(* ################################################################# *)
(** * Inductively Defined Propositions *)

(** In the [Logic] chapter, we looked at several ways of writing
    propositions, including conjunction, disjunction, and quantifiers.
    In this chapter, we bring a new tool into the mix: _inductive
    definitions_.

    Recall that we have seen two ways of stating that a number [n] is
    even: We can say (1) [evenb n = true], or (2) [exists k, n =
    double k].  Yet another possibility is to say that [n] is even if
    we can establish its evenness from the following rules:

       - Rule [ev_0]:  The number [0] is even.
       - Rule [ev_SS]: If [n] is even, then [S (S n)] is even. *)

(** To illustrate how this definition of evenness works, let's
    imagine using it to show that [4] is even. By rule [ev_SS], it
    suffices to show that [2] is even. This, in turn, is again
    guaranteed by rule [ev_SS], as long as we can show that [0] is
    even. But this last fact follows directly from the [ev_0] rule. *)

(** We will see many definitions like this one during the rest
    of the course.  For purposes of informal discussions, it is
    helpful to have a lightweight notation that makes them easy to
    read and write.  _Inference rules_ are one such notation: *)
(**

                              ------------                        (ev_0)
                                 ev 0

                                  ev n
                             --------------                      (ev_SS)
                              ev (S (S n))
*)

(** Each of the textual rules above is reformatted here as an
    inference rule; the intended reading is that, if the _premises_
    above the line all hold, then the _conclusion_ below the line
    follows.  For example, the rule [ev_SS] says that, if [n]
    satisfies [ev], then [S (S n)] also does.  If a rule has no
    premises above the line, then its conclusion holds
    unconditionally.

    We can represent a proof using these rules by combining rule
    applications into a _proof tree_. Here's how we might transcribe
    the above proof that [4] is even: *)
(**

                             ------  (ev_0)
                              ev 0
                             ------ (ev_SS)
                              ev 2
                             ------ (ev_SS)
                              ev 4
*)

(** Why call this a "tree" (rather than a "stack", for example)?
    Because, in general, inference rules can have multiple premises.
    We will see examples of this below. *)

(** Putting all of this together, we can translate the definition of
    evenness into a formal Coq definition using an [Inductive]
    declaration, where each constructor corresponds to an inference
    rule: *)

Inductive ev : nat -> Prop :=
| ev_0 : ev 0
| ev_SS : forall n : nat, ev n -> ev (S (S n)).(* ################################################################# *)
(** * Inductively Defined Propositions *)

(** In the [Logic] chapter, we looked at several ways of writing
    propositions, including conjunction, disjunction, and quantifiers.
    In this chapter, we bring a new tool into the mix: _inductive
    definitions_.

    Recall that we have seen two ways of stating that a number [n] is
    even: We can say (1) [evenb n = true], or (2) [exists k, n =
    double k].  Yet another possibility is to say that [n] is even if
    we can establish its evenness from the following rules:

       - Rule [ev_0]:  The number [0] is even.
       - Rule [ev_SS]: If [n] is even, then [S (S n)] is even. *)

(** To illustrate how this definition of evenness works, let's
    imagine using it to show that [4] is even. By rule [ev_SS], it
    suffices to show that [2] is even. This, in turn, is again
    guaranteed by rule [ev_SS], as long as we can show that [0] is
    even. But this last fact follows directly from the [ev_0] rule. *)

(** We will see many definitions like this one during the rest
    of the course.  For purposes of informal discussions, it is
    helpful to have a lightweight notation that makes them easy to
    read and write.  _Inference rules_ are one such notation: *)
(**

                              ------------                        (ev_0)
                                 ev 0

                                  ev n
                             --------------                      (ev_SS)
                              ev (S (S n))
*)

(** Each of the textual rules above is reformatted here as an
    inference rule; the intended reading is that, if the _premises_
    above the line all hold, then the _conclusion_ below the line
    follows.  For example, the rule [ev_SS] says that, if [n]
    satisfies [ev], then [S (S n)] also does.  If a rule has no
    premises above the line, then its conclusion holds
    unconditionally.

    We can represent a proof using these rules by combining rule
    applications into a _proof tree_. Here's how we might transcribe
    the above proof that [4] is even: *)
(**

                             ------  (ev_0)
                              ev 0
                             ------ (ev_SS)
                              ev 2
                             ------ (ev_SS)
                              ev 4
*)

(** Why call this a "tree" (rather than a "stack", for example)?
    Because, in general, inference rules can have multiple premises.
    We will see examples of this below. *)

(** Putting all of this together, we can translate the definition of
    evenness into a formal Coq definition using an [Inductive]
    declaration, where each constructor corresponds to an inference
    rule: *)

Inductive ev : nat -> Prop :=
| ev_0 : ev 0
| ev_SS : forall n : nat, ev n -> ev (S (S n)).(* ################################################################# *)
(** * Inductively Defined Propositions *)

(** In the [Logic] chapter, we looked at several ways of writing
    propositions, including conjunction, disjunction, and quantifiers.
    In this chapter, we bring a new tool into the mix: _inductive
    definitions_.

    Recall that we have seen two ways of stating that a number [n] is
    even: We can say (1) [evenb n = true], or (2) [exists k, n =
    double k].  Yet another possibility is to say that [n] is even if
    we can establish its evenness from the following rules:

       - Rule [ev_0]:  The number [0] is even.
       - Rule [ev_SS]: If [n] is even, then [S (S n)] is even. *)

(** To illustrate how this definition of evenness works, let's
    imagine using it to show that [4] is even. By rule [ev_SS], it
    suffices to show that [2] is even. This, in turn, is again
    guaranteed by rule [ev_SS], as long as we can show that [0] is
    even. But this last fact follows directly from the [ev_0] rule. *)

(** We will see many definitions like this one during the rest
    of the course.  For purposes of informal discussions, it is
    helpful to have a lightweight notation that makes them easy to
    read and write.  _Inference rules_ are one such notation: *)
(**

                              ------------                        (ev_0)
                                 ev 0

                                  ev n
                             --------------                      (ev_SS)
                              ev (S (S n))
*)

(** Each of the textual rules above is reformatted here as an
    inference rule; the intended reading is that, if the _premises_
    above the line all hold, then the _conclusion_ below the line
    follows.  For example, the rule [ev_SS] says that, if [n]
    satisfies [ev], then [S (S n)] also does.  If a rule has no
    premises above the line, then its conclusion holds
    unconditionally.

    We can represent a proof using these rules by combining rule
    applications into a _proof tree_. Here's how we might transcribe
    the above proof that [4] is even: *)
(**

                             ------  (ev_0)
                              ev 0
                             ------ (ev_SS)
                              ev 2
                             ------ (ev_SS)
                              ev 4
*)

(** Why call this a "tree" (rather than a "stack", for example)?
    Because, in general, inference rules can have multiple premises.
    We will see examples of this below. *)

(** Putting all of this together, we can translate the definition of
    evenness into a formal Coq definition using an [Inductive]
    declaration, where each constructor corresponds to an inference
    rule: *)

Inductive ev : nat -> Prop :=
| ev_0 : ev 0
| ev_SS : forall n : nat, ev n -> ev (S (S n)).(** This definition is different in one crucial respect from
    previous uses of [Inductive]: its result is not a [Type], but
    rather a function from [nat] to [Prop] -- that is, a property of
    numbers.  Note that we've already seen other inductive definitions
    that result in functions, such as [list], whose type is [Type ->
    Type].  What is new here is that, because the [nat] argument of
    [ev] appears _unnamed_, to the _right_ of the colon, it is allowed
    to take different values in the types of different constructors:
    [0] in the type of [ev_0] and [S (S n)] in the type of [ev_SS].

    In contrast, the definition of [list] names the [X] parameter
    _globally_, to the _left_ of the colon, forcing the result of
    [nil] and [cons] to be the same ([list X]).  Had we tried to bring
    [nat] to the left in defining [ev], we would have seen an error: *)

Fail Inductive wrong_ev (n : nat) : Prop :=
| wrong_ev_0 : wrong_ev 0
| wrong_ev_SS : forall n, wrong_ev n -> wrong_ev (S (S n)).(** This definition is different in one crucial respect from
    previous uses of [Inductive]: its result is not a [Type], but
    rather a function from [nat] to [Prop] -- that is, a property of
    numbers.  Note that we've already seen other inductive definitions
    that result in functions, such as [list], whose type is [Type ->
    Type].  What is new here is that, because the [nat] argument of
    [ev] appears _unnamed_, to the _right_ of the colon, it is allowed
    to take different values in the types of different constructors:
    [0] in the type of [ev_0] and [S (S n)] in the type of [ev_SS].

    In contrast, the definition of [list] names the [X] parameter
    _globally_, to the _left_ of the colon, forcing the result of
    [nil] and [cons] to be the same ([list X]).  Had we tried to bring
    [nat] to the left in defining [ev], we would have seen an error: *)

Fail Inductive wrong_ev (n : nat) : Prop :=
| wrong_ev_0 : wrong_ev 0
| wrong_ev_SS : forall n, wrong_ev n -> wrong_ev (S (S n)).(** This definition is different in one crucial respect from
    previous uses of [Inductive]: its result is not a [Type], but
    rather a function from [nat] to [Prop] -- that is, a property of
    numbers.  Note that we've already seen other inductive definitions
    that result in functions, such as [list], whose type is [Type ->
    Type].  What is new here is that, because the [nat] argument of
    [ev] appears _unnamed_, to the _right_ of the colon, it is allowed
    to take different values in the types of different constructors:
    [0] in the type of [ev_0] and [S (S n)] in the type of [ev_SS].

    In contrast, the definition of [list] names the [X] parameter
    _globally_, to the _left_ of the colon, forcing the result of
    [nil] and [cons] to be the same ([list X]).  Had we tried to bring
    [nat] to the left in defining [ev], we would have seen an error: *)

Fail Inductive wrong_ev (n : nat) : Prop :=
| wrong_ev_0 : wrong_ev 0
| wrong_ev_SS : forall n, wrong_ev n -> wrong_ev (S (S n)).

Theorem ev_even_iff : forall n, ev n <-> exists k, n = double k.