Embedded functions in F#

F# allows us to declare a function within a function. The embedded function will only be visible within the encasing function. Indentation is important in this case as the embedded function can also span multiple rows.

Consider the following function:

let calculate x = 
    printfn "Your input parameter was %i" x
    let embedded y z = 
        printfn "%s" "Running the embedded function"
        let a = (x + y) * 2
        x + y + a
    let finalRet = embedded 5 4
    printfn "The result is %i" (finalRet)
    finalRet

let calculateRes = calculate 5

The function “calculate” which accepts a paremeter x and runs some arguably pointless mathematical operations has an inline function called “embedded”. Embedded in turn accepts 2 parameters y and z. This function can only be called within the calculate function. Note how the code lines that make up “embedded” are indented so that they run within the inline function.

Running calculate in the interactive window will give the following output:

Your input parameter was 5
Running the embedded function
The result is 30

View all F# related articles here.