在此示例中，如何使编译器插入正确的类型? [英] How can I get the compiler to insert the correct type in this example?

问题描述

这是对该问题的后续跟进

this is somehow follow-up on this question How can I get rid of this "Can not be generalised" error?

有以下几种类型

type IModel<'value, 'target when 'target :> IModel<'value, 'target>> =
    abstract value: 'value with get

type IModelSimple<'value, 'target> =
    inherit IModel<'value, IModelSimple<'value, 'target>>
    abstract ReInitWith:  #IModel<_ , _ > -> 'target

此函数返回一个对象表达式

and this function returning an object expression

let rec mkModelSimple<'value, 'target> vctor value =
    {
        new IModelSimple<'value, 'target> with
            member this.value = value
            member this.ReInitWith m = mkModelSimple vctor this.value
    }

我收到此错误

Type mismatch. Expecting a
    ''target'
but given a
    'IModelSimple<'value,'target>'

在ReInitWith的实现中.

推荐答案

编译器报告时，您正在定义一个函数，其中推断的类型包含其自身并且将是无限的.解决此问题的方法是引入一些破坏递归的命名类型.一种选择是使用递归类型定义新记录:

As the compiler reports, you are defining a function where the inferred type contains itself and would be infinite. The way to fix this is to introduce some named type that breaks the recursion. One option is to define a new record with recursive type:

type IModel<'value, 'target when 'target :> IModel<'value, 'target>> =
    abstract value: 'value with get

type IModelSimple<'value, 'target> =
    inherit IModel<'value, IModelSimple<'value, 'target>>
    abstract ReInitWith :  #IModel<_, _> -> 'target

type Model<'value, 'target> =
  { Model : IModelSimple<'value, Model<'value, 'target>> }

let rec mkModelSimple<'value, 'target> vctor value =
    { Model = 
        { new IModelSimple<'value, Model<'value, 'target>> with
              member this.value = value
              member this.ReInitWith m = 
                mkModelSimple<_, _> vctor this.value } }

仅作记录，我认为这段代码非常难以理解.无疑，这是泛型可以明智地做的事情，我建议避免在生产中这样做.

Just for the record, I think this code is pretty incomprehensible. It is definitely stretching what generics can sensibly do and I'd recommend avoiding doing things like this in production.

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