Lambda Expressions: The fun Keyword (F#)

The fun keyword is used to define a lambda expression, that is, an anonymous function.

Syntax

fun parameter-list -> expression

Or using the _.Property shorthand notation:

_.

fun, parameter-list, and lambda arrow (->) are omitted, and the _. is a part of expression where _ replaces the parameter symbol.

The following snippets are equivalent:

(fun x -> x.Property) _.Property

Remarks

The parameter-list typically consists of names and, optionally, types of parameters. More generally, the parameter-list can be composed of any F# patterns. For a full list of possible patterns, see Pattern Matching. Lists of valid parameters include the following examples.

// Lambda expressions with parameter lists.
fun a b c -> ...
fun (a: int) b c -> ...
fun (a : int) (b : string) (c:float) -> ...

// A lambda expression with a tuple pattern.
fun (a, b) -> …

// A lambda expression with a cons pattern.
// (note that this will generate an incomplete pattern match warning)
fun (head :: tail) -> …

// A lambda expression with a list pattern.
// (note that this will generate an incomplete pattern match warning)
fun [_; rest] -> …

The expression is the body of the function, the last expression of which generates a return value. Examples of valid lambda expressions include the following:

fun x -> x + 1
fun a b c -> printfn "%A %A %A" a b c
fun (a: int) (b: int) (c: int) -> a + b * c
fun x y -> let swap (a, b) = (b, a) in swap (x, y)

Using Lambda Expressions

Lambda expressions are especially useful when you want to perform operations on a list or other collection and want to avoid the extra work of defining a function. Many F# library functions take function values as arguments, and it can be especially convenient to use a lambda expression in those cases. The following code applies a lambda expression to elements of a list. In this case, the anonymous function checks if an element is text that ends with specified characters.

let fullNotation = [ "a"; "ab"; "abc" ] |> List.find ( fun text -> text.EndsWith("c") )
printfn "%A" fullNotation         // Output: "abc"

let shorthandNotation = [ "a"; "ab"; "abc" ] |> List.find ( _.EndsWith("b") )
printfn "%A" shorthandNotation    // Output: "ab"

The previous code snippet shows both notations: using the fun keyword, and the shorthand _.Property notation.

See also

• Functions