F# keyword 'Some'

Question

Some is not a keyword. There is an option type however, which is a discriminated union containing two things:

1. Some which holds a value of some type.
2. None which represents lack of value.

It's defined as:

type 'a option =
    | None
    | Some of 'a

It acts kind of like a nullable type, where you want to have an object which can hold a value of some type or have no value at all.

let stringRepresentationOfSomeObject (x : 'a option) =
    match x with
    | None -> "NONE!"
    | Some(t) -> t.ToString()

Can check out Discriminated Unions in F# for more info on DUs in general and the option type (Some, None) in particular. As a previous answer says, Some is just a union-case of the option<'a> type, which is a particularly common/useful example of an algebraic data type.

Some is used to specify an option type, or in other words, a type that may or may not exist.

F# is different from most languages in that control flow is mostly done through pattern matching as opposed to traditional if/else logic.

In traditional if/else logic, you may see something like this:

if (isNull(x)) {
   do ...  
} else {         //x exists
   do ...  
}

With pattern matching logic, matching we need a similar way to execute certain code if a value is null, or in F# syntax, None

Thus we would have the same code as

match x with 
  | None -> do ...
  | Some x -> do ...