CODEWITHSUNDEEP
f#
zhengbli
zhengbli

Reputation: 712

F# type definition syntax

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!

Upvotes: 2

Views: 1035

Answers (3)

scrwtp
scrwtp

Reputation: 13577

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.

Upvotes: 3

Christopher Stevenson
Christopher Stevenson

Reputation: 2871

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.

Upvotes: 4

John Palmer
John Palmer

Reputation: 25526

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.

Upvotes: 2

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