Node.js + Nginx - 现在呢？(Node.js + Nginx - What now?)

 [过时... 
 
 您可以得到的最接近的定义签名为： 
 
module type SOURCE = sig
  type 'a ctx = 'a constraint 'a = #CouchDB.ctx
  type source
  val get : source -> 'a ctx -> string 
end
 
 但是，当然，你也可以写下： 
 
module type SOURCE = sig
  type source
  val get : source -> #CouchDB.ctx -> string 
end
 
 编辑：请注意，OCaml使用结构打字的对象。 这意味着，即使你想要，你也不能比上述更具限制性。 它甚至没有限制参数成为CouchDB.ctx或派生类的实例 - 任何具有（至少）相同方法的对象都是兼容的。 即使你写作 
 
  val get : source -> CouchDB.ctx -> string 
 
 你可以传递任何具有相同方法的对象。 类型CouchDB.ctx只是一个特定结构对象类型的缩写，恰好与相同名称的类生成的对象相匹配。 它不限于那些。 可以肯定的是：这被认为是一项功能。 
 
 ======] 
 
 编辑2：通过扩展示例，我现在可以看到你想要什么以及为什么。 不幸的是，这在OCaml中是不可能的。 你需要部分抽象类型。 也就是说，你需要能够写作 
 
module type SOURCE = sig
  type ctx < CouchDB.ctx
  ...
end
 
 这在OCaml中不可用。 但是，如果您愿意在签名中提供明确的上传，则可以关闭： 
 
module type SOURCE = sig
  type ctx
  val up : ctx -> CouchDB.ctx
  type source = string
  val get : source -> ctx -> string monad
end
 
 然后，在Cache ，您必须用ctx#get替换(S.up ctx)#get ，并且对于ctx#put也是如此。 
 
module Cache = functor (S:SOURCE) -> struct
  type ctx = S.ctx
  type source = S.source
  let get source ctx = 
     bind ((S.up ctx)#get source) ...
end

module SomeSource = struct
  type ctx = AsyncDB.ctx
  let up ctx = (ctx : ctx :> CouchDB.ctx)
  type source = string
  let get s ctx = ...
end

module SomeCache = Cache (SomeSource)
 
 请注意，我还在签名SOURCE type source = string 。 没有这一点，我看不出ctx#get source可能会如何在Cache函数中进行类型检查。 

[Obsolete...
 
The closest you can get is defining the signature as:
 
module type SOURCE = sig
  type 'a ctx = 'a constraint 'a = #CouchDB.ctx
  type source
  val get : source -> 'a ctx -> string 
end
 
But of course, you could as well just write:
 
module type SOURCE = sig
  type source
  val get : source -> #CouchDB.ctx -> string 
end
 
Edit: Note that OCaml uses structural typing for objects. That means that even if you wanted, you cannot get any more restrictive than the above. It does not even limit arguments to get to be instances of CouchDB.ctx or a derived class -- any object that has (at least) the same methods will be compatible. Even when you write
 
  val get : source -> CouchDB.ctx -> string 
 
you can pass any object that has the same methods. The type CouchDB.ctx is just an abbreviation for a specific structural object type that happens to match the objects generated by the class of the same name. It is not restricted to those. And just to be sure: that is considered a feature.
 
======]
 
Edit 2: With the extended example, I now see what you want and why. Unfortunately, that isn't possible directly in OCaml. You would need partially abstract types. Namely, you would need to be able to write
 
module type SOURCE = sig
  type ctx < CouchDB.ctx
  ...
end
 
This is not available in OCaml. However, you can get close if you are willing to provide an explicit upcast in the signature:
 
module type SOURCE = sig
  type ctx
  val up : ctx -> CouchDB.ctx
  type source = string
  val get : source -> ctx -> string monad
end
 
Then, in Cache, you have to replace occurrences of ctx#get with (S.up ctx)#get, and likewise for ctx#put.
 
module Cache = functor (S:SOURCE) -> struct
  type ctx = S.ctx
  type source = S.source
  let get source ctx = 
     bind ((S.up ctx)#get source) ...
end

module SomeSource = struct
  type ctx = AsyncDB.ctx
  let up ctx = (ctx : ctx :> CouchDB.ctx)
  type source = string
  let get s ctx = ...
end

module SomeCache = Cache (SomeSource)
 
Note that I also made type source = string transparent in the signature SOURCE. Without that, I cannot see how ctx#get source can possibly type-check in the Cache functor.