以下代码来自《F# 4.0 设计模式》第 5 章。
let a = 1,"car"
type System.Tuple<'T1,'T2> with
member t.AsString() =
sprintf "[[%A]:[%A]]" t.Item1 t.Item2
(a |> box :?> System.Tuple<int,string>).AsString()
期望的输出是[[1]:["car"]]
但是，下面会出现一条红色波浪线AsString()。“字段、构造函数或成员‘AsString’未定义。也许您需要以下之一：ToString”
下面的代码来自https://www.manning.com/books/real-world-function-programming的第8章
当我运行代码时，我在 testClientTree 中收到空引用异常。我检查了这本书的勘误表，但没有找到任何与此相关的内容。
type Client =
{ Name : string
Income : int
YearsInJob : int
UsesCreditCard : bool
CriminalRecord : bool }
let john =
{ Name = "John Doe"
Income = 40000
YearsInJob = 1
UsesCreditCard = true
CriminalRecord = false }
type ClientTests =
{ Check : Client -> bool
Report : Client -> unit }
type QueryInfo =
{ Title : string
Test : Client -> bool
Positive : Decision
Negative : Decision } …下面的 SML/NJ 代码会生成“val Grove(whatTree) = glen”的绑定非详尽警告。F# 等效代码不会产生警告。为什么？
新泽西州标准 ML（32 位）v110.99.2 [构建时间：2021 年 9 月 28 日星期二 13:04:14]：
datatype tree = Oak|Elem|Maple|Spruce|Fir|Pine|Willow
datatype vegetable = Carrot|Zucchini|Tomato|Cucumber|Lettuce
datatype grain = Wheat|Oat|Barley|Maize
datatype plot = Grove of tree|Garden of vegetable|Field of grain|Vacant
val glen = Grove(Oak)
val Grove(whatTree) = glen
F# 6.0.0 警告级别 5：
type Tree = Oak|Elem|Maple|Spruce|Fir|Pine|Willow
type Vegetable = Carrot|Zucchini|Tomato|Cucumber|Lettuce
type Grain = Wheat|Oat|Barley|Maize
type Plot = Grove of Tree|Garden of Vegetable|Field of Grain|Vacant
let glen = Grove(Oak)
let Grove(whatTree) = …我正在尝试将以下模式匹配函数转换为匹配表达式：
let reverse ls =
let rec rev acc =
function
| h :: t -> rev (h :: acc) t
| [] -> acc
rev [] ls
当我尝试转换为等效匹配表达式类型时，会发生不匹配错误：
let reverse ls =
let rec rev acc =
match acc with
| h :: t -> rev (h :: acc) t
| [] -> acc
rev [] ls
两者所需的输出是：
reverse [ 1; 2; 3 ]
// val it : int list = [3; 2; 1]