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Match expressions
Learn how the F# match expression provides branching control that is based on the comparison of an expression with a set of patterns.
04/19/2018

Match expressions

The match expression provides branching control that is based on the comparison of an expression with a set of patterns.

Syntax

// Match expression.
match test-expression with
| pattern1 [ when condition ] -> result-expression1
| pattern2 [ when condition ] -> result-expression2
| ...

// Pattern matching function.
function
| pattern1 [ when condition ] -> result-expression1
| pattern2 [ when condition ] -> result-expression2
| ...

Remarks

The pattern matching expressions allow for complex branching based on the comparison of a test expression with a set of patterns. In the match expression, the test-expression is compared with each pattern in turn, and when a match is found, the corresponding result-expression is evaluated and the resulting value is returned as the value of the match expression.

The pattern matching function shown in the previous syntax is a lambda expression in which pattern matching is performed immediately on the argument. The pattern matching function shown in the previous syntax is equivalent to the following.

fun arg ->
    match arg with
    | pattern1 [ when condition ] -> result-expression1
    | pattern2 [ when condition ] -> result-expression2
    | ...

For more information about lambda expressions, see Lambda Expressions: The fun Keyword.

The whole set of patterns should cover all the possible matches of the input variable. Frequently, you use the wildcard pattern (_) as the last pattern to match any previously unmatched input values.

The following code illustrates some of the ways in which the match expression is used. For a reference and examples of all the possible patterns that can be used, see Pattern Matching.

[!code-fsharpMain]

Guards on patterns

You can use a when clause to specify an additional condition that the variable must satisfy to match a pattern. Such a clause is referred to as a guard. The expression following the when keyword is not evaluated unless a match is made to the pattern associated with that guard.

The following example illustrates the use of a guard to specify a numeric range for a variable pattern. Note that multiple conditions are combined by using Boolean operators.

[!code-fsharpMain]

Note that because values other than literals cannot be used in the pattern, you must use a when clause if you have to compare some part of the input against a value. This is shown in the following code:

[!code-fsharpMain]

Note that when a union pattern is covered by a guard, the guard applies to all of the patterns, not just the last one. For example, given the following code, the guard when a > 41 applies to both A a and B a:

type Union =
    | A of int
    | B of int

let foo() =
    let test = A 42
    match test with
    | A a
    | B a when a > 41 -> a // the guard applies to both patterns
    | _ -> 1

foo() // returns 42

See also