Anonymous functions in F#

Anonymous functions in F# are used to build functions that are meant to be used within a statement. They have no names so other parts of the application can’t call them.

The following named function…:

let increment x = x + 1

…has the following anonymous counterpart:

fun x -> x + 1

The input parameter x is followed by the -> operator as opposed to the ‘=’ function assignment operator. Here’s how this declaration can be used within another statement:

printfn "%i" ((fun x -> x + 1) 5)

The anonymous function is followed by the input parameter 5 which will take the place of ‘x’.

We can declare the input parameter type:

printfn "%i" ((fun (x:int) -> x + 1) 5)

We can have multiple input parameters:

printfn "%i" ((fun (x:int) (y:int) -> x + y) 5 9)

5 will be assigned to x and 9 will be assigned to y.

We can have multiple statements in the function body. Take care of the indentation:

[<EntryPoint>]
let main argv =  
    printfn "%i" ((fun (x:int) (y:int) ->
        printfn "%s" "Calling an anonymous function!!!"
        printfn "You have called this anonymous function with %i and %i" x y
        x + y) 5 9)

This prints the following message:

Calling an anonymous function!!!
You have called this anonymous function with 5 and 9
14

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