Result (Either) Functor

Result (the Result type in F#) has two cases: success (Ok) and failure (Error). As a Functor, it provides Result.map to apply a function only in the success case.

Example in F#

let okValue = Ok 10
let mappedOk = okValue |> Result.map (fun x -> x * 2)
// mappedOk = Ok 20

let errorValue = Error "error"
let mappedError = errorValue |> Result.map (fun x -> x * 2)
// mappedError = Error "error"

Result.map applies the function only in the Ok case and returns Error unchanged.