Idiom of using in F#

最新推荐文章于 2017-01-01 16:21:08 发布

最新推荐文章于 2017-01-01 16:21:08 发布 · 160 阅读

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本文通过一个HTTP请求的示例，展示了F#语言如何简化代码并确保资源安全释放。使用了[b]using[/b]函数使得代码更加简洁安全。

In Don Syme's [url=http://blogs.msdn.com/dsyme/archive/2006/12/18/DraftChaptersUpdateDec2006.aspx]excellent book drafts[/url] of his upcoming book [i]Expert F#[/i], the following code is used to demonstrate the quickness and easiness of F# interactive development.

[code]
> 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>"
[/code]

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

[code]#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())) [/code]

The arguments of [b]using[/b] are:
[code]val it : 'a -> ('a -> 'b) -> 'b when 'a :> IDisposable = <fun:clo@0>[/code]
Clearly, the first argument requires a resource that implements the IDisposable interface ( [b]:>[/b] is a special operator in F# for up-casting); then [b]using[/b] will feed the resource to a function to process further and finally will guarantee to release the resource when everything is done.