Describing a state machine / approval workflow in F#

states.fs

type State = Initial | Draft | PendingApproval | Approved | Cancelled | Completed
type Action = Create | Cancel | SendForApproval | Approve | Reject | Complete
type Role = Creator | Approver | Completor
type Transition = { Action:Action; From:State; To:State; Roles:Role list }
module Transitions =
    let create      = { Action=Create;          From=Initial;         To=Draft;           Roles=[ Creator   ] }
    let cancel      = { Action=Cancel;          From=Draft;           To=Cancelled;       Roles=[ Creator   ] }
    let sendForAppr = { Action=SendForApproval; From=Draft;           To=PendingApproval; Roles=[ Creator   ] }
    let approve     = { Action=Approve;         From=PendingApproval; To=Approved;        Roles=[ Approver  ] }
    let reject      = { Action=Reject;          From=PendingApproval; To=Draft;           Roles=[ Approver  ] }
    let complete    = { Action=Complete;        From=Approved;        To=Completed;       Roles=[ Completor ] }
    let all = [ create; sendForAppr; approve; reject; complete; cancel; ]

Transitions.all
|> List.mapi (fun i t -> i, t.Action, t.From, t.To, t.Roles)

(*
val it : (int * Action * State * State * Role list) list =
 [(0, Create, Initial, Draft, [Creator]);
  (1, SendForApproval, Draft, PendingApproval, [Creator]);
  (2, Approve, PendingApproval, Approved, [Approver]);
  (3, Reject, PendingApproval, Draft, [Approver]);
  (4, Complete, Approved, Completed, [Completor]);
  (5, Cancel, Draft, Cancelled, [Creator])]
 *)