Module Support.State

State monad definition.

The state monad is used to define stateful actions, i.e. actions that can read or write to some auxiliary data structure. In general, using the state monad is slower than using side effects because of extra memory allocations and eager closure constructions. For helpers to operations defined with respect to mutable states, see Imperative_state.

In order to effectively use the state monad, you should define:

1. An abstract module type definition for the state data structure. This acts as an interface between concrete implementations of the state data structure and modules that are defined with respect to that state. Even if only an immutable data structure is intended to be implemented for this abstract definition, its operations should be implementable using a mutable state.
2. A concrete implementation for that abstract state definition.
3. A functor taking an abstract state definition as argument, and producing operations that use that state.

For instance:

  module type DISAMBIGUATION_STATE = sig
    include State.STATE

    val with_bound_lf_variable : Identifier.t -> 'a t -> 'a t
    (* ... *)
  end

  module Immutable_disambiguation_state : sig
    include DISAMBIGUATION_STATE

    val initial_state : state
  end = struct
    (* ... *)
  end

  module Mutable_disambiguation_state : sig
    include DISAMBIGUATION_STATE

    val create_initial_state : unit -> state
  end = struct
    (* ... *)
  end

  module type LF_DISAMBIGUATION = sig
    include State.STATE

    val disambiguate_lf_kind : Synprs.lf_object -> Synext.lf_kind t
    (* ... *)
  end

  module Make_lf_disambiguation
      (Disambiguation_state : DISAMBIGUATION_STATE) :
    LF_DISAMBIGUATION with type state = Disambiguation_state.state =
  struct
    (* ... *)
  end