It is my understanding that from the standpoint of .NET runtime F# modules are classes (static classes). Therefore the let bindings are static members on the class. Initialzers for such members are supposed to be called only once 'before the first use of the class'.

My guess is that enforcing the 'once before first use' rule cannot be done without some sort of locks on the class definition. Which makes me wonder how happy can I be about constructs like this:

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module public Utilities =

    /// valid format of an int    
    let internal int_re = new Regex(@"^-?\d+$", RegexOptions.Compiled)

    /// valid format of a float
    let internal float_re = new Regex(@"^-?\d+(?:\.\d+)?$", RegexOptions.Compiled)

It seems innocent, but in a heavily multithreaded envornment if my suspictions are correct it can create some serious contention