F中的自定义计算表达式#

type Identity<'a> = 
    | Identity of 'a

type IdentityBuilder() =
    member x.Bind (Identity v) f  = f(v)
    member x.Return v = Identity v
let identity = new IdentityBuilder()

let getInt() = identity { return Int32.Parse(Console.ReadLine()) }

let calcs() = identity {
    let! a = getInt()    // <- I get an error here
    let! b = getInt()
    return a + b }

我不明白我在标记行中遇到的错误:

此表达式应具有类型标识<'a>但这里有“b*”c型

我认为这毫无意义,因为getInt()显然是一个类型的值 Identity<'a> .

谁能告诉我我做错了什么?