F# recursive function in strange endless loop

Question

I am very green when it comes to F#, and I have run across a small issue dealing with recursive functions that I was hoping could help me understand.

I have a function that is supposed to spit out the next even number:

let rec nextEven(x) =
    let y = x + 1
    if y % 2 = 0 then y
    else nextEven y


// This never returns..
nextEven 3;;

I use the 'rec' keyword so that it will be recursive, although when I use it, it will just run in an endless loop for some reason. If I rewrite the function like this:

let nextEven(x) =
    let y = x + 1
    if y % 2 = 0 then y
    else nextEven y

Then everything works fine (no rec keyword). For some reason I though I needed 'rec' since the function is recursive (so why don't I?) and why does the first version of the function run forever ?

EDIT
Turns out this was a total noob mistake. I had created multiple definitions of the function along the way, as is explained in the comments + answers.

Answer 1

I suspect you have multiple definitions of nextEven. That's the only explanation for your second example compiling. Repro:

module A =
  let rec nextEven(x) =
    let y = x + 1
    if y % 2 = 0 then y
    else nextEven y

open A //the function below will not compile without this

let nextEven(x) =
    let y = x + 1
    if y % 2 = 0 then y
    else nextEven y //calling A.nextEven

Try resetting your FSI session.