# Library Ind

(* Induction
Version of 4/28/2009
*)

Require Export Poly.

• Midterm I is Wednesday Feb 18
• New convention for "exercise stars"...
• one star: very easy exercises that should be considered as REQUIRED (ideally they should be done while reading the lecture notes), but that are NOT TO BE HANDED IN (and will not be graded)
• two, three, or four stars: real homework problems that (unless explicitly marked "optional") should be done and handed in for grading
The 30,000 foot view...

What we've seen so far:
• inductive definitions of datatypes
• Fixpoints over inductive datatypes
• higher-order functions (map, filter, etc.)
• polymorphism
• basic Coq
• inductive proofs
• several fundamental tactics
Still to come (before Midterm I):
• more on induction: generalizing the IH, induction principles
• "programming with propositions"
• logical connectives as inductive propositions

# Programming with Propositions

A PROPOSITION is a factual claim. In Coq, propositions are written as expressions of type Prop.
(* Check (plus 2 2 = 4). *)
(* Check (ble_nat 3 2 = false). *)

Both provable and unprovable claims are perfectly good propositions. Simply BEING a proposition is one thing; being PROVABLE is something else!
(* Check (plus 2 2 = 4). *)
(* Check (plus 2 2 = 5). *)

We've seen one way that propositions can be used in Coq: in Theorem declarations.
Theorem plus_2_2_is_4 :
plus 2 2 = 4.
Proof. reflexivity. Qed.

Coq allows us to do many other things with propositions. For example, we can give a name to a proposition using a Definition.
Definition plus_fact : Prop := plus 2 2 = 4.

(* Check plus_fact. *)

Theorem plus_fact_is_true :
plus_fact.
Proof. unfold plus_fact. reflexivity. Qed.
Note that we need an unfold in the proof because plus_fact was introduced as a Definition.

So far, all the propositions we have seen are equality propositions. But we can build on equality propositions to make other sorts of claims. For example, what does it mean to claim that "a number n is even"? We have a function that (we believe) tests evenness, so one possible definition is "n is even iff (evenb n = true)."
Definition even (n:nat) :=
evenb n = true.

even is a PARAMETERIZED PROPOSITION. Think of it as a FUNCTION that, when applied to a number n, yields a proposition asserting that n is even.
(* Check even. *)
(* Check (even 4). *)

The type of even, nat->Prop, can be pronounced in two ways: either simply "even is a function from numbers to propositions" or, perhaps more helpfully, "even is a FAMILY of propositions, indexed by a number n."

Functions returning propositions are 100-percent first-class citizens in Coq. We can use them in other definitions:
Definition even_n__even_SSn (n:nat) :=
(even n) -> (even (S (S n))).

We can define them to take multiple arguments...
Definition between (n m o: nat) : Prop :=
andb (ble_nat n o) (ble_nat o m) = true.

... and then partially apply them:
Definition teen : nat->Prop := between 13 19.

We can pass propositions -- and even parameterized propositions -- as arguments to functions:
Definition true_for_zero (P:nat->Prop) : Prop :=
P 0.

Definition true_for_n__true_for_Sn (P:nat->Prop) (n:nat) : Prop :=
P n -> P (S n).

Definition preserved_by_S (P:nat->Prop) : Prop :=
forall n', P n' -> P (S n').

Definition true_for_all_numbers (P:nat->Prop) : Prop :=
forall n, P n.

Definition nat_induction (P:nat->Prop) : Prop :=
(true_for_zero P)
-> (preserved_by_S P)
-> (true_for_all_numbers P).

Let's unravel what this means in concrete terms:
Example nat_induction_example : forall (P:nat->Prop),
nat_induction P
= ( (P 0)
-> (forall n', P n' -> P (S n'))
-> (forall n, P n)).
Proof.
unfold nat_induction, true_for_zero, preserved_by_S, true_for_all_numbers.
reflexivity. Qed.

Theorem our_nat_induction_works : forall (P:nat->Prop),
nat_induction P.
Proof.
intros P.
unfold nat_induction.
intros TFZ PPS.
unfold true_for_all_numbers. intros n.
induction n as [| n'].
Case "n = O". apply TFZ.
Case "n = S n'". apply PPS. apply IHn'. Qed.

## Induction axioms

You may be puzzled by the last proof because it seems like we're using a built-in reasoning principle of induction over natural numbers to prove a theorem that basically just says the same thing. Indeed, this is exactly what we just did.

In general, every time we declare a new datatype with Inductive, Coq automatically generates an induction principle as an axiom (a theorem that we do not need to prove).

The induction principle for a type t is called t_ind. Here is the one for natural numbers:
(* Check nat_ind. *)
The ":" here can be pronounced "...is a theorem proving the proposition..."

Here's a more direct proof that our induction principle is valid, using the nat_ind axiom directly (with apply instead of induction).
Theorem our_nat_induction_works' :
forall P, nat_induction P.
Proof.
intros P.
unfold nat_induction, true_for_zero,
preserved_by_S, true_for_all_numbers.
apply nat_ind. Qed.

Indeed, we can apply nat_ind (instead of using induction) in ANY inductive proof.
Theorem mult_0_r' : forall n:nat,
mult n 0 = 0.
Proof.
apply nat_ind.
Case "O". reflexivity.
Case "S". simpl. intros n IHn. rewrite -> IHn.
simpl. reflexivity. Qed.

Some things to note:

• In the induction step of the proof we have to do a little manual bookkeeping (the intros)
• We do not introduce n into the context before applying nat_ind
• The apply tactic automatically chooses variable names for us (in the second subgoal, here), whereas induction gives us a way to specify what names should be used. The automatic choice is suboptimal.
All this makes inductive nicer in practice than using induction principles directly.

#### Exercise: 2 stars

Theorem plus_one_r' : forall n:nat,
plus n 1 = S n.
Proof.
(* Complete this proof without using the induction tactic. *)

#### Exercise: 2 stars

Our formulation of induction (the nat_induction proposition and the theorem stating that it works) can also be used directly to carry out proofs by induction.
Theorem plus_one_r'' : forall n:nat,
plus n 1 = S n.
Proof.
Prove the same theorem again without induction or apply nat_ind.

### Induction principles for other datatypes

We've looked in depth now at the induction principle for natural numbers. The induction principles that Coq generates for other datatypes defined with Inductive follow a very similar pattern. If we define a type t with constructors c1 ... cn, Coq generates an induction principle with this shape:

t_ind : forall P : t -> Prop, ... case for c1 ...
• > ... case for c2 ...
• > ...
• > ... case for cn ...
• > forall n : t, P n
The specific shape of each case depends on the arguments to the corresponding constructor. Before trying to write down a general rule, let's look at some more examples.

Inductive yesno : Set :=
| yes : yesno
| no : yesno.

(* Check yesno_ind. *)
Yields: yesno_ind : forall P : yesno -> Prop, P yes
• > P no
• > forall y : yesno, P y

#### Exercise: 1 star

Write out the induction principle that Coq will generate for the following datatype. Write down your answer on paper, and then compare it with what Coq prints.
Inductive rgb : Set :=
| red : rgb
| green : rgb
| blue : rgb.
(* Check rgb_ind. *)

Inductive natlist : Set :=
| nnil : natlist
| ncons : nat -> natlist -> natlist.
(* Check natlist_ind. *)
Yields (modulo a little tidying): natlist_ind : forall P : natlist -> Prop, P nnil
• > (forall (n : nat) (l : natlist), P l -> P (ncons n l))
• > forall n : natlist, P n

#### Exercise: 1 star

Suppose we had written the above definition a little differently:
Inductive natlist1 : Set :=
| nnil1 : natlist1
| nsnoc1 : natlist1 -> nat -> natlist1.
Now what will the induction principle look like?

From these examples, we can extract this general rule:

• each constructor c takes argument types a1...an
• each ai can be either t (the datatype we are defining) or some other type s
• the corresponding case of the induction principle says (in English),
"for all values x1...xn of types a1...an, if P holds for each of the xs of type t, then P holds for (c x1 ... xn)"

#### Exercise: 1 star (ExSet)

Here is an induction principle for an inductively defined set s.

ExSet_ind : forall P : ExSet -> Prop, (forall b : bool, P (con1 b))
• > (forall (n : nat) (e : ExSet), P e -> P (con2 n e))
• > forall e : ExSet, P e
Give an Inductive definition of ExSet:

Inductive ExSet : Set := (* FILL IN HERE

*)

(** Now, what about polymorphic datatypes?

The inductive definition of polymorphic lists

Inductive list (X:Set) : Set :=
| nil : list X
| cons : X -> list X -> list X.

is very similar.  The main difference is that, here, the
whole definition is PARAMTERIZED on a set X -- i.e., we
are defining a FAMILY of inductive types list X, one
for each X.  Note that, wherever list appears in the
body of the declaration, it is always applied to the
parameter X.  The induction principle is likewise
parameterized on X:

list_ind :
forall (X : Set) (P : list X -> Prop),
P
-> (forall (x : X) (l : list X), P l -> P (x :: l))
-> forall l : list X, P l

Note the wording here (and, accordingly, the form of
list_ind): The WHOLE induction principle is
parameterized on X.  That is, list_ind can be thought
of as a polymorphic function that, when applied to a set
X, gives us back an induction principle specialized to
list X. *)

(** **** Exercise: 1 star *)
(** Write out the induction principle that Coq will
generate for the following datatype.  Compare your
answer with what Coq prints. *)

Inductive tree (X:Set) : Set :=
| leaf : X -> tree X
| node : tree X -> tree X -> tree X.
(* Check tree_ind. *)

#### Exercise: 1 star (mytype)

Find an inductive definition that gives rise to the following induction principle:

mytype_ind : forall (X : Set) (P : mytype X -> Prop), (forall x : X, P (constr1 X x))
• > (forall n : nat, P (constr2 X n))
• > (forall m : mytype X, P m -> forall n : nat, P (constr3 X m n))
• > forall m : mytype X, P m

#### Exercise: 1 star (foo)

Find an inductive definition that gives rise to the following induction principle:

foo_ind : forall (X Y : Set) (P : foo X Y -> Prop), (forall x : X, P (bar X Y x))
• > (forall y : Y, P (baz X Y y))
• > (forall f1 : nat -> foo X Y, (forall n : nat, P (f1 n)) -> P (quux X Y f1))
• > forall f2 : foo X Y, P f2

#### Exercise: 1 star

Consider the following inductive definition:
Inductive foo' (X:Set) : Set :=
| C1 : list X -> foo' X -> foo' X
| C2 : foo' X.

What induction principle will Coq generate for foo'? (FILL IN THE BLANKS, then check your answer with Coq.)

foo'_ind : forall (X : Set) (P : foo' X -> Prop), (forall (l : list X) (f : foo' X), ______________________ -> _______________________)
• > _________________________________________________
• > forall f : foo' X, _______________________________

### Induction hypotheses

The induction principle for numbers

forall P : nat -> Prop, P 0
• > (forall n : nat, P n -> P (S n))
• > forall n : nat, P n
is a generic statement that holds for all propositions P (strictly speaking, for all families of propositions P indexed by a number n). Each time we use this principle, we are choosing P to be a particular expression of type nat->Prop.

We can make the proof more explicit by giving this expression a name.

Definition P_m0r (n:nat) : Prop :=
mult n 0 = 0.

... or equivalently...

Definition P_m0r' : nat->Prop :=
fun n => mult n 0 = 0.

Theorem mult_0_r'' : forall n:nat,
P_m0r n.
Proof.
apply nat_ind.
Case "n = O". unfold P_m0r. reflexivity.
Case "n = S n'".
(* Note the proof state at this point! *)
unfold P_m0r. simpl. intros n IHn.
rewrite -> IHn. reflexivity. Qed.

### A closer look at the induction tactic

The induction tactic actually does quite a bit of low-level bookkeeping for us.

Recall the informal statement of the induction principle for natural numbers:

If P n is some proposition involving a natural number n, and we want to show that P holds for ALL numbers n, we can reason like this:

• show that P O holds
• show that, if P n' holds, then so does P (S n')
• conclude that P n holds for all n.
So, when we begin a proof with intros n and then induction n, we are first telling Coq to consider a PARTICULAR n (by introducing it into the context) and then telling it to prove something about ALL numbers (by using induction).

What Coq actually does in this situation, internally, is to "re-generalize" the variable we perform induction on. For example, in the proof above that plus is associative...
Theorem plus_assoc' : forall n m p : nat,
plus n (plus m p) = plus (plus n m) p.
Proof.
...we first introduce all 3 variables into the context, which amounts to saying "Consider an arbitrary n, m, and p..."
intros n m p.
...We now use the induction tactic to prove P n (that is, plus n (plus m p) = plus (plus n m) p) for ALL n, and hence also for the particular n that is in the context at the moment.
induction n as [| n'].
Case "n = O". reflexivity.
Case "n = S n'".
In the second subgoal generated by induction -- the "inductive step" -- we must prove that P n' implies P (S n') for all n'. The induction tactic automatically introduces n' and P n' into the context for us, leaving just P (S n') as the goal.
simpl. rewrite -> IHn'. reflexivity. Qed.

It also works to apply induction to a variable that is quantified in the goal.
Theorem plus_comm' : forall n m : nat,
plus n m = plus m n.
Proof.
induction n as [| n'].
Case "n = O". intros m. rewrite -> plus_0_r. reflexivity.
Case "n = S n'". intros m. simpl. rewrite -> IHn'.
rewrite <- plus_n_Sm. reflexivity. Qed.
Note that induction n leaves m still bound in the goal -- i.e., what we are proving inductively is a statement beginning with forall m.

If we do induction on a variable that is quantified in the goal AFTER some other quantifiers, the induction tactic will automatically introduce these quantifiers into the context.
Theorem plus_comm'' : forall n m : nat,
plus n m = plus m n.
Proof.
Let's do induction on m this time, instead of n...
induction m as [| m'].
Case "m = O". simpl. rewrite -> plus_0_r. reflexivity.
Case "m = S m'". simpl. rewrite <- IHm'.
rewrite <- plus_n_Sm. reflexivity. Qed.

#### Exercise: 1 star (plus_comm)

Rewrite the previous two theorems and their proofs in the same style as mult_0_r'' above -- i.e., give, for each, an explicit Definition of the proposition being proved by induction and state the theorem and proof in terms of this defined proposition.

A quick digression, for adventurous souls... If we can define parameterized propositions using Definition, then can we also use Fixpoint? Of course we can! However, this kind of "recursive parameterization" doesn't correspond to anything very familiar from everyday mathematics. The following exercise gives a slightly contrived example.

#### Exercise: 4 stars, optional

```(* Define a recursive function [true_upto_n_implies_true_everywhere]
that makes [true_upto_n_example] work. *)
Fixpoint true_upto_n__true_everywhere
(* OPTEXERCISE...*)
(n:nat) (P:nat->Prop) {struct n} : Prop :=
match n with
| 0 =>    (forall m, P m)
| S n' => (P (S n') -> true_upto_n__true_everywhere n' P)
end.

Example true_upto_n_example :
(true_upto_n__true_everywhere 3 (fun n => even n))
= (even 3 -> even 2 -> even 1 -> forall m : nat, even m).
Proof. reflexivity.  Qed.
```

## Generalizing induction hypotheses

Last week's homework included a proof that the double function is injective. The way we START this proof is a little bit delicate: if we begin it with intros n. induction n., all is well. But if we begin it with intros n m. induction n., we get stuck in the middle of the inductive case...
Theorem double_injective_FAILED : forall n m,
double n = double m
-> n = m.
Proof.
intros n m. induction n as [| n'].
Case "n = O". simpl. intros eq. destruct m as [| m'].
SCase "m = O". reflexivity.
SCase "m = S m'". inversion eq.
Case "n = S n'". intros eq. destruct m as [| m'].
SCase "m = O". inversion eq.
SCase "m = S m'".
assert (n' = m') as H.
SSCase "Proof of assertion".
Here we are stuck. We need the assertion in order to rewrite the final goal (subgoal 2 at this point) to an identity. But the induction hypothesis, IHn', does not give us n' = m' -- there is an extra S in the way -- so the assertion is not provable.
What went wrong here?

The problem is that, at the point we invoke the induction hypothesis, we have already introduced m into the context -- intuitively, we have told Coq, "Let's consider some particular n and m..." and we now have to prove that, if double n = double m for this *this particular* n and m, then n = m.

The next tactic, induction n says to Coq: We are going to show the goal by induction on n. That is, we are going to prove that

P n = "if double n = double m, then n = m"

holds for all n by showing

• P O (i.e., "if double O = double m then O = m")
• P n -> P (S n) (i.e., "if double n = double m then n = m" implies "if double (S n) = double m then S n = m").
If we look closely at the second statement, it is saying something rather strange: it says that, for any *particular* m, if we know

"if double n = double m then n = m"

then we can prove

"if double (S n) = double m then S n = m".

To see why this is strange, let's think of a particular m -- say, 5. The statement is then saying that, if we can prove

Q = "if double n = 10 then n = 5"

then we can prove

R = "if double (S n) = 10 then S n = 5".

But knowing Q doesn't give us any help with proving R! (If we tried to prove R from Q, we would say something like "Suppose double (S n) = 10..." but then we'd be stuck: knowing that double (S n) is 10 tells us nothing about whether double n is 10, so Q is useless at this point.)

To summarize: Trying to carry out this proof by induction on n when m is already in the context doesn't work because we are trying to prove a relation involving *every* n but just a *single* m.

The good proof of double_injective leaves m in the goal statement at the point where the induction tactic is invoked on n:
Theorem double_injective' : forall n m,
double n = double m
-> n = m.
Proof.
intros n. induction n as [| n'].
Case "n = O". simpl. intros m eq. destruct m as [| m'].
SCase "m = O". reflexivity.
SCase "m = S m'". inversion eq.
Case "n = S n'".
Notice that both the goal and the induction hypothesis have changed: the goal asks us to prove something more general (i.e., to prove the statement for *every* m), but the IH is correspondingly more flexible, allowing us to choose any m we like when we apply the IH.
intros m eq.
Now we choose a particular m and introduce the assumption that double n = double m. Since we are doing a case analysis on n, we need a case analysis on m to keep the two "in sync".
destruct m as [| m'].
SCase "m = O". inversion eq. (* The 0 case is trivial *)
SCase "m = S m'".
At this point, since we are in the second branch of the destruct m, the m' mentioned in the context at this point is actually the predecessor of the one we started out talking about. Since we are also in the S branch of the induction, this is perfect: if we instantiate the generic m in the IH with the m' that we are talking about right now (this instantiation is performed automatically by apply), then IHn' gives us exactly what we need to finish the proof.
assert (n' = m') as H.
SSCase "Proof of assertion". apply IHn'.
inversion eq. reflexivity.
rewrite -> H. reflexivity. Qed.

So what we've learned is that we need to be careful about using induction to try to prove something too specific: If we're proving a property of n and m by induction on n, we may need to leave m generic.

However, this strategy doesn't always apply directly; sometimes a little rearrangement is needed. Suppose, for example, that we had decided we wanted to prove double_injective by induction on m instead of n.
Theorem double_injective_take2_FAILED : forall n m,
double n = double m
-> n = m.
Proof.
intros n m. induction m as [| m'].
Case "m = O". simpl. intros eq. destruct n as [| n'].
SCase "n = O". reflexivity.
SCase "n = S n'". inversion eq.
Case "m = S m'". intros eq. destruct n as [| n'].
SCase "n = O". inversion eq.
SCase "n = S n'".
assert (n' = m') as H.
SSCase "Proof of assertion".
Here we are stuck again, just like before.

The problem is that, to do induction on m, we must first introduce n. (If we simply say induction m without introducing anything first, Coq will automatically introduce n for us!) What can we do about this?

One possibility is to rewrite the statement of the lemma so that m is quantified before n. This will work, but it's not nice: We don't want to have to mangle the statements of lemmas to fit the needs of a particular strategy for proving them -- we want to state them in the most clear and natural way.

What we can do instead is to first introduce all the quantified variables and then RE-GENERALIZE one or more of them, taking them out of the context and putting them back at the beginning of the goal. The generalize dependent tactic does this.
Theorem double_injective_take2 : forall n m,
double n = double m
-> n = m.
Proof.
intros n m.
n and m are both in the context
generalize dependent n.
Now n is back in the goal and we can do induction on m and get a sufficiently general IH.
induction m as [| m'].
Case "m = O". simpl. intros n eq. destruct n as [| n'].
SCase "n = O". reflexivity.
SCase "n = S n'". inversion eq.
Case "m = S m'". intros n eq. destruct n as [| n'].
SCase "n = O". inversion eq.
SCase "n = S n'".
assert (n' = m') as H.
SSCase "Proof of assertion".
apply IHm'. inversion eq. reflexivity.
rewrite -> H. reflexivity. Qed.
Let's look at an informal proof of this theorem. Note that the proposition we prove by induction leaves n quantified, corresponding to the use of generalize dependent in our formal proof.

Theorem: For any nats n and m, if double n = double m, then n = m

Proof: Let a nat m be given. We prove:

For any n, if double n = double m then n = m

by induction on m.

In the base case, we have m = 0. Let a nat n be given such that double n = double m. Since m = 0, we have double n = 0. If n = S n' for some n', then double n = S (S (double n')) by the definition of double. This would be a contradiction of the assumption that double n = 0, so n = 0, and thus n = m.

In the inductive case, we have m = S m' for some nat m'. Let a nat n be given such that double n = double m. By the definition of double, we therefore have: double n = S (S (double m'))

If n = 0, then double n = 0 (by the definition of double), which we have just seen is not the case. Thus, n = S n' for some n', and we have: S (S (double n')) = S (S (double m')) which implies that double n' = double m'.

But observe that our inductive hypothesis here is: for any n, if double n = double m' then n = m'

Applying this for n' then yields n' = m', and it follows directly that S n' = S m'. Since S n' = n and S m' = m, this is just what we wanted to show.

#### Exercise: 3 stars (gen_dep_practice)

Theorem plus_n_n_injective_take2 : forall n m,
plus n n = plus m m
-> n = m.
Proof.
Carry out this proof by induction on m.

Theorem index_after_last : forall (n : nat) (X : Set)
(l : list X),
length l = n
-> index (S n) l = None.
Proof.
(* Prove this by induction on l *)

#### Exercise: 3 stars (index_after_last_informal)

(* Write an informal proof corresponding to your coq proof
of index_after_last:

Theorem: For any nat n and list l, if length l = n then
index (S n) l = None.

Proof:
(* FILL IN HERE *)
*)

#### Exercise: 3 stars (gen_dep_practice_opt)

Theorem length_snoc''' : forall (n : nat) (X : Set)
(v : X) (l : list X),
length l = n
-> length (snoc l v) = S n.
Proof.
(* Prove this by induction on l. *)

#### Exercise: 3 stars, optional

Theorem eqnat_false_S : forall n m,
beq_nat n m = false -> beq_nat (S n) (S m) = false.
Proof.
(* Prove this by induction on m. *)

#### Exercise: 3 stars, optional

Theorem length_append_cons : forall (X : Set) (l1 l2 : list X)
(x : X) (n : nat),
length (l1 ++ (x :: l2)) = n
-> S (length (l1 ++ l2)) = n.
Proof.
(* Prove this by induction on l1, without using app_length. *)

#### Exercise: 4 stars

Theorem length_appendtwice : forall (X:Set) (n:nat)
(l:list X),
length l = n
-> length (l ++ l) = plus n n.
Proof.
(* Prove this by induction on l, without using app_length. *)

## Constructing Evidence

One of the first examples in the discussion of propositions involved the concept of evenness. It is important to notice that we have two rather different ways of talking about this concept:

• a computation evenb n that CHECKS evenness, yielding a boolean
• a proposition even n (defined in terms of evenb) that ASSERTS that n is even.
There is another way of defining what it means to say that a number is even. Instead of going "indirectly" via the evenb function, we can give a "direct" definition of evenness by saying, straight out, what we would be willing to accept as EVIDENCE that a given number is even.

Inductive ev : nat -> Prop :=
| ev_0 : ev O
| ev_SS : forall n:nat, ev n -> ev (S (S n)).

Informally, this definition says that there are two ways to give evidence that a number m is even: observe that it is zero, or observe that n = S (S m) for some m, if we have evidence that m itself is even. The constructors ev_0 and ev_SS are names for these different ways of giving evidence for evenness.

The role of ":" here is interesting. Before, we've written "e : t" in situations where either e is an expression denoting a value and t is the type of this value or else where e is an expression describing some set and t is Set. In both cases, the ":" is pronounced "is a" or, more formally, "is classified by" or "has type." Here, the type is a proposition and the thing that has that type is evidence for the proposition.

In other words, we are thinking of a proposition as analogous to a set... a set of PROOFS (or evidence)! Saying "e has type P" (where P is a proposition) and saying "e is a proof of P" are exactly the same thing.

The analogy

propositions ~ sets proofs ~ data values

is called the CURRY-HOWARD CORRESPONDENCE (or, sometimes, the CURRY-HOWARD ISOMORPHISM).

A great many things follow from it.

First, just as a set can be empty, a singleton, finite, or infinite, a proposition may be "inhabited" by zero, one, many, or infinitely many proofs. This makes sense: each inhabitant of a proposition P is a different way of giving evidence for P. If there are none, then P is not provable. If there are many, then P has many different proofs.

Second, the Inductive construction means exactly the same thing whether we are using it to define sets of data values (in the Set world) or sets of evidence. Let's look at the parts of the inductive definition of ev above:
• The first line declares that ev is a proposition indexed by a natural number
• The second line declares that the constructor ev_0 can be taken as evidence for the assertion ev O.
• The third line declares that, if n is a number and E is evidence for the assertion ev n), then ev_SS n E can be taken as evidence for ev (S (S n)). That is, we can construct evidence that S (S n) is even by applying the constructor ev_SS to evidence that n is even.
For example, here is a proof that 4 is even:

Definition four_ev : ev 4 :=
ev_SS 2 (ev_SS 0 ev_0).

#### Exercise: 1 star

What are the 2 and 0 doing there?

Hang on a minute! Until now, we've constructed proofs by giving sequences of tactics that eventually caused Coq to say "Proof completed." Is this something new?

No, it is not. Here's the real story. In Coq, a proof of a proposition P is a precisely term of type P. So the fact that the above Definition typechecks means that four_ev really is a proof of ev 4. The other way of giving proofs in Coq -- using Theorem... Proof... Qed. -- is just a different notation for doing exactly the same thing. To see this, let's write a proof that four is even in the more familiar way.

Theorem four_ev' :
ev 4.
Proof.
apply ev_SS. apply ev_SS. apply ev_0. Qed.
It's worth working through this proof slowly to make sure you understand what each apply is doing and why it makes sense to use apply with the constructors of ev instead of with hypotheses and theorems, as we've been doing.

Notice the similarity between the first two lines of this theorem and the previous Definition of four_ev: they can be read "four_ev' is evidence for the proposition ev 4." But there's more. The theorem actually builds a PROOF OBJECT that embodies the evidence that 4 is even. Let's look at it:

(* Print four_ev'. *)

Proof objects are the "bedrock" of reasoning in Coq. Tactic proofs are convenient shorthands that instruct Coq how to construct proof objects for us instead of our writing them out explicitly. (Here, of course, the proof object is actually shorter than the tactic proof. But the proof objects for more interesting proofs can become quite large and complex -- building them by hand would be painful.)

Third, the correspondence between values and proofs induces a strong correpondence between FUNCTIONS and PROOFS OF HYPOTHETICAL PROPOSITIONS. For example, consider the proposition

forall n, ev n -> ev (plus 4 n).

This can be read "assuming we have evidence that n is even, we can construct evidence that plus 4 n is even." Viewed differently, it is the type of a function taking a number n and evidence that n is even and yielding evidence that plus 4 n is even.

Definition ev_plus4 : forall n, ev n -> ev (plus 4 n) :=
fun (n : nat) =>
fun (E : ev n) =>
ev_SS (S (S n)) (ev_SS n E).

For comparison, here is a tactic proof of the same proposition.
Theorem ev_plus4' : forall n,
ev n -> ev (plus 4 n).
Proof.
intros n E. simpl. apply ev_SS. apply ev_SS. apply E. Qed.

Again, the proof object constructed by this tactic proof is identical to the one that we built by hand.
(* Print ev_plus4'. *)

Fourth, just as hypothetical propositions have "arrow implication types", the USE of a hypothetical proposition exactly corresponds to function application.

For example, consider this theorem:
Definition ev_ten : ev 10 :=
ev_plus4 6 (ev_plus4 2 (ev_SS 0 ev_0)).

Fifth, and finally, SIMPLIFICATION of expressions denoting data values also has an analog in the world of propositions and proofs. Just as the computational expression plus 4 (plus 4 2) simplifies to 10...
(* Eval simpl in (plus 4 (plus 4 2)). *)
... the proof term ev_ten, which contains two applications of the theorem ev_plus4 (which we defined above as a function), can be simplified to a direct proof of the same proposition:
(* Eval compute in (ev_plus4 6 (ev_plus4 2 (ev_SS 0 ev_0))). *)
(Note that we have to use the more aggressive Eval compute rather than just Eval simpl to get Coq to unfold the definitions here.)

To summarize: The Curry-Howard Correspondence is a powerful conceptual tool, establishing deep and fruitful links between programming languages and mathematical logic. Closer to home, it is the backbone for the organization of Coq.

PICTURE GOES HERE

Of course, this is just the backbone. There are lots of other types (and their inhabitants) that this picture doesn't show. For example, there is Set->Set, which is the type of things like list, there is (nat->Prop)->Prop, the type of things like nat_ind, and so on.

#### Exercise: 2 stars

Construct a tactic proof of the following proposition.
Theorem double_even : forall n,
ev (double n).
Proof.

#### Exercise: 4 stars, optional (double_even_pfobj)

Try to predict what proof object is constructed by the above tactic proof. (Before checking your answer, you'll want to strip out any uses of Case, as these will make the proof object a bit cluttered.)

### Manipulating Evidence

Besides CONSTRUCTING evidence of evenness, we can also REASON ABOUT evidence of evenness. The fact that we introduced ev with an Inductive declaration tells us not only that the constructors ev_O and ev_SS are ways to build evidence of evenness, but also that these two constructors are the ONLY ways that evidence of evenness can be built.

In other words, if someone gives us evidence E justifying the assertion ev n, then we know that E can only have one of two forms: either E is ev_0 (and n is O), or E is ev_SS n' E' (and n is S (S n')) and E' is evidence that n' is even.

Thus, it makes sense to use all the tactics that we have already seen for inductively defined DATA to reason instead about inductively defined EVIDENCE.

For example, here we use a destruct on evidence that n is even in order to show that ev n implies ev (n-2). (Recall that pred 0 is 0.)
Theorem ev_minus2: forall n,
ev n -> ev (pred (pred n)).
Proof.
intros n E.
destruct E as [| n' E'].
Case "E = ev_0". simpl. apply ev_0.
Case "E = ev_SS n' E'". simpl. apply E'. Qed.

#### Exercise: 1 star (ev_minus2_n)

What happens if we try to destruct on n instead of E?

We can also perform INDUCTION on evidence that n is even. Here we use it show that the old evenb function returns true on n when n is even according to ev.
Theorem ev_even : forall n,
ev n -> even n.
Proof.
intros n E. induction E as [| n' E'].
Case "E = ev_0".
unfold even. reflexivity.
Case "E = ev_SS n' E'".
unfold even. simpl. unfold even in IHE'. apply IHE'. Qed.

#### Exercise: 1 star (ev_even_n)

Could this proof be carried out by induction on n instead of E?

#### Exercise: 3 stars (l_fails)

(* The following proof attempt will not succeed.

Theorem l : forall n,
ev n.
Proof.
intros n. induction n.
Case "O". simpl. apply ev_0.
Case "S".
...

Briefly explain why.

(* FILL IN HERE *)
*)

#### Exercise: 2 stars

Here's another exercise requiring induction.
Theorem ev_sum : forall n m,
ev n -> ev m -> ev (n+m).
Proof.

Here's another situation where we want analyze evidence for evenness and build two sub-goals.
Theorem SSev_ev_firsttry : forall n,
ev (S (S n)) -> ev n.
Proof.
intros n E.
destruct E as [| n' E'].
(* Oops. destruct destroyed far too much!
In the first sub-goal, we don't know that n is 0. *)

The right thing to use here is inversion (!)
Theorem SSev_even : forall n,
ev (S (S n)) -> ev n.
Proof.
intros n E. inversion E as [| n' E']. apply E'. Qed.
(* Print SSev_even. *)
This use of inversion may seem a bit mysterious at first. Until now, we've only used inversion on equality propositions, to utilize injectivity of constructors or to discriminate between different constructors. But we see here that inversion can also be applied to analyzing evidence for inductively defined propositions.

Here's how inversion works in general. Suppose the name I refers to an assumption P in the current context, where P has been defined by an Inductive declaration. Then, for each of the constructors of P, inversion I generates a subgoal in which I has been replaced by the exact, specific conditions under which this constructor could have been used to prove P. Some of these subgoals will be self-contradictory; inversion throws these away. The ones that are left represent the cases that must be proved to establish the original goal.

In this particular case, the inversion analyzed the construction ev (S (S n)), determined that this could only have been constructed using ev_SS, and generated a new subgoal with the arguments of that constructor as new hypotheses. (It also produced an auxiliary equality, which happens to be useless.) We'll begin exploring this more general behavior of inversion in what follows.

#### Exercise: 1 star (inversion_practice)

Theorem SSSSev_even : forall n,
ev (S (S (S (S n)))) -> ev n.
Proof.

inversion can also be used to derive goals by showing absurdity of a hypothesis.
Theorem even5_nonsense :
ev 5 -> plus 2 2 = 9.
Proof.

We can generally use inversion instead of destruct on inductive propositions. This illustrates that in general, we get one case for each possible constructor. Again, we also get some auxiliary equalities which are rewritten in the goal but not in the other hypotheses.
Theorem ev_minus2': forall n,
ev n -> ev (pred (pred n)).
Proof.
intros n E. inversion E as [| n' E'].
Case "E = ev_0". simpl. apply ev_0.
Case "E = ev_SS n' E'". simpl. apply E'. Qed.

#### Exercise: 3 stars

Finding the appropriate thing to do induction on is a bit tricky here.
Theorem ev_ev_even : forall n m,
ev (n+m) -> ev n -> ev m.
Proof.

### More on inductively defined propositions

We have seen that the proposition "some number is even" can be phrased in two different ways -- indirectly, via a testing function evenb, or directly, by inductively describing what constitutes evidence for evenness. These two ways of defining evenness are about equally easy to state and work with.

However, for many other properties of interest, the direct inductive definition is preferable, since writing a testing function may be awkward or even impossible. For example, consider the property MyProp defined as follows:

1) the number 4 has property MyProp 2) if n has property MyProp, then so does 4+n 3) if n+2 has property MyProp, then so does n 4) no other numbers have property MyProp

This is a perfectly sensible definition of a set of numbers, but we cannot translate this definition directly as a Coq Fixpoint (or translate it directly into a recursive function in any other programming language). We might be able to find a clever way of testing this property using a Fixpoint (indeed, it is not even too hard to find one in this case), but in general this could require arbitrarily much thinking.

In fact, if the property we are interested in is uncomputable, then we cannot define it as a Fixpoint no matter how hard we try, because Coq requires that all Fixpoints correspond to terminating computations.

On the other hand, writing an inductive definition of what it means to give evidence for the property MyProp is straightforward:
Inductive MyProp : nat -> Prop :=
| MyProp1 : MyProp 4
| MyProp2 : forall n:nat, MyProp n -> MyProp (plus 4 n)
| MyProp3 : forall n:nat, MyProp (plus 2 n) -> MyProp n.
The first three clauses in the informal definition of MyProp above are reflected in the first three clauses of the inductive definition. The fourth clause is the precise force of the keyword Inductive.

As we did with evenness, we can now construct evidence that certain numbers satisfy MyProp.
Theorem MyProp_ten : MyProp 10.
Proof.
apply MyProp3. simpl.
assert (12 = plus 4 8) as H12.
Case "Proof of assertion". reflexivity.
rewrite -> H12.
apply MyProp2.
assert (8 = plus 4 4) as H8.
Case "Proof of assertion". reflexivity.
rewrite -> H8.
apply MyProp2.
apply MyProp1. Qed.

#### Exercise: 2 stars (MyProp)

Here are two useful facts about MyProp. (The proofs are left to you.)
Theorem MyProp_0 : MyProp 0.
Proof.

Theorem MyProp_plustwo : forall n:nat, MyProp n -> MyProp (S (S n)).
Proof.

With these, we can show that MyProp holds of all even numbers, and vice versa.
Theorem MyProp_ev : forall n:nat,
ev n -> MyProp n.
Proof.
intros n E.
induction E as [| n' E'].
Case "E = ev_0".
apply MyProp_0.
Case "E = ev_SS n' E'".
apply MyProp_plustwo. apply IHE'. Qed.

Here's an informal proof of this theorem:

Theorem : For any nat n, if ev n then MyProp n.

Proof: Let a nat n and a derivation E of ev n be given. We go by induction on E. There are two case:

1) If the last step in E is a use of ev_0, then n is 0. But we know by lemma MyProp_0 that MyProp 0, so the theorem is satisfied in this case.

2) If the last step in E is by ev_SS, then n = S (S n') for some n' and there is a derivation of ev n'. By lemma MyProp_plustwo, it's enough to show MyProp n'. This follows directly from the inductive hypothesis for the derivation of (ev n').

#### Exercise: 3 stars

Theorem ev_MyProp : forall n:nat,
MyProp n -> ev n.
Proof.

(* EXERCISE: Write an informal proof corresponding to your
formal proof of ev_MyProp:

Theorem: For any nat n, if MyProp n then ev n.

Proof:
(* FILL IN HERE *)
*)

#### Exercise: 4 stars, optional (MyProp_pfobj)

(* Prove MyProp_ev and ev_MyProp again by constructing
explicit proof objects by hand (as we did above in
ev_plus4, for example). *)

(* FILL IN HERE *)

#### Exercise: 3 stars

Theorem ev_MyProp' : forall n:nat,
MyProp n -> ev n.
Proof.
(* Complete this proof using apply MyProp_ind instead
of the induction tactic. *)

(* --------------------------------------------------- *)
(* Induction principles for inductively defined Props *)

The induction principles for inductively defined propositions like ev are a tiny bit more complicated than the ones for inductively defined sets. Intuitively, this is because we want to use them to prove things by inductively considering the possible shapes that something in ev can have -- either it is evidence that 0 is even, or else it is evidence that, for some n, S (S n) is even, and it includes evidence that n itself is. But the things we want to prove are not statements about EVIDENCE, they are statements about NUMBERS. So we want an induction principle that allows reasoning by induction on the former to prove properties of the latter.

In English, the induction principle for ev goes like this:

• Suppose, P is a predicate on natural numbers (that is, P n is a Prop for every n). To show that P n holds whenever n is even, it suffices to show:
• P holds for 0
• for any n, if n is even and P holds for n, then P holds for S (S n).
Formally:
(* Check ev_ind. *)

We can apply ev_ind directly instead of using induction, following pretty much the same pattern as above.
Theorem ev_even' : forall n,
ev n -> even n.
Proof.
apply ev_ind.
Case "ev_0". unfold even. reflexivity.
Case "ev_SS". intros n H IHE. unfold even. apply IHE. Qed.

#### Exercise: 3 stars, optional (prop_ind)

Write out the induction principles that Coq will generate for the inductive declarations list and MyProp. Compare your answers against the results Coq prints for the following queries.
(* Check list_ind. *)
(* Check MyProp_ind. *)

#### Exercise: 2 stars, optional

The following proof attempt is not going to succeed. Briefly explain why.
Lemma failed_proof : forall n,
ev n.
Proof.
intros n. induction n as [| n' ].
Case "n = O". simpl. apply ev_0.
Case "n = S n'".
(*
(* FILL IN HERE *)

*)

#### Exercise: 4 stars

A palindrome is a sequence that reads the same backwards as forwards.

(1) Define an inductive proposition pal on list nat that captures what it means to be a palindrome. (Hint: You'll need three cases.)

(2) Prove that

forall l, pal(l++(rev l))

(3) Prove that

forall l, pal l => l = rev l.

Note: The converse thoerem

forall l, l = rev l => pal l.

is much harder! We won't have the tools to attack this for some time...

(* FILL IN HERE *)