F# Function doesn't let me mutate mutable value

Probably another dumb F# beginner's question... But it's bugging me all the same

I can't seem to find any answers to this online... might be 'cause I search the wrong terms but eh

anyway my code goes as follows:

let counter() = 
    let mutable x = 0

    let increment(y :int) =
        x <- x + y // this line is giving me trouble
        printfn "%A" x // and this one too

    increment // return the function

Visual Studio is telling me that x is used in an invalid way, that mutable variables can't be captured by closures

why is that? and what can I do to allow me to mutate it?

Solution

As the error message indicates, you can use a ref cell instead:

let counter() = 
    let x = ref 0

    let increment(y :int) =
        x := !x + y // this line is giving me trouble
        printfn "%A" !x // and this one too

    increment // return the function

This does exactly what your code would do if it were legal. The ! operator gets the value out of the ref cell and := assigns a new value. As to why this is required, it's because the semantics of capturing a mutable value by a closure have proven to be confusing; using a ref cell makes things somewhat more explicit and less error-prone (see http://lorgonblog.wordpress.com/2008/11/12/on-lambdas-capture-and-mutability/ for further elaboration).