为什么F＃推断这种类型？

This is my code:

这是我的代码:

type Cell<'t>(initial : 't) =
    let mutable v = initial
    let callbacks = new List<'t -> unit>()
    member x.register c = callbacks.Add(c)
    member x.get () = v
    member x.set v' = 
        if v' <> v 
        then v <- v'
             for callback in callbacks do callback v'
    member x.map f = 
        let c = new Cell<_>(f v)
        x.register(fun v' -> c.set (f v')) ; c
type Cell<'t>(init