package irmin

Types representing permissions 'perms for performing operations on a certain type 'perms t.

They are intended to be used as phantom parameters of the types that they control access to. As an example, consider the following type of references with permissions:

module Ref : sig
  type (+'a, -'perms) t

  val create : 'a -> ('a, read_write) t
  val get : ('a, [> read ]) t -> 'a
  val set : ('a, [> write ]) t -> 'a -> unit
end

This type allows references to be created with arbitrary read-write access. One can then create weaker views onto the reference – with access to fewer operations – by upcasting:

let read_only t = (t :> (_, read) Ref.t)
let write_only t = (t :> (_, write) Ref.t)

Note that the 'perms phantom type parameter should be contravariant: it's safe to discard permissions, but not to gain new ones.