F#:此表达式应具有 DateTime 类型，但此处具有类型 unit

Question

下面的 F# 函数不编译

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"new"和“旧”

编译失败并出现以下错误:

Script1.fsx(3,59): error FS0001: This expression was expected to have type

DateTime    

but here has type

unit    

当我只是想通过 printfn 打印一些东西时，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) = 

第一个有签名 (DateTime -> DateTime) 因为你明确地对函数输出做了这个约束。输入已由编译器推断。

第二个有签名(DateTime -> unit)。输入已明确约束，输出 unit 推断

添加:

完整的显式签名应该是这样的

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"