Question

我似乎在最内层for循环的查询中丢失了intellisense，这让我相信我写的是错误的代码。这似乎是匹配声明的问题。当我尝试通过简单捕获值来匹配时，一切似乎都有效。当我尝试通过比较值进行匹配时，代码似乎打破了。这是代码：

let createOrderFromStdOrder () =

    // retrieve standard orders
    let stdOrders = 
        Queries.retrieveStdOrders null |> Seq.toList

    // retrieve orders
    let orders = Queries.retrieveOrders null |> Seq.toList

    // current date
    let curDate = DateTime.Now

    // iterate and create order
    for i in stdOrders do

        let validDates = MyUtil.stdDeadlineCheck i

        // process standard order
        let orderDate = 
            if not validDates.IsEmpty then
                let daysOfWeek = 
                    query{
                        for row in validDates do
                        sortBy row
                        take 2
                    }

                for j in daysOfWeek do
                    match j with
                    | c -> // this seems to work
                        let order =
                            query{
                                for o in orders do
                                where(o.Works = i.Works) // and I get dot intellisense o and i
                            }

但如果我尝试匹配这样似乎打破了：

                for j in daysOfWeek do
                    match j with
                    | DateTime.Now.DayOfWeek -> // this does not work
                        let order =
                            query{
                                for o in orders do
                                where(o.NOTHING = i.Works) // NO dot intellisense on o
                            }

我怀疑这可能与嵌套有关。我怎么能纠正这个？

Answer 1

模式匹配只能测试与常量的相等性，而不能测试DateTime.Now.DayOfWeek等值。原因是当值是一个简单的变量名时会使它变得模糊，因为模式中的变量用于将此位置的任何值绑定到名称，而不是将其与现有变量进行比较：

match j with
| x ->
    // in here, x is equal to j.

在您的具体情况下，您可以使用if：

for j in daysOfWeek do
    if j = DateTime.Now.DayOfWeek then
        let order = // ...

但是，如果在更复杂的情况下，您希望与内部模式进行比较，则可以使用guard：

match someRecord with
| { someField = d } when d = DateTime.Now.DayOfWeek ->
    let order = // ...

在嵌套for循环中使用match表达式

1 个答案: