Idiom of using in F#

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In Don Syme's excellent book drafts of his upcoming book Expert F#, the following code is used to demonstrate the quickness and easiness of F# interactive development.

> let req = WebRequest.Create("http://www.microsoft.com");; 
val req : WebRequest 
> let resp = req.GetResponse();; 
val resp : WebResponse 
> let stream = resp.GetResponseStream();; 
val stream : Stream 
> let reader = new StreamReader(stream) ;; 
val reader : StreamReader 
> let html = reader.ReadToEnd();; 
val html : string = "..." 

However, writing codes in such way would be too imperative. By using the idiomatic using function, we can write it in a more concise and safer form:

#light
let http (url: string) =  
    let req = WebRequest.Create(url)  
    using (req.GetResponse()) 
          (fun res -> using (res.GetResponseStream()) 
                            (fun stream -> let reader = new StreamReader(stream)  
                                           reader.ReadToEnd()))  

The arguments of using are:

val it : 'a -> ('a -> 'b) -> 'b when 'a :> IDisposable = 

Clearly, the first argument requires a resource that implements the IDisposable interface ( :> is a special operator in F# for up-casting); then using will feed the resource to a function to process further and finally will guarantee to release the resource when everything is done.