F# 检查算术范围

Question

F# 允许通过打开 Checked 来使用检查算法模块，它将标准运算符重新定义为检查运算符，例如:

open Checked
let x = 1 + System.Int32.MaxValue // overflow

会导致算术溢出异常。

但是如果我想在一些小范围内使用检查算术，比如 C# 允许使用关键字 checked 怎么办？ :

int x = 1 + int.MaxValue;             // ok
int y = checked { 1 + int.MaxValue }; // overflow

如何通过打开 Checked 来控制运算符重定义的范围模块或使其尽可能小？

Answer 1

你总是可以定义一个单独的操作符，或者使用阴影，或者使用括号来为临时阴影创建一个内部范围:

let f() =
    // define a separate operator
    let (+.) x y = Checked.(+) x y
    try 
        let x = 1 +. System.Int32.MaxValue
        printfn "ran ok"
    with e ->
        printfn "exception"
    try 
        let x = 1 + System.Int32.MaxValue
        printfn "ran ok"
    with e ->
        printfn "exception"
    // shadow (+)
    let (+) x y = Checked.(+) x y
    try 
        let x = 1 + System.Int32.MaxValue
        printfn "ran ok"
    with e ->
        printfn "exception"
    // shadow it back again
    let (+) x y = Operators.(+) x y
    try 
        let x = 1 + System.Int32.MaxValue
        printfn "ran ok"
    with e ->
        printfn "exception"
    // use parens to create a scope
    (
        // shadow inside
        let (+) x y = Checked.(+) x y
        try 
            let x = 1 + System.Int32.MaxValue
            printfn "ran ok"
        with e ->
            printfn "exception"
    )            
    // shadowing scope expires
    try 
        let x = 1 + System.Int32.MaxValue
        printfn "ran ok"
    with e ->
        printfn "exception"


f()    
// output:
// exception
// ran ok
// exception
// ran ok
// exception
// ran ok

最后，另见 --checked+编译器选项:

http://msdn.microsoft.com/en-us/library/dd233171(VS.100).aspx