如何修复以下错误并获得所需的结果

Question

我下面有代码，但没有得到想要的正确结果

type expr
= Val of value 
| App of expr * expr
| MultiApp of expr * expr list
and value 
= Int of int 
| Bool of bool

let rec t(e:expr) : expr = match e with
| Val _ -> e
| App(f,a) -> App(t f, t a)

| MultiApp(f,[]) -> raise (Failure "no")
| MultiApp(f,lst) -> match lst with
                        |[]-> raise (Failure "no")
                        |x::xs -> let rest = MultiApp(f,xs) in 

                             App(f,x) 

let e1 = Val(Int 1)
let e2 = Val(Int 3)
let e3 = Val(Bool true)
let e4 = Val(Bool false)

例如

MultiApp(fe,[e1;e2;e3])

应用有趣t之后，它会得到类似

App(App(App(t fe, t e1), t e2) t e3)