately, F# compiler will issue a warning when it can determine that pattern match rules are incomplete.

> let testAND x y = match x, y with | true, true -> true // Commented | true, false -> false | false, true -> false | false, false -> true > testAND true false;; Microsoft.Fsharp.Core.MatchFailureExcption: The match cases were incomplete at:.... Stopped due to error

Pattern matching rules are checked in the order they are declared.