使用选项(Option)
type 'T option =
| None
| Some of 'T
下面看一个例子：
> let people = [ ("Adam", None);
("Eve" , None);
("Cain", Some("Adam","Eve"));
("Abel", Some("Adam","Eve")) ];;
val people : (string * (string *string) option) list
使用模式匹配(Pattern matching)来生成option：
> let showParents (name,parents) =
match parents with
| Some(dad,mum) -> printfn "%s has father %s, mother %s" name dad mum
| None -> printfn "%s has no parents!" name;;
val showParents : (string * (string * string) option) -> unit
> showParents people.[0];;
Adam has no parents
val it : unit = ()
Option的一些有用的方法：
|
方法 |
类型 |
描述 |
|
Option.get |
'T option -> 'T |
返回一个Some类型的值。或抛异常 |
|
Option.isNone |
'T option -> bool |
返回一个Option是否是None |
|
Option.map |
('T -> 'U) -> 'T option -> 'U option |
如果是None，就返回None。如果是Some(x)，返回Some(f x)，f是给定的函数 |
|
Option.iter |
('T -> unit) -> 'T option -> unit |
对Some类型的Option执行指定的方法。 |
一些例子：
> Option.map(fun x->x) a;;
val it : (string * string) option = Some ("aa", "bb")
> Option.map(fun x-> match x with | (first,second) -> first) a;;
val it : string option = Some "aa"
> Option.map(fun x-> match x with | (first,second) -> second) a;;
val it : string option = Some "bb"
>
> Option.iter(fun x-> match x with | (first:string,second) -> printfn "%s" (first+second)) a;;
aabb
val it : unit = ()
>
使用Option类型进行控制
看这个例子：
let fetch url =
try Some (http url)
with :? System.Net.WebException -> None
http函数是在之前章节定义的获取html的那个方法。在None的情况下抛出一个exception。成功的访问会返回一个Some值，也就是Option类型的值。然后我们就可以使用Option值来进行模式匹配：
> match (fetch "http://www.nature.com") with
| Some text -> printfn "text = %s" text
| None -> printfn "**** no web page found";;
text = <HTML> ... </HTML> (note: the HTML is shown here if connected to the web)
val it : unit = ()
使用条件判断：&&和||
基本的F#控制符为if/then/elif/else。举例：
let round x =
if x >= 100 then 100
elif x < 0 then 0
else x
条件判断其实是模式匹配(pattern matching)的缩写；上例可以写为下面的形式：
let round x =
match x with
| _ when x >= 100 -> 100
| _ when x < 0 -> 0
| _ -> x
使用&&和||：
let round2 (x, y) =
if x >= 100 || y >= 100 then 100,100
elif x < 0 || y < 0 then 0,0
else x,y