相关疑难解决方法(0)

我决定查看FParsec,并尝试为λ表达式编写解析器.事实证明,渴望使递归解析变得困难.我怎么解决这个问题？

码:

open FParsec

type ?Expr =
    | Variable of char
    | Application of ?Expr * ?Expr
    | Lambda of char * ?Expr

let rec FV = function
    | Variable v -> Set.singleton v
    | Application (f, x) -> FV f + FV x
    | Lambda (x, m) -> FV m - Set.singleton x

let ?0 = FV >> (=) Set.empty

let apply f p =
    parse
        { let! v = p
          return f v }

let ? e = …