F# type definition syntax

问题

I'm working on the '99 problems of F#', and saw the following definition of NestedList:

type 'a NestedList = List of 'a NestedList list | Elem of 'a

I'm a little confused by the syntax here, as normally there is only a type name after the type. Can someone clarify it? Thanks!

回答1:

This is a discriminated union in a condensed syntax. You can also write it as:

type 'a NestedList =
    | List of 'a NestedList list
    | Elem of 'a

or

type NestedList<'a> =
    | List of NestedList<'a> list
    | Elem of 'a

This is using generics, where the type itself is taking another type as an argument.

回答2:

F# has two ways of specifying a generic type:

• C#/.NET style - SomeType<'a, 'b>
• OCaml/ML style - ('a * 'b) SomeType

Those are really two ways of saying the same thing and can be used interchangeably. Which one to use is a matter of preference and code standards. Usually people use OCaml style for basic F# types like list, array or option, and C# style for user-defined and BCL ones.

回答3:

So this is a definition of a generic discriminated union with generic type 'a.

The key idea is that each element has the type 'a.

来源：https://stackoverflow.com/questions/26568037/f-type-definition-syntax