parsing - F# 中的事件模式的递归如何工作？

Question

我有以下用于解析整数的函数:

let rec digits = function
    | head::tail when System.Char.IsDigit(head) ->
        let result = digits tail
        (head::(fst result), snd result)
    | rest -> ([], rest)

如果我将此函数更改为主动识别器，它将不再编译。

let rec (|Digits|) = function
    | head::tail when System.Char.IsDigit(head) ->
        let result = Digits tail
        (head::(fst result), snd result)
        //          ^^^^^^       ^^^^^^ see error*
    | rest -> ([], rest)

*error FS0001: This expression was expected to have type char list * 'a but here has type char list

Answer 1

let rec (|Digits|) = function
    | head::(Digits (a, b)) when System.Char.IsDigit(head) -> (head::a, b)
    | rest -> ([], rest)

注意:如果您想使用事件模式作为功能，您仍然可以这样做:

let rec (|Digits|) = function
    | head::tail when System.Char.IsDigit(head) ->
        let a, b = (|Digits|) tail
        (head::a, b)
    | rest -> ([], rest)