F#中的函数..为什么不编译 [英] functions in F# .. why is it not compiling

问题描述

我写了两个版本的代码.第一个按预期方式工作并显示"Hi".第二个错误给我一个错误，在此之后的块未完成"

I have written two versions of code. The first one works as expected and print "Hi". the second one gives me error that "block following this let is unfinished"

第一个版本

#light 

let samplefn() =
            let z = 2
            let z = z * 2
            printfn "hi"

samplefn()

第二版

#light 

let samplefn() =
            let z = 2
            let z = z * 2           

samplefn()

唯一的区别是第二个版本中没有printfn.我正在使用Visual Studio 2010作为我的IDE.我是F#的新手，但这个错误对我来说似乎很奇怪.我想我缺少一些非常重要的概念.请解释.

Only difference is the printfn is absent in the second version. I am using Visual Studio 2010 as my IDE. I am very new to F# but this error seems very strange to me. I guess I am missing some very important concept. Please explain.

同样，如果我在函数外部执行此操作，即使使用第一版代码，也会出现错误.

Also if I do it outside the function I get error even with the first version of code.

#light
let z = 2
let z = z * 2
printfn "Error: Duplicate definition of value z"

推荐答案

一个不在顶层(例如，缩进的)的let必须具有称为a的语句(实际上是一个表达式，如pst所指出的)作业后的正文".在第一个示例中，主体为printfn "hi"，而第二个示例中没有主体.这就是编译器所抱怨的.

A let that is not at the top level (e.g. your indented ones) has to have a statement (actually an expression, as pst notes) called a "body" following the assignment. In the first example the body is printfn "hi", while the second example has no body. That's what the compiler is complaining about.

请注意，在函数定义中，内部let表达式实际上会创建嵌套作用域.也就是说，let z = z * 2实际上创建一个名为z的新值，并将其与外部z的值乘以2，然后在let的主体(即printfn这个案例).嵌套的let将始终具有主体.这是嵌套，它允许看似重复的定义.

Note that in your function definitions the inner let expressions actually create nested scopes. That is, the let z = z * 2 actually creates a new value called z and binds to it the value of the outer z times 2, then uses it in the body of the let (which is the printfn in this case). A nested let will always have a body. It is the nesting which allows the seemingly duplicate definition.

由于最外面的let不需要主体，因此编译器认为您正在尝试在同一范围内重新定义z，这是错误的.您可以使用括号告诉编译器如何正确解释最后一个示例:

Since an outermost let does not need a body, the compiler thinks you're trying to redefine z in the same scope, which is an error. You can use parentheses to tell the compiler how to properly interpret the last example:

let z = 2
(let z = z * 2
printfn "z = %d" z)
printfn "z = %d" z

上面将打印

z = 4
z = 2

这篇关于F#中的函数..为什么不编译的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！