F#：此表达式应为 DateTime 类型，但此处为 unit 类型

Question

以下 F# 函数无法编译

open System
let checkCreation time : DateTime = 
    if (time > DateTime.UtcNow.AddDays(-7.0)) then printfn "New"
    else printfn "Old"

checkCreation time

错误标记指向“新”和“旧”

编译器失败并出现以下错误：

Script1.fsx(3,59)：错误 FS0001：此表达式应具有类型

DateTime    

但是这里有类型

unit    

当我试图通过 printfn 打印某些内容时，为什么编译器需要 DateTime？

Answer 1

替换这个

let checkCreation time: DateTime = 

有了这个

let checkCreation (time: DateTime) = 

第一个具有签名(DateTime -> DateTime)，因为您明确地对函数输出进行了此约束。输入已被编译器推断出来。

第二个有签名(DateTime -> unit)。输入已被显式约束，输出unit推断

添加：

完整的显式签名应如下所示

let checkCreation (time: DateTime) : unit = 
    ...

您可以删除每个显式类型约束并让编译器完成工作：

//because time argument compared to DateTime it is inferred to be DateTime
//No explicit constrain needed
let checkCreation time = //DateTime -> unit

    //because if expression is the last one, its output will be used as function output
    if (time > DateTime.UtcNow.AddDays(-7.0))
    //because then branch has unit output, function output will be inferred as unit
    then printfn "New" 
    //else branch output MUST match with then branch. Your code pass :)
    else printfn "Old"