I'm just starting to learn F# using VS2010 and below is my first attempt at generating the Fibonacci series. What I'm trying to do is to build a list of all numbers less than 400.

let fabList = 
    let l =  [1;2;]
    let mutable a = 1
    let mutable b = 2
    while l.Tail < 400 do
        let c = a + b
        l.Add(c)
        let a = b
        let b = c

My first problem is that on the last statement, I'm getting an error message "Incomplete structured construct at or before this point in expression" on the last line. I don't understand what I'm doing wrong here.

While this seems to be an obvious way to build the list in a fairly efficient way (from a c++/C# programmer), from what little I know of f#, this doesn't seem to feel to be the right way to do the program. Am I correct in this feeling?