introduction.md

The ability for something to be defined in terms of itself is called recursion. In F#, recursion is most commonly found in recursive functions, which are functions that call themselves.

A recursive function needs to have at least one base case and at least one recursive case. A base case returns a value without calling the function again. A recursive case calls the function again, modifying the input so that it will at some point match the base case.

Recursive functions are defined like regular functions, but with the rec modifier. Without this modifier, a function will not be able to call itself and any attempt to do so will result in a compilation error. Recursion thus has to be explicitly opted into.

let rec factorial x =
    // Base case
    if x = 1 then
        1
    // Recursive case
    else
        x * factorial (x - 1)

F# also supports recursive types through discriminated union types. A recursive discriminated union type has one or more of its cases refer to the discriminated union type itself in their data. Like recursive functions, recursive types must have a base case. Unlike recursive functions, recursive types don't use the rec keyword.

type RussianDoll
    | Doll                 // Base case
    | Layer of RussianDoll // Recursive case