option.v 7 KB
Newer Older
1 2 3 4
(* Copyright (c) 2012, Robbert Krebbers. *)
(* This file is distributed under the terms of the BSD license. *)
(** This file collects general purpose definitions and theorems on the option
data type that are not in the Coq standard library. *)
5
Require Export base tactics decidable.
6 7 8

(** * General definitions and theorems *)
(** Basic properties about equality. *)
Robbert Krebbers's avatar
Robbert Krebbers committed
9 10 11 12 13 14
Lemma None_ne_Some `(a : A) : None  Some a.
Proof. congruence. Qed.
Lemma Some_ne_None `(a : A) : Some a  None.
Proof. congruence. Qed.
Lemma eq_None_ne_Some `(x : option A) a : x = None  x  Some a.
Proof. congruence. Qed.
15
Instance Some_inj {A} : Injective (=) (=) (@Some A).
Robbert Krebbers's avatar
Robbert Krebbers committed
16 17
Proof. congruence. Qed.

18 19
(** The non dependent elimination principle on the option type. *)
Definition option_case {A B} (f : A  B) (b : B) (x : option A) : B :=
Robbert Krebbers's avatar
Robbert Krebbers committed
20 21 22 23 24
  match x with
  | None => b
  | Some a => f a
  end.

25 26 27
(** The [maybe] function allows us to get the value out of the option type
by specifying a default value. *)
Definition maybe {A} (a : A) (x : option A) : A :=
28 29
  match x with
  | None => a
30
  | Some b => b
31 32
  end.

33 34
(** An alternative, but equivalent, definition of equality on the option
data type. This theorem is useful to prove that two options are the same. *)
Robbert Krebbers's avatar
Robbert Krebbers committed
35 36 37 38
Lemma option_eq {A} (x y : option A) :
  x = y   a, x = Some a  y = Some a.
Proof.
  split.
39
  { intros. by subst. }
40
  intros E. destruct x, y.
41 42 43 44
  + by apply E.
  + symmetry. by apply E.
  + by apply E.
  + done.
Robbert Krebbers's avatar
Robbert Krebbers committed
45 46
Qed.

47 48
Inductive is_Some {A} : option A  Prop :=
  make_is_Some x : is_Some (Some x).
Robbert Krebbers's avatar
Robbert Krebbers committed
49

50 51 52 53 54 55 56 57 58 59 60
Lemma make_is_Some_alt `(x : option A) a : x = Some a  is_Some x.
Proof. intros. by subst. Qed.
Hint Resolve make_is_Some_alt.
Lemma is_Some_None {A} : ¬is_Some (@None A).
Proof. by inversion 1. Qed.
Hint Resolve is_Some_None.

Lemma is_Some_alt `(x : option A) : is_Some x   y, x = Some y.
Proof. split. inversion 1; eauto. intros [??]. by subst. Qed.

Ltac inv_is_Some := repeat
Robbert Krebbers's avatar
Robbert Krebbers committed
61
  match goal with
62
  | H : is_Some _ |- _ => inversion H; clear H; subst
63
  end.
Robbert Krebbers's avatar
Robbert Krebbers committed
64

65
Definition is_Some_sigT `(x : option A) : is_Some x  { a | x = Some a } :=
Robbert Krebbers's avatar
Robbert Krebbers committed
66
  match x with
67 68 69 70 71 72 73 74
  | None => False_rect _  is_Some_None
  | Some a => λ _, a  eq_refl
  end.

Instance is_Some_dec `(x : option A) : Decision (is_Some x) :=
  match x with
  | Some x => left (make_is_Some x)
  | None => right is_Some_None
Robbert Krebbers's avatar
Robbert Krebbers committed
75 76 77
  end.

Lemma eq_None_not_Some `(x : option A) : x = None  ¬is_Some x.
78 79 80
Proof. split. by destruct 2. destruct x. by intros []. done. Qed.
Lemma not_eq_None_Some `(x : option A) : x  None  is_Some x.
Proof. rewrite eq_None_not_Some. intuition auto using dec_stable. Qed.
Robbert Krebbers's avatar
Robbert Krebbers committed
81

82
Lemma make_eq_Some {A} (x : option A) a :
Robbert Krebbers's avatar
Robbert Krebbers committed
83
  is_Some x  ( b, x = Some b  b = a)  x = Some a.
84
Proof. destruct 1. intros. f_equal. auto. Qed.
Robbert Krebbers's avatar
Robbert Krebbers committed
85

86 87 88
(** Equality on [option] is decidable. *)
Instance option_eq_dec `{dec :  x y : A, Decision (x = y)}
    (x y : option A) : Decision (x = y) :=
Robbert Krebbers's avatar
Robbert Krebbers committed
89 90 91 92 93 94
  match x with
  | Some a =>
    match y with
    | Some b =>
      match dec a b with
      | left H => left (f_equal _ H)
95
      | right H => right (H  injective Some _ _)
Robbert Krebbers's avatar
Robbert Krebbers committed
96 97 98 99 100 101 102 103 104 105
      end
    | None => right (Some_ne_None _)
    end
  | None =>
    match y with
    | Some _ => right (None_ne_Some _)
    | None => left eq_refl
    end
  end.

106
(** * Monadic operations *)
107 108
Instance option_ret: MRet option := @Some.
Instance option_bind: MBind option := λ A B f x,
Robbert Krebbers's avatar
Robbert Krebbers committed
109 110 111 112
  match x with
  | Some a => f a
  | None => None
  end.
113
Instance option_join: MJoin option := λ A x,
Robbert Krebbers's avatar
Robbert Krebbers committed
114 115 116 117
  match x with
  | Some x => x
  | None => None
  end.
118 119 120
Instance option_fmap: FMap option := @option_map.
Instance option_guard: MGuard option := λ P dec A x,
  if dec then x else None.
Robbert Krebbers's avatar
Robbert Krebbers committed
121

122 123
Lemma option_fmap_is_Some {A B} (f : A  B) (x : option A) :
  is_Some x  is_Some (f <$> x).
124
Proof. split; inversion 1. done. by destruct x. Qed.
125 126
Lemma option_fmap_is_None {A B} (f : A  B) (x : option A) :
  x = None  f <$> x = None.
127
Proof. unfold fmap, option_fmap. by destruct x. Qed.
Robbert Krebbers's avatar
Robbert Krebbers committed
128

129
Tactic Notation "simplify_option_equality" "by" tactic3(tac) := repeat
130
  match goal with
131 132 133 134 135 136 137
  | _ => first [progress simpl in * | progress simplify_equality]
  | H : mbind (M:=option) (A:=?A) ?f ?o = ?x |- _ =>
    let x := fresh in evar (x:A);
    let x' := eval unfold x in x in clear x;
    let Hx := fresh in
    assert (o = Some x') as Hx by tac;
    rewrite Hx in H; clear Hx
138
  | H : mbind (M:=option) ?f ?o = ?x |- _ =>
139 140
    match o with Some _ => fail 1 | None => fail 1 | _ => idtac end;
    match x with Some _ => idtac | None => idtac | _ => fail 1 end;
141
    destruct o eqn:?
142 143 144 145 146 147
  | |- mbind (M:=option) (A:=?A) ?f ?o = ?x =>
    let x := fresh in evar (x:A);
    let x' := eval unfold x in x in clear x;
    let Hx := fresh in
    assert (o = Some x') as Hx by tac;
    rewrite Hx; clear Hx
148 149 150 151 152 153
  | H : context C [@mguard option _ ?P ?dec _ ?x] |- _ =>
    let X := context C [ if dec then x else None ] in
    change X in H; destruct dec
  | |- context C [@mguard option _ ?P ?dec _ ?x] =>
    let X := context C [ if dec then x else None ] in
    change X; destruct dec
154
  end.
155
Tactic Notation "simplify_option_equality" := simplify_option_equality by eauto.
156 157

(** * Union, intersection and difference *)
Robbert Krebbers's avatar
Robbert Krebbers committed
158 159 160 161 162 163 164
Instance option_union: UnionWith option := λ A f x y,
  match x, y with
  | Some a, Some b => Some (f a b)
  | Some a, None => Some a
  | None, Some b => Some b
  | None, None => None
  end.
165
Instance option_intersection: IntersectionWith option := λ A f x y,
Robbert Krebbers's avatar
Robbert Krebbers committed
166 167 168 169
  match x, y with
  | Some a, Some b => Some (f a b)
  | _, _ => None
  end.
170 171 172 173 174 175
Instance option_difference: DifferenceWith option := λ A f x y,
  match x, y with
  | Some a, Some b => f a b
  | Some a, None => Some a
  | None, _ => None
  end.
Robbert Krebbers's avatar
Robbert Krebbers committed
176

177
Section option_union_intersection.
Robbert Krebbers's avatar
Robbert Krebbers committed
178 179 180
  Context {A} (f : A  A  A).

  Global Instance: LeftId (=) None (union_with f).
181
  Proof. by intros [?|]. Qed.
Robbert Krebbers's avatar
Robbert Krebbers committed
182
  Global Instance: RightId (=) None (union_with f).
183
  Proof. by intros [?|]. Qed.
Robbert Krebbers's avatar
Robbert Krebbers committed
184
  Global Instance: Commutative (=) f  Commutative (=) (union_with f).
185 186
  Proof.
    intros ? [?|] [?|]; compute; try reflexivity.
187
    by rewrite (commutative f).
188
  Qed.
Robbert Krebbers's avatar
Robbert Krebbers committed
189
  Global Instance: Associative (=) f  Associative (=) (union_with f).
190 191
  Proof.
    intros ? [?|] [?|] [?|]; compute; try reflexivity.
192
    by rewrite (associative f).
193
  Qed.
Robbert Krebbers's avatar
Robbert Krebbers committed
194
  Global Instance: Idempotent (=) f  Idempotent (=) (union_with f).
195 196
  Proof.
    intros ? [?|]; compute; try reflexivity.
197
    by rewrite (idempotent f).
198 199 200 201 202
  Qed.

  Global Instance: Commutative (=) f  Commutative (=) (intersection_with f).
  Proof.
    intros ? [?|] [?|]; compute; try reflexivity.
203
    by rewrite (commutative f).
204 205 206 207
  Qed.
  Global Instance: Associative (=) f  Associative (=) (intersection_with f).
  Proof.
    intros ? [?|] [?|] [?|]; compute; try reflexivity.
208
    by rewrite (associative f).
209 210 211 212
  Qed.
  Global Instance: Idempotent (=) f  Idempotent (=) (intersection_with f).
  Proof.
    intros ? [?|]; compute; try reflexivity.
213
    by rewrite (idempotent f).
214
  Qed.
215 216 217 218 219 220
End option_union_intersection.

Section option_difference.
  Context {A} (f : A  A  option A).

  Global Instance: RightId (=) None (difference_with f).
221
  Proof. by intros [?|]. Qed.
222
End option_difference.