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. Here's 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

Let's take AND truth table example using type inference on pattern matches. Here's 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.

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)