Lambda Expressions: The fun Keyword (F#)

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

fun parameter-list -> expression


The parameter-list consists of names and, optionally, types of parameters. Examples of valid parameter lists include the following:

fun a b c -> ...
fun (a: int) b c -> ...
fun (a : int) (b : string) (c:float) -> ...

The parameter-list can be composed of any F# patterns. For a full list of possible patterns, see Pattern Matching (F#).

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 adds 1 to every element of a list.

let list = (fun i -> i + 1) [1;2;3]
printfn "%A" list

See Also


Functions (F#)