option.opa

/*
    Copyright © 2011 MLstate

    This file is part of OPA.

    OPA is free software: you can redistribute it and/or modify it under the
    terms of the GNU Affero General Public License, version 3, as published by
    the Free Software Foundation.

    OPA is distributed in the hope that it will be useful, but WITHOUT ANY
    WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS
    FOR A PARTICULAR PURPOSE.  See the GNU Affero General Public License for
    more details.

    You should have received a copy of the GNU Affero General Public License
    along with OPA.  If not, see <http://www.gnu.org/licenses/>.
*/

/**
 * @author Valentin Gatien-Baron (documentation)
 * @destination public
 * @category Data
 * @stability Stable
 */

/**
 * {1 About this module}
 *
 * Contains the 'option' data type.
 * This data type allows you to represent arbitrary type with the ability
 * to indicate lack of value.
 * It is a type-safe(!) version of null/void values in many imperative languages.
 * It is most useful to create functions that may fail, or values that need a
 * dummy case.
 *
 * {1 Where should I start?}
 *
 * {1 What if I need more?}
 */

/**
 * {1 Types defined in this module}
 */

type either('a,'b) = {left:'a} / {right:'b}

@opacapi
type option('a) = { none } / { some : 'a }

/**
 * {1 Interface}
 */

@expand `?`(o, d) = match o : option with
    | { ~some } -> some
    | { none } -> d

Option = {{
  /**
   * A shorthand for [{none}]
   */
  none = {none} : option('a)

  /**
   * Wraps a value in the [some] case of an option
   */
  some = (v -> {some = v}) : 'a -> option('a)

  /**
   * [default(d,o)] returns the value contained in [o] if any, or [d]
   */
  default(d, o) = o ? d

  /**
   * [default(d,o)] returns the value contained in [o] if any, or [d()]
   */
  lazy_default = (d, o -> match o : option with
    | { ~some } -> some
    | { none } -> d()
  ) : (-> 'a), option('a) -> 'a

  /**
   * [lazy_switch(some_cb,none_cb,o)] returns
   * [lazy_default(none_cb,map(some_cb,o))]
   */
  lazy_switch = (some_callback, none_callback, o ->
    match o with
      | { ~some } -> some_callback(some)
      | { none } -> none_callback()
  ) : ('a -> 'b), (-> 'b), option('a) -> 'b

  /**
   * [switch(some_cb,none,o)] returns
   * [default(none,map(some_cb,o))]
   */
  switch = (some_callback, if_none, o ->
    match o with
      | { ~some } -> some_callback(some)
      | { none } -> if_none
  ) : ('a -> 'b), 'b, option('a) -> 'b

  /**
   * For continuation writing, when trying to get a some case
   */
  lazy_try = (callbacks, o ->
    match o with
    | {some = _ } -> o
    | {none} ->
      match callbacks with
      | [] -> none
      | [thunk|callbacks] -> lazy_try(callbacks,thunk())
  ) : list(-> option('a)), option('a) -> option('a)

  /**
   * [map(f,o)] returns [{none}] when [o] is [{none}], or [{some=f(a)}] when
   * [o] is [{some=a}]
   */
  map = (f, o -> match o : option with
    | { some = item } -> some(f(item))
    | { none = _ } -> none
  ) : ('a -> 'b), option('a) -> option('b)

  /**
   * [iter(f,o)] calls [f] on the content of [o], if it has any
   */
  iter = (f, o -> match o : option with
    | { some = item } -> f(item)
    | { none } -> void
  ) : ('a -> void), option('a) -> void

  /**
   * [bind(f, o)] returns [none] if [o] is [none] or [f(a)] if [o] is [some(a)]
   */
  bind = (f, o -> match o : option with
    | { some = item } -> f(item)
    | { none = _ } -> none
  ) : ('a -> option('b)), option('a) -> option('b)

  /**
   * Works like [get], except that you can choose the error message
   */
  get_msg = (fun_msg, o -> match o : option with
    | { ~some } -> some
    | { none = _ } -> error(fun_msg())
  ) : (-> string), option('a) -> 'a

  /**
   * [get(o)] returns the value of [o]
   * If [o] is [none], it exits with an error
   */
  get = get_msg(-> "Option.get called on \{none}",_) : option('a) -> 'a

  /**
   * [is_some(o)]
   * @return [{true}] if [o] is a [{some=_}]
   */
  is_some(o : option('a)) : bool =
    match o with
    | { some = _ } -> true
    | _ -> false

  /**
   * [is_none(o)]
   * @return [{true}] if [o] is [{none=_}]
   */
  is_none(o : option('a)) : bool =
    match o with
    | { none = _ } -> true
    | _ -> false

  ordering(using: order('a, 'b), x:option('a), y:option('a)): Order.ordering =
     match (x, y) with
          | ({none}, {none}) -> {eq}
          | ({none}, _) -> {lt}
          | (_, {none}) -> {gt}
          | ({some=a}, {some=b}) -> Order.compare(a,b,using)

  make_order(using: order('a, 'b)): order(option('a), 'b) =
     Order.make(ordering(using, _, _))

  merge( f:('a,'a -> 'a), x:option('a), y:option('a) ): option('a) =
     match (x, y) with
     | (_, {none}) -> x
     | ({none}, {some=_}) -> y
     | ({some=a}, {some=b}) -> {some=f(a,b)}


}}

@opacapi some    = Option.some
@opacapi none    = Option.none
default = Option.default