Idiom of using in F#
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.
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:
The arguments of using are:
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.
> 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 = "<html>...</html>"
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 = <fun:clo@0>
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.