Concept of Pattern Matching in FSharp

Introduction:

Pattern Matching is one of the most important tools of F Sharp programming. It allows a programmer to make different computations, depending on matching patterns. It decomposes data structures into their component parts. It is similar to a if..else or switch statement in C or C++. It allows the programmer to test a value against a series of conditions and extract information depending on which conditions match.

Syntax for Pattern Matching

match exp with
| pattern1 -> result
| pattern2 -> result
|------------------
|------------------
| _ -> default

Here match and with are keyword. Each rule of match is represented by | followed by arrow (->). Like | pattern1 -> result meaning - if pattern is matched (condition is true) then return result. ( _ ) denotes the default pattern.

Different Ways to Form Patterns:-

Constant Pattern:

Input is compared with Constant pattern like, numeric, character, string constants. If matched, then the corresponding value is returned. Note here, the type of literal must be same type of input. We write a simple program for constant matching.

//Constant pattern
let cons n =
  match n with
 
  | 1 -> "One"
  | 2 -> "Two"
  | 3 -> "Three"
  | 4 -> "Four"
  | 5 -> "Five"

for i in 1..5 do
printfn "%s" (cons i)

Output:



Variable Pattern:

The variable pattern assigns the matching value to a variable which is used as result.

let vari x =
    match x with
    | (fisrt, second) when first > second -> "first number is greater"
    | (first, second) when first < second -> "second number is greater"
    | (first, second) -> "both are equal"
 
printfn "%s" ( vari( 1 , 2) )

Output:



OR Pattern:

The Or pattern works the same as the logical OR operator in C, C++. It is used when the input data has multiple pattern and we want the same result for all. It is denoted with the | operator between matching pattern.

AND Pattern:

The AND pattern works the same as logical AND operator. It is used when input has to match with more than one pattern.

List Pattern:

The list pattern decomposes the list into the number of elements. The list pattern matches with only the specific number of elements.

let  listpattrn n =
    match n with

    | [] -> 0
    | [_] -> 1
    | [_; _] -> 2
    | [_; _; _] -> 3

printfn " Number of element is %d" (listpattrn [ ])
printfn " Number of element is %d" (listpattrn [ 1 ])
printfn " Number of element is %d" (listpattrn [ 1; 1 ])
printfn " Number of element is %d" (listpattrn [ 1; 1; 1; ])

Output:

Array pattern:

The array pattern is the same as the list pattern.

let arrpattrn n =
 match n with

    | [| |] -> 0
    | [|_|] -> 1
    | [|_; _|] -> 2
    | [|_; _; _|] -> 3
 
printfn " Number of Array element is %d" (arrpattrn [| |])
printfn " Number of Array element is %d" (arrpattrn [| 1 |])
printfn " Number of Array element is %d" (arrpattrn [| 1; 1 |])
printfn " Number of Array element is %d" (arrpattrn [| 1; 1; 1; |])

Output: