Returning a function with a closure

Question

The following code:

let CreateFunc=
    let counter = ref 0
    fun () -> counter := !counter + 1; !counter

let f1 = CreateFunc
let f2 = CreateFunc

printfn "%d" (f1())
printfn "%d" (f1())
printfn "%d" (f2())
printfn "%d" (f2())

Outputs:

1
2
3
4

So, basically, what we see here is f1 and f2 being the same function - as they're obviously sharing the same instance of 'counter'.

The expected output is:

1
2
1
2

QUESTION: Shouldn't f1 and f2 be two separate instances? After all they are created by the two different calls to 'CreateFunc'???

Thanks

Answer 1

let CreateFunc() =
    let counter = ref 0
    fun () -> counter := !counter + 1; !counter

let f1 = CreateFunc()
let f2 = CreateFunc()

printfn "%d" (f1())
printfn "%d" (f1())
printfn "%d" (f2())
printfn "%d" (f2())

Output is

1
2
1
2

Explanation:

In your original solution, CreateFunc was a function, but always the same function (CreateFunc, f1 and f2 were all synonyms, all pointing to the same function). In my solution CreateFunc is a function which returns a new function whenever it is called, thus each function has its own state (i.e. counter).

In short: the original CreateFunc was a value, always the same value.