F# 中的自定义计算表达式
我一直在玩弄 F# 中的 monad（又名计算表达式），并且编写了这个简单的 Identity monad：
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：
该表达式的类型应为 Identity<'a>但这里有类型 'b * 'c
我认为这没有意义，因为 getInt() 显然是 Identity<'a> 类型的值。
谁能告诉我我做错了什么？
I've been toying with monads in F# (aka computation expressions) and I wrote this simple Identity monad:
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 }
I don't understand the error I'm getting in the marked line:
This expression was expected to have type Identity<'a> but here has type 'b * 'c
I think this makes no sense as getInt() is clearly a value of type Identity<'a>.
Can anyone tell me what am I doing wrong?
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计算表达式语法希望 Bind 有一个元组参数，而不是柯里化参数。
因此，
请参阅本文了解所有签名。
The computation expression syntax wants
Bindto have a tupled, not curried argument.So
See this article for all signatures.
问题在于您的 Bind 函数的类型 - 它不应该采用柯里化参数。如果你将其更改为：
那么它应该可以工作。
The problem is the type of your
Bindfunction - it shouldn't take curried arguments. If you change it to:then it should work.