Elixir/Erlang Dialyzer:为什么行为回调的param类型应该是子类型而不是超类型? [英] Elixir / Erlang Dialyzer : Why behaviour callback's param type should be subtype instead of supertype?
问题描述
我有一个行为X和一个带有参数类型的回调函数:
I have a behaviour X and a callback function with parameter type:
%{a: any}
模块Y实现行为X，而实现模块Y中的回调函数具有参数类型:
Module Y implements behaviour X and the callback function in implementing module Y has parameter type:
%{a: any, b: any}
Dialyzer不喜欢它并抱怨:
Dialyzer doesn't like that and complains:
(#{'a':=_, 'b':=_, _=>_})
is not a supertype of
#{'a':=_}
这意味着透析器尝试确定实现模块Y中的回调参数的类型是否为行为X中的参数类型的超类型.换句话说，它询问:
This implies dialyzer attempts to determine if callback parameter's type in implementing module Y is a supertype of param type in behaviour X. In other words, it asks:
行为X的回调参数类型是
%{a: any}的子类型吗? 实现模块Y的参数类型%{a: any, b: any}?
Is behaviour X's callback param type
%{a: any}a subtype of implementing module Y's param type%{a: any, b: any}?
为什么透析仪期望行为回调的参数类型是子类型而不是超类型?
在编程语言类型理论的上下文中，定义了子类型为:
In the context of programming language type theory, subtype is defined as:
类型S是类型T的子类型，如果表达式为，则写为S< ;:T. 类型S可以在任何需要类型T的上下文中使用. 另一种表达方式是，任何类型为S的表达式都可以 假装成T型表达.
type S is a subtype of a type T, written S <: T, if an expression of type S can be used in any context that expects an element of type T. Another way of putting this is that any expression of type S can masquerade as an expression of type T.
根据以上定义，行为回调的参数类型为T而实现模块的参数类型为S对我来说很有意义.因为执行模块仍然保持行为契约.但是，我对为什么透析仪期望出现相反的情况一无所知.
In light of the definition above, it makes sense to me if parameter type of behaviour callback is T and that of implementing module is S. Because implementing module still keeps the behaviour contract. However, I'm clueless as to why dialyzer expects the other way around.
请帮助我理解这一点.
注意:此问题是后续问题，但独立于另一个SO问题 Erlang( Elixir)Dialyzer-令人困惑的超类型错误.
Note: This question is a follow-up but independent of another SO question Erlang (Elixir) Dialyzer - confusing supertype error.
推荐答案
Dialyzer是正确的.如果存在具有%{a: any}类型的回调的行为X，则用户应能够调用声称实现此行为的任何模块的该函数，例如%{a: 1}.模块的函数采用%{a: any, b: any}，它是%{a: any}的子类型，这意味着该函数不能再用%{a: 1}调用，这与行为不符.
Dialyzer is correct. If there is a behaviour X with a callback of type %{a: any}, the user should be able to call that function of any module that claims to implement this behaviour with e.g. %{a: 1}. Your module's function takes %{a: any, b: any} which is a subtype of %{a: any}, which means that function cannot be called with %{a: 1} anymore which does not comply with the behaviour.
另一方面，如果行为的回调的类型为%{a: any, b: any}，而您的模块的函数的类型为%{a: any}，那会很好，因为%{a: any}是%{a: any, b: any}的超类型，并且您的模块可以是用%{a: 1, b: 2}调用-它可以忽略多余的字段.
On the other hand, if the behaviour's callback had the type %{a: any, b: any} and your module's function had the type %{a: any}, that would have been fine because %{a: any} is a supertype of %{a: any, b: any} and your module can be called with %{a: 1, b: 2} -- it can just ignore the extra field.
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