lens.ml

(*
   Copyright (c) 2011-2012 Alessandro Strada

   Permission is hereby granted, free of charge, to any person obtaining a copy
   of this software and associated documentation files (the "Software"), to deal
   in the Software without restriction, including without limitation the rights
   to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
   copies of the Software, and to permit persons to whom the Software is
   furnished to do so, subject to the following conditions:

   The above copyright notice and this permission notice shall be included in all
   copies or substantial portions of the Software.

   THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
   IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
   FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
   AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
   LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
   OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
   SOFTWARE.
*)

let (|-) f g x = g (f x)

type ('a, 'b) t = {
  get : 'a -> 'b;
  set : 'b -> 'a -> 'a
}

let modify l f a =
  let value = l.get a in
  let new_value = f value in
  l.set new_value a

let _get a l = l.get a

let _set v a l = l.set v a

let _modify f l = modify l f

let compose l1 l2 = {
  get = l2.get |- l1.get;
  set = l1.set |- modify l2
}

let pair l1 l2 = {
  get = (fun (a, b) -> (l1.get a, l2.get b));
  set = (fun (a, c) (b, d) -> (l1.set a b, l2.set c d))
}

let pair3 l1 l2 l3 = {
  get = (fun (a, b, c) -> (l1.get a, l2.get b, l3.get c));
  set = (fun (a, c, e) (b, d, f) -> (l1.set a b, l2.set c d, l3.set e f))
}

let cond pred lt lf =
  let choose a = if pred a then lt else lf in
  { get = (fun a -> choose a |> _get a);
    set = (fun b a -> choose a |> _set b a)
  }

let get_state l =
  fun a -> _get a l, a

let put_state l v =
  fun a -> (), _set v a l

let modify_state l f =
  fun a -> (), _modify f l a

let ignore = {
  get = ignore;
  set = (fun _ a -> a)
}

let id = {
  get = (fun a -> a);
  set = (fun b a -> b)
}

let first = {
  get = fst;
  set = (fun v a -> v, snd a)
}

let second = {
  get = snd;
  set = (fun v a -> fst a, v)
}

let head = {
  get = List.hd;
  set = (fun v xs -> v :: List.tl xs)
}

let tail = {
  get = List.tl;
  set = (fun v xs -> List.hd xs :: v)
}

let for_hash key = {
  get = (fun h ->
      try
        Some (Hashtbl.find h key)
      with Not_found -> None);
  set = (fun v h ->
      match v with
        Some value -> Hashtbl.add h key value; h
      | None -> Hashtbl.remove h key; h)
}

let for_assoc key = {
  get = (fun l ->
      try
        Some (List.assoc key l)
      with Not_found -> None);
  set = (fun v l ->
      match v with
        Some value ->
        let l' = List.remove_assoc key l in
        (key, value) :: l'
      | None -> List.remove_assoc key l)
}

let for_array i = {
  get = (fun a -> Array.get a i);
  set = (fun v a -> let a' = Array.copy a in Array.set a' i v; a')
}

let for_list i = {
  get = (fun xs -> List.nth xs i);
  set = (fun v xs ->
      List.fold_left
        (fun (xs', j) x ->
           (if i = j then v::xs' else x::xs'), (j+1))
        ([], 0)
        xs |> fst |> List.rev)
}

let option_get = {
  get = (function None -> raise Not_found | Some v -> v);
  set = (fun v _ -> Some v)
}

let list_map l = {
  get = List.map l.get;
  set = List.map2 l.set
}

(* TODO: array_map *)

let xmap f g l = {
  get = l.get |- f;
  set = g |- l.set
}

module Infix =
struct
  let (|.) = _get

  let (^=) l v = fun a -> _set v a l

  let (^%=) = modify

  let (|--) l1 l2 = compose l2 l1

  let (--|) = compose

  let ( *** ) l1 l2 = pair l1 l2

  let (+=) l v = _modify ((+) v) l

  let (-=) l v = _modify ((-) v) l

end

module StateInfix =
struct
  let (^=!) l v = put_state l v

  let (+=!) l v = modify_state l ((+) v)

  let (-=!) l v = modify_state l ((-) v)

  let (@=!) l v = modify_state l (fun a -> a @ v)

end