F# Pattern Matching allows you to “compare data with a logical structure or structures, decompose data into constituent parts, or extract information from data in various ways”.
In other terms, it provides a more flexible and powerful way of testing data against a series of conditions and performing some computations based on the condition met.
Conceptually, it is like a series of if… then statements.
Syntax of F# Pattern Matching
In high-level terms, pattern matching follows this syntax in F# −
match expr with | pat1 - result1 | pat2 -> result2 | pat3 when expr2 -> result3 | _ -> defaultResult
Where,
- Each | symbol defines a condition.
- The -> symbol means “if the condition is true, return this value…”.
- The _ symbol provides the default pattern, meaning that it matches all other things like a wildcard.
Example 1
The following example calculates the Fibonacci numbers using pattern matching syntax −
let rec fib n = match n with | 0 -> 0 | 1 -> 1 | _ -> fib (n - 1) + fib (n - 2) for i = 1 to 10 do printfn "Fibonacci %d: %d" i (fib i)
When you compile and execute the program, it yields the following output −
Fibonacci 1: 1 Fibonacci 2: 1 Fibonacci 3: 2 Fibonacci 4: 3 Fibonacci 5: 5 Fibonacci 6: 8 Fibonacci 7: 13 Fibonacci 8: 21 Fibonacci 9: 34 Fibonacci 10: 55
You can also chain together multiple conditions, which return the same value. For example −
Example 2
let printSeason month = match month with | "December" | "January" | "February" -> printfn "Winter" | "March" | "April" -> printfn "Spring" | "May" | "June" -> printfn "Summer" | "July" | "August" -> printfn "Rainy" | "September" | "October" | "November" -> printfn "Autumn" | _ -> printfn "Season depends on month!" printSeason "February" printSeason "April" printSeason "November" printSeason "July"
When you compile and execute the program, it yields the following output −
Winter Spring Autumn Rainy
Pattern Matching Functions
F# allows you to write pattern matching functions using the function keyword −
let getRate = function | "potato" -> 10.00 | "brinjal" -> 20.50 | "cauliflower" -> 21.00 | "cabbage" -> 8.75 | "carrot" -> 15.00 | _ -> nan (* nan is a special value meaning "not a number" *) printfn "%g"(getRate "potato") printfn "%g"(getRate "brinjal") printfn "%g"(getRate "cauliflower") printfn "%g"(getRate "cabbage") printfn "%g"(getRate "carrot")
When you compile and execute the program, it yields the following output −
10 20.5 21 8.75 15
Adding Filters or Guards to Patterns
You can add filters, or guards, to patterns using the when keyword.
Example 1
let sign = function | 0 -> 0 | x when x < 0 -> -1 | x when x > 0 -> 1 printfn "%d" (sign -20) printfn "%d" (sign 20) printfn "%d" (sign 0)
When you compile and execute the program, it yields the following output −
-1 1 0
Example 2
let compareInt x = match x with | (var1, var2) when var1 > var2 -> printfn "%d is greater than %d" var1 var2 | (var1, var2) when var1 < var2 -> printfn "%d is less than %d" var1 var2 | (var1, var2) -> printfn "%d equals %d" var1 var2 compareInt (11,25) compareInt (72, 10) compareInt (0, 0)
When you compile and execute the program, it yields the following output −
11 is less than 25 72 is greater than 10 0 equals 0
Pattern Matching with Tuples
The following example demonstrates the pattern matching with tuples −
let greeting (name, subject) =
match (name, subject) with
| ("Zara", _) -> "Hello, Zara"
| (name, "English") -> "Hello, " + name + " from the department of English"
| (name, _) when subject.StartsWith("Comp") -> "Hello, " + name + " from the department of Computer Sc."
| (_, "Accounts and Finance") -> "Welcome to the department of Accounts and Finance!"
| _ -> "You are not registered into the system"
printfn "%s" (greeting ("Zara", "English"))
printfn "%s" (greeting ("Raman", "Computer Science"))
printfn "%s" (greeting ("Ravi", "Mathematics"))
When you compile and execute the program, it yields the following output −
Hello, Zara Hello, Raman from the department of Computer Sc. You are not registered into the system
Pattern Matching with Records
The following example demonstrates pattern matching with records −
type Point = { x: float; y: float }
let evaluatePoint (point: Point) =
match point with
| { x = 0.0; y = 0.0 } -> printfn "Point is at the origin."
| { x = xVal; y = 0.0 } -> printfn "Point is on the x-axis. Value is %f." xVal
| { x = 0.0; y = yVal } -> printfn "Point is on the y-axis. Value is %f." yVal
| { x = xVal; y = yVal } -> printfn "Point is at (%f, %f)." xVal yVal
evaluatePoint { x = 0.0; y = 0.0 }
evaluatePoint { x = 10.0; y = 0.0 }
evaluatePoint { x = 0.0; y = 10.0 }
evaluatePoint { x = 10.0; y = 10.0 }
When you compile and execute the program, it yields the following output −
Point is at the origin. Point is on the x-axis. Value is 10.000000. Point is on the y-axis. Value is 10.000000. Point is at (10.000000, 10.000000).
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