# ListsProducts, Lists and Options

(* \$Date: 2011-02-09 15:47:56 -0500 (Wed, 09 Feb 2011) \$ *)

The next line imports all of our definitions from the previous chapter.

Require Export Basics.

For it to work, you need to compile Basics.v into Basics.vo. (This is like making a .class file from a .java file, or a .o file from a .c file.)
Here are two ways to compile your code:
• CoqIDE:
Open Basics.v. In the "Compile" menu, click on "Compile Buffer".
• Command line:
Run coqc Basics.v
In this file, we again use the Module feature to wrap all of the definitions for pairs and lists of numbers in a module so that, later, we can reuse the same names for improved (generic) versions of the same operations.

Module NatList.

# Pairs of Numbers

In an Inductive type definition, each constructor can take any number of parameters — none (as with true and O), one (as with S), or more than one, as in this definition:

Inductive natprod : Type :=
pair : nat nat natprod.

This declaration can be read: "There is just one way to construct a pair of numbers: by applying the constructor pair to two arguments of type nat."
Here are some simple function definitions illustrating pattern matching on two-argument constructors:

Definition fst (p : natprod) : nat :=
match p with
| pair x y => x
end.
Definition snd (p : natprod) : nat :=
match p with
| pair x y => y
end.

Since pairs are used quite a bit, it is nice to be able to write them with the standard mathematical notation (x,y) instead of pair x y. We can tell Coq to allow this with a Notation declaration.

Notation "( x , y )" := (pair x y).

The new notation can be used both in expressions and in pattern matches (indeed, we've seen it already in the previous chapter — this notation is provided as part of the standard library):

Eval simpl in (fst (3,4)).

Definition fst' (p : natprod) : nat :=
match p with
| (x,y) => x
end.
Definition snd' (p : natprod) : nat :=
match p with
| (x,y) => y
end.

Definition swap_pair (p : natprod) : natprod :=
match p with
| (x,y) => (y,x)
end.

Let's try and prove a few simple facts about pairs. If we state the lemmas in a particular (and slightly peculiar) way, we can prove them with just reflexivity (and its built-in simplification):

Theorem surjective_pairing' : (n m : nat),
(n,m) = (fst (n,m), snd (n,m)).
Proof.
reflexivity. Qed.

But reflexivity is not enough if we state the lemma in a more natural way:

Theorem surjective_pairing_stuck : (p : natprod),
p = (fst p, snd p).
Proof.
simpl. (* Doesn't reduce anything! *)

We have to expose the structure of p so that simpl can perform the pattern match in fst and snd. We can do this with destruct.
Notice that, unlike for nats, destruct doesn't generate an extra subgoal here. That's because natprods can only be constructed in one way.

Theorem surjective_pairing : (p : natprod),
p = (fst p, snd p).
Proof.
intros p. destruct p as (n,m). simpl. reflexivity. Qed.

Notice that Coq allows us to use the notation we introduced for pairs in the "as..." pattern telling it what variables to bind.

#### Exercise: 1 star (snd_fst_is_swap)

Theorem snd_fst_is_swap : (p : natprod),
(snd p, fst p) = swap_pair p.
Proof.
(* FILL IN HERE *) Admitted.

#### Exercise: 1 star, optional (fst_swap_is_snd)

Theorem fst_swap_is_snd : (p : natprod),
fst (swap_pair p) = snd p.
Proof.
(* FILL IN HERE *) Admitted.

# Lists of Numbers

Generalizing the definition of pairs a little, we can describe the type of lists of numbers like this: "A list is either the empty list or else a pair of a number and another list."

Inductive natlist : Type :=
| nil : natlist
| cons : nat natlist natlist.

For example, here is a three-element list:

Definition l_123 := cons 1 (cons 2 (cons 3 nil)).

As with pairs, it is more convenient to write lists in familiar programming notation. The following two declarations allow us to use :: as an infix cons operator and square brackets as an "outfix" notation for constructing lists.

Notation "x :: l" := (cons x l) (at level 60, right associativity).
Notation "[ ]" := nil.
Notation "[ x , .. , y ]" := (cons x .. (cons y nil) ..).

It is not necessary to fully understand these declarations, but in case you are interested, here is roughly what's going on.
The right associativity annotation tells Coq how to parenthesize expressions involving several uses of :: so that, for example, the next three declarations mean exactly the same thing:

Definition l_123' := 1 :: (2 :: (3 :: nil)).
Definition l_123'' := 1 :: 2 :: 3 :: nil.
Definition l_123''' := [1,2,3].

The at level 60 part tells Coq how to parenthesize expressions that involve both :: and some other infix operator. For example, since we defined + as infix notation for the plus function at level 50,
Notation "x + y" := (plus x y)
(at level 50, left associativity).
The + operator will bind tighter than ::, so 1 + 2 :: [3] will be parsed, as we'd expect, as (1 + 2) :: [3] rather than 1 + (2 :: [3]).
(By the way, it's worth noting in passing that expressions like "1 + 2 :: [3]" can be a little confusing when you read them in a .v file. The inner brackets, around 3, indicate a list, but the outer brackets are there to instruct the "coqdoc" tool that the bracketed part should be displayed as Coq code rather than running text. These brackets don't appear in the generated HTML.)
The second and third Notation declarations above introduce the standard square-bracket notation for lists; the right-hand side of the third one illustrates Coq's syntax for declaring n-ary notations and translating them to nested sequences of binary constructors.
A number of functions are useful for manipulating lists. For example, the repeat function takes a number n and a count and returns a list of length count where every element is n.

Fixpoint repeat (n count : nat) : natlist :=
match count with
| O => nil
| S count' => n :: (repeat n count')
end.

The length function calculates the length of a list.

Fixpoint length (l:natlist) : nat :=
match l with
| nil => O
| h :: t => S (length t)
end.

The app ("append") function concatenates two lists.

Fixpoint app (l1 l2 : natlist) : natlist :=
match l1 with
| nil => l2
| h :: t => h :: (app t l2)
end.

Actually, app will be used a lot in some parts of what follows, so it is convenient to have an infix operator for it.

Notation "x ++ y" := (app x y)
(right associativity, at level 60).

Example test_app1: [1,2,3] ++ [4,5] = [1,2,3,4,5].
Proof. reflexivity. Qed.
Example test_app2: nil ++ [4,5] = [4,5].
Proof. reflexivity. Qed.
Example test_app3: [1,2,3] ++ nil = [1,2,3].
Proof. reflexivity. Qed.

Here are two more small examples of programming with lists. The hd function returns the first element (the "head") of the list, while tail returns everything but the first element. Of course, the empty list has no first element, so we must pass a default value to be returned in that case.

Definition hd (default:nat) (l:natlist) : nat :=
match l with
| nil => default
| h :: t => h
end.

Definition tail (l:natlist) : natlist :=
match l with
| nil => nil
| h :: t => t
end.

Example test_hd1: hd 0 [1,2,3] = 1.
Proof. reflexivity. Qed.
Example test_hd2: hd 0 [] = 0.
Proof. reflexivity. Qed.
Example test_tail: tail [1,2,3] = [2,3].
Proof. reflexivity. Qed.

#### Exercise: 2 stars, recommended (list_funs)

Complete the definitions of nonzeros, oddmembers and countoddmembers below.

Fixpoint nonzeros (l:natlist) : natlist :=
(* FILL IN HERE *) admit.

Example test_nonzeros: nonzeros [0,1,0,2,3,0,0] = [1,2,3].
(* FILL IN HERE *) Admitted.

Fixpoint oddmembers (l:natlist) : natlist :=
(* FILL IN HERE *) admit.

Example test_oddmembers: oddmembers [0,1,0,2,3,0,0] = [1,3].
(* FILL IN HERE *) Admitted.

Fixpoint countoddmembers (l:natlist) : nat :=
(* FILL IN HERE *) admit.

Example test_countoddmembers1: countoddmembers [1,0,3,1,4,5] = 4.
(* FILL IN HERE *) Admitted.
Example test_countoddmembers2: countoddmembers [0,2,4] = 0.
(* FILL IN HERE *) Admitted.
Example test_countoddmembers3: countoddmembers nil = 0.
(* FILL IN HERE *) Admitted.

#### Exercise: 2 stars (alternate)

Complete the definition of alternate, which "zips up" two lists into one, alternating between elements taken from the first list and elements from the second. See the tests below for more specific examples.
Note: one natural way of writing alternate will fail to satisfy Coq's requirement that all Fixpoint definitions be "obviously terminating." If you find yourself in this rut, look for a slightly more verbose solution that considers elements of both lists at the same time.

Fixpoint alternate (l1 l2 : natlist) : natlist :=
(* FILL IN HERE *) admit.

Example test_alternate1: alternate [1,2,3] [4,5,6] = [1,4,2,5,3,6].
(* FILL IN HERE *) Admitted.
Example test_alternate2: alternate [1] [4,5,6] = [1,4,5,6].
(* FILL IN HERE *) Admitted.
Example test_alternate3: alternate [1,2,3] [4] = [1,4,2,3].
(* FILL IN HERE *) Admitted.
Example test_alternate4: alternate [] [20,30] = [20,30].
(* FILL IN HERE *) Admitted.

## Bags via Lists

A bag (or multiset) is like a set, but each element can appear multiple times instead of just once. One reasonable implementation of bags is to represent a bag of numbers as a list.

Definition bag := natlist.

#### Exercise: 3 stars (bag_functions)

Complete the following definitions for the functions count, sum, add, and member for bags.

Fixpoint count (v:nat) (s:bag) : nat :=
(* FILL IN HERE *) admit.

All these proofs can be done just by reflexivity.

Example test_count1: count 1 [1,2,3,1,4,1] = 3.
(* FILL IN HERE *) Admitted.
Example test_count2: count 6 [1,2,3,1,4,1] = 0.
(* FILL IN HERE *) Admitted.

Multiset sum is similar to set union: sum a b contains all the elements of a and of b. (Mathematicians usually define union on multisets a little bit differently, which is why we don't use that name for this operation.) For sum we're giving you a header that does not give explicit names to the arguments. Moreover, it uses the keyword Definition instead of Fixpoint, so even if you had names for the arguments, you wouldn't be able to process them recursively. The point of stating the question this way is to encourage you to think about whether sum can be implemented in another way — perhaps by using functions that have already been defined.

Definition sum : bag bag bag :=
(* FILL IN HERE *) admit.

Example test_sum1: count 1 (sum [1,2,3] [1,4,1]) = 3.
(* FILL IN HERE *) Admitted.

Definition add (v:nat) (s:bag) : bag :=
(* FILL IN HERE *) admit.

(* FILL IN HERE *) Admitted.
(* FILL IN HERE *) Admitted.

Definition member (v:nat) (s:bag) : bool :=
(* FILL IN HERE *) admit.

Example test_member1: member 1 [1,4,1] = true.
(* FILL IN HERE *) Admitted.
Example test_member2: member 2 [1,4,1] = false.
(* FILL IN HERE *) Admitted.

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

Here are some more bag functions for you to practice with.

Fixpoint remove_one (v:nat) (s:bag) : bag :=
(* When remove_one is applied to a bag without the number to remove,
it should return the same bag unchanged. *)

(* FILL IN HERE *) admit.

Example test_remove_one1: count 5 (remove_one 5 [2,1,5,4,1]) = 0.
(* FILL IN HERE *) Admitted.
Example test_remove_one2: count 5 (remove_one 5 [2,1,4,1]) = 0.
(* FILL IN HERE *) Admitted.
Example test_remove_one3: count 4 (remove_one 5 [2,1,4,5,1,4]) = 2.
(* FILL IN HERE *) Admitted.
Example test_remove_one4:
count 5 (remove_one 5 [2,1,5,4,5,1,4]) = 1.
(* FILL IN HERE *) Admitted.

Fixpoint remove_all (v:nat) (s:bag) : bag :=
(* FILL IN HERE *) admit.

Example test_remove_all1: count 5 (remove_all 5 [2,1,5,4,1]) = 0.
(* FILL IN HERE *) Admitted.
Example test_remove_all2: count 5 (remove_all 5 [2,1,4,1]) = 0.
(* FILL IN HERE *) Admitted.
Example test_remove_all3: count 4 (remove_all 5 [2,1,4,5,1,4]) = 2.
(* FILL IN HERE *) Admitted.
Example test_remove_all4: count 5 (remove_all 5 [2,1,5,4,5,1,4,5,1,4]) = 0.
(* FILL IN HERE *) Admitted.

Fixpoint subset (s1:bag) (s2:bag) : bool :=
(* FILL IN HERE *) admit.

Example test_subset1: subset [1,2] [2,1,4,1] = true.
(* FILL IN HERE *) Admitted.
Example test_subset2: subset [1,2,2] [2,1,4,1] = false.
(* FILL IN HERE *) Admitted.

#### Exercise: 3 stars, recommended (bag_theorem)

Write down an interesting theorem about bags involving the functions count and add, and prove it. Note that, since this problem is somewhat open-ended, it's possible that you may come up with a theorem which is true, but whose proof requires techniques you haven't learned yet. Feel free to ask for help if you get stuck!
(* FILL IN HERE *)

Just as with numbers, simple facts about list-processing functions can sometimes be proved entirely by simplification. For example, the simplification performed by reflexivity is enough for this theorem...

Theorem nil_app : l:natlist,
[] ++ l = l.
Proof.
reflexivity. Qed.

... because the [] is substituted into the match position in the definition of app, allowing the match itself to be simplified.
Also, as with numbers, it is sometimes helpful to perform case analysis on the possible shapes (empty or non-empty) of an unknown list.

Theorem tl_length_pred : l:natlist,
pred (length l) = length (tail l).
Proof.
intros l. destruct l as [| n l'].
Case "l = nil".
reflexivity.
Case "l = cons n l'".
reflexivity. Qed.

Here, the nil case works because we've chosen to define tl nil = nil. Notice that the as annotation on the destruct tactic here introduces two names, n and l', corresponding to the fact that the cons constructor for lists takes two arguments (the head and tail of the list it is constructing).
Usually, though, interesting theorems about lists require induction for their proofs.

## Micro-Sermon

Simply reading example proofs will not get you very far! It is very important to work through the details of each one, using Coq and thinking about what each step of the proof achieves. Otherwise it is more or less guaranteed that the exercises will make no sense.

## Induction on Lists

Proofs by induction over datatypes like natlist are perhaps a little less familiar than standard natural number induction, but the basic idea is equally simple. Each Inductive declaration defines a set of data values that can be built up from the declared constructors: a boolean can be either true or false; a number can be either O or S applied to a number; a list can be either nil or cons applied to a number and a list.
Moreover, applications of the declared constructors to one another are the only possible shapes that elements of an inductively defined set can have, and this fact directly gives rise to a way of reasoning about inductively defined sets: a number is either O or else it is S applied to some smaller number; a list is either nil or else it is cons applied to some number and some smaller list; etc. So, if we have in mind some proposition P that mentions a list l and we want to argue that P holds for all lists, we can reason as follows:
• First, show that P is true of l when l is nil.
• Then show that P is true of l when l is cons n l' for some number n and some smaller list l', asssuming that P is true for l'.
Since larger lists can only be built up from smaller ones, eventually reaching nil, these two things together establish the truth of P for all lists l. Here's a concrete example:

Theorem app_ass : l1 l2 l3 : natlist,
(l1 ++ l2) ++ l3 = l1 ++ (l2 ++ l3).
Proof.
intros l1 l2 l3. induction l1 as [| n l1'].
Case "l1 = nil".
reflexivity.
Case "l1 = cons n l1'".
simpl. rewrite IHl1'. reflexivity. Qed.

Again, this Coq proof is not especially illuminating as a static written document — it is easy to see what's going on if you are reading the proof in an interactive Coq session and you can see the current goal and context at each point, but this state is not visible in the written-down parts of the Coq proof. So a natural-language proof — one written for human readers — will need to include more explicit signposts; in particular, it will help the reader stay oriented if we remind them exactly what the induction hypothesis is in the second case.
Theorem: For all lists l1, l2, and l3, (l1 ++ l2) ++ l3 = l1 ++ (l2 ++ l3).
Proof: By induction on l1.
• First, suppose l1 = []. We must show
([] ++ l2) ++ l3 = [] ++ (l2 ++ l3),
which follows directly from the definition of ++.
• Next, suppose l1 = n::l1', with
(l1' ++ l2) ++ l3 = l1' ++ (l2 ++ l3)
(the induction hypothesis). We must show
((n :: l1') ++ l2) ++ l3 = (n :: l1') ++ (l2 ++ l3).
By the definition of ++, this follows from
n :: ((l1' ++ l2) ++ l3) = n :: (l1' ++ (l2 ++ l3)),
which is immediate from the induction hypothesis.
Here is an exercise to be worked together in class:

Theorem app_length : l1 l2 : natlist,
length (l1 ++ l2) = (length l1) + (length l2).
Proof.
(* WORKED IN CLASS *)
intros l1 l2. induction l1 as [| n l1'].
Case "l1 = nil".
reflexivity.
Case "l1 = cons".
simpl. rewrite IHl1'. reflexivity. Qed.

For a slightly more involved example of an inductive proof over lists, suppose we define a "cons on the right" function snoc like this...

Fixpoint snoc (l:natlist) (v:nat) : natlist :=
match l with
| nil => [v]
| h :: t => h :: (snoc t v)
end.

... and use it to define a list-reversing function rev like this:

Fixpoint rev (l:natlist) : natlist :=
match l with
| nil => nil
| h :: t => snoc (rev t) h
end.

Example test_rev1: rev [1,2,3] = [3,2,1].
Proof. reflexivity. Qed.
Example test_rev2: rev nil = nil.
Proof. reflexivity. Qed.

Now let's prove some more list theorems using our newly defined snoc and rev. For something a little more challenging than the inductive proofs we've seen so far, let's prove that reversing a list does not change its length. Our first attempt at this proof gets stuck in the successor case...

Theorem rev_length_firsttry : l : natlist,
length (rev l) = length l.
Proof.
intros l. induction l as [| n l'].
Case "l = []".
reflexivity.
Case "l = n :: l'".
simpl. (* Here we are stuck: the goal is an equality involving
snoc, but we don't have any equations in either the
immediate context or the global environment that have
anything to do with snoc! *)

So let's take the equation about snoc that would have enabled us to make progress and prove it as a separate lemma.

Theorem length_snoc : n : nat, l : natlist,
length (snoc l n) = S (length l).
Proof.
intros n l. induction l as [| n' l'].
Case "l = nil".
reflexivity.
Case "l = cons n' l'".
simpl. rewrite IHl'. reflexivity. Qed.

Now we can complete the original proof.

Theorem rev_length : l : natlist,
length (rev l) = length l.
Proof.
intros l. induction l as [| n l'].
Case "l = nil".
reflexivity.
Case "l = cons".
simpl. rewrite length_snoc.
rewrite IHl'. reflexivity. Qed.

For comparison, here are informal proofs of these two theorems:
Theorem: For all numbers n and lists l, length (snoc l n) = S (length l).
Proof: By induction on l.
• First, suppose l = []. We must show
length (snoc [] n) = S (length []),
which follows directly from the definitions of length and snoc.
• Next, suppose l = n'::l', with
length (snoc l' n) = S (length l').
We must show
length (snoc (n' :: l'n) = S (length (n' :: l')).
By the definitions of length and snoc, this follows from
S (length (snoc l' n)) = S (S (length l')),
which is immediate from the induction hypothesis.
Theorem: For all lists l, length (rev l) = length l.
Proof: By induction on l.
• First, suppose l = []. We must show
length (rev []) = length [],
which follows directly from the definitions of length and rev.
• Next, suppose l = n::l', with
length (rev l') = length l'.
We must show
length (rev (n :: l')) = length (n :: l').
By the definition of rev, this follows from
length (snoc (rev l'n) = S (length l')
which, by the previous lemma, is the same as
S (length (rev l')) = S (length l').
This is immediate from the induction hypothesis.
Obviously, the style of these proofs is rather longwinded and pedantic. After the first few, we might find it easier to follow proofs that give a little less detail overall (since we can easily work them out in our own minds or on scratch paper if necessary) and just highlight the non-obvious steps. In this more compressed style, the above proof might look more like this:
Theorem: For all lists l, length (rev l) = length l.
Proof: First, observe that
length (snoc l n) = S (length l)
for any l. This follows by a straightforward induction on l. The main property now follows by another straightforward induction on l, using the observation together with the induction hypothesis in the case where l = n'::l'.
Which style is preferable in a given situation depends on the sophistication of the expected audience and on how similar the proof at hand is to ones that the audience will already be familiar with. The more pedantic style is a good default for present purposes.

We've seen that proofs can make use of other theorems we've already proved, using rewrite, and later we will see other ways of reusing previous theorems. But in order to refer to a theorem, we need to know its name, and remembering the names of all the theorems we might ever want to use can become quite difficult! It is often hard even to remember what theorems have been proven, much less what they are named.
Coq's SearchAbout command is quite helpful with this. Typing SearchAbout foo will cause Coq to display a list of all theorems involving foo. For example, try uncommenting the following to see a list of theorems that we have proved about rev:

Keep SearchAbout in mind as you do the following exercises and throughout the rest of the course; it can save you a lot of time!

## List Exercises, Part 1

#### Exercise: 3 stars, recommended (list_exercises)

More practice with lists.

Theorem app_nil_end : l : natlist,
l ++ [] = l.
Proof.
(* FILL IN HERE *) Admitted.

Theorem rev_involutive : l : natlist,
rev (rev l) = l.
Proof.
(* FILL IN HERE *) Admitted.

Theorem distr_rev : l1 l2 : natlist,
rev (l1 ++ l2) = (rev l2) ++ (rev l1).
Proof.
(* FILL IN HERE *) Admitted.

There is a short solution to the next exercise. If you find yourself getting tangled up, step back and try to look for a simpler way.

Theorem app_ass4 : l1 l2 l3 l4 : natlist,
l1 ++ (l2 ++ (l3 ++ l4)) = ((l1 ++ l2) ++ l3) ++ l4.
Proof.
(* FILL IN HERE *) Admitted.

Theorem snoc_append : (l:natlist) (n:nat),
snoc l n = l ++ [n].
Proof.
(* FILL IN HERE *) Admitted.

Lemma nonzeros_length : l1 l2 : natlist,
nonzeros (l1 ++ l2) = (nonzeros l1) ++ (nonzeros l2).
Proof.
(* FILL IN HERE *) Admitted.

## List Exercises, Part 2

#### Exercise: 2 stars, recommended (list_design)

Design exercise:
• Write down a non-trivial theorem involving cons (::), snoc, and append (++).
• Prove it.

(* FILL IN HERE *)

#### Exercise: 2 stars, optional (bag_proofs)

If you did the optional exercise about bags above, here are a couple of little theorems to prove about your definitions.

Theorem count_member_nonzero : (s : bag),
ble_nat 1 (count 1 (1 :: s)) = true.
Proof.
(* FILL IN HERE *) Admitted.

Theorem ble_n_Sn : n,
ble_nat n (S n) = true.
Proof.
intros n. induction n as [| n'].
Case "0".
simpl. reflexivity.
Case "S n'".
simpl. rewrite IHn'. reflexivity. Qed.

Theorem remove_decreases_count: (s : bag),
ble_nat (count 0 (remove_one 0 s)) (count 0 s) = true.
Proof.
(* FILL IN HERE *) Admitted.

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

Write down an interesting theorem about bags involving the functions count and sum, and prove it.
(* FILL IN HERE *)

# Options

Here is another type definition that is often useful in day-to-day programming:

Inductive natoption : Type :=
| Some : nat natoption
| None : natoption.

One use of natoption is as a way of returning "error codes" from functions. For example, suppose we want to write a function that returns the nth element of some list. If we give it type nat natlist nat, then we'll have to return some number when the list is too short!

Fixpoint index_bad (n:nat) (l:natlist) : nat :=
match l with
| nil => 42 (* arbitrary! *)
| a :: l' => match beq_nat n O with
| true => a
| false => index_bad (pred n) l'
end
end.

On the other hand, if we give it type nat natlist natoption, then we can return None when the list is too short and Some a when the list has enough members and a appears at position n.

Fixpoint index (n:nat) (l:natlist) : natoption :=
match l with
| nil => None
| a :: l' => match beq_nat n O with
| true => Some a
| false => index (pred n) l'
end
end.

Example test_index1 : index 0 [4,5,6,7] = Some 4.
Proof. reflexivity. Qed.
Example test_index2 : index 3 [4,5,6,7] = Some 7.
Proof. reflexivity. Qed.
Example test_index3 : index 10 [4,5,6,7] = None.
Proof. reflexivity. Qed.

This example is also an opportunity to introduce one more small feature of Coq's programming language: conditional expressions...

Fixpoint index' (n:nat) (l:natlist) : natoption :=
match l with
| nil => None
| a :: l' => if beq_nat n O then Some a else index (pred n) l'
end.

Coq's conditionals are exactly like those found in any other language, with one small generalization. Since the boolean type is not built in, Coq actually allows conditional expressions over any inductively defined type with exactly two constructors. The guard is considered true if it evaluates to the first constructor in the Inductive definition and false if it evaluates to the second.
The function below pulls the nat out of a natoption, returning a supplied default in the None case.

Definition option_elim (o : natoption) (d : nat) : nat :=
match o with
| Some n' => n'
| None => d
end.

#### Exercise: 2 stars (hd_opt)

Using the same idea, fix the hd function from earlier so we don't have to pass a default element for the nil case.

Definition hd_opt (l : natlist) : natoption :=
(* FILL IN HERE *) admit.

Example test_hd_opt1 : hd_opt [] = None.
(* FILL IN HERE *) Admitted.

Example test_hd_opt2 : hd_opt [1] = Some 1.
(* FILL IN HERE *) Admitted.

Example test_hd_opt3 : hd_opt [5,6] = Some 5.
(* FILL IN HERE *) Admitted.

#### Exercise: 2 stars, optional (option_elim_hd)

This exercise relates your new hd_opt to the old hd.

Theorem option_elim_hd : (l:natlist) (default:nat),
hd default l = option_elim (hd_opt l) default.
Proof.
(* FILL IN HERE *) Admitted.

#### Exercise: 2 stars, recommended (beq_natlist)

Fill in the definition of beq_natlist, which compares lists of numbers for equality. Prove that beq_natlist l l yields true for every list l.

Fixpoint beq_natlist (l1 l2 : natlist) : bool :=
(* FILL IN HERE *) admit.

Example test_beq_natlist1 : (beq_natlist nil nil = true).
(* FILL IN HERE *) Admitted.
Example test_beq_natlist2 : beq_natlist [1,2,3] [1,2,3] = true.
(* FILL IN HERE *) Admitted.
Example test_beq_natlist3 : beq_natlist [1,2,3] [1,2,4] = false.
(* FILL IN HERE *) Admitted.

Theorem beq_natlist_refl : l:natlist,
true = beq_natlist l l.
Proof.
(* FILL IN HERE *) Admitted.

# The apply Tactic

We often encounter situations where the goal to be proved is exactly the same as some hypothesis in the context or some previously proved lemma.

Theorem silly1 : (n m o p : nat),
n = m
[n,o] = [n,p]
[n,o] = [m,p].
Proof.
intros n m o p eq1 eq2.
rewrite eq1.
(* At this point, we could finish with "rewrite eq2. reflexivity."
as we have done several times above.  But we can achieve the same
effect in a single step by using the apply tactic instead: *)

apply eq2. Qed.

The apply tactic also works with conditional hypotheses and lemmas: if the statement being applied is an implication, then the premises of this implication will be added to the list of subgoals needing to be proved.

Theorem silly2 : (n m o p : nat),
n = m
( (q r : nat), q = r [q,o] = [r,p])
[n,o] = [m,p].
Proof.
intros n m o p eq1 eq2.
apply eq2. apply eq1. Qed.

You may find it instructive to experiment with this proof and see if there is a way to complete it using just rewrite instead of apply.
Typically, when we use apply H, the statement H will begin with a binding some universal variables. When Coq matches the current goal against the conclusion of H, it will try to find appropriate values for these variables. For example, when we do apply eq2 in the following proof, the universal variable q in eq2 gets instantiated with n and r gets instantiated with m.

Theorem silly2a : (n m : nat),
(n,n) = (m,m)
( (q r : nat), (q,q) = (r,r) [q] = [r])
[n] = [m].
Proof.
intros n m eq1 eq2.
apply eq2. apply eq1. Qed.

#### Exercise: 2 stars, optional (silly_ex)

Complete the following proof without using simpl.

Theorem silly_ex :
( n, evenb n = true oddb (S n) = true)
evenb 3 = true
oddb 4 = true.
Proof.
(* FILL IN HERE *) Admitted.
To use the apply tactic, the (conclusion of the) fact being applied must match the goal exactly — for example, apply will not work if the left and right sides of the equality are swapped.

Theorem silly3_firsttry : (n : nat),
true = beq_nat n 5
beq_nat (S (S n)) 7 = true.
Proof.
intros n H.
simpl.
(* here we cannot use apply directly *)

In this case we can use the symmetry tactic, which switches the left and right sides of an equality in the goal.

Theorem silly3 : (n : nat),
true = beq_nat n 5
beq_nat (S (S n)) 7 = true.
Proof.
intros n H.
symmetry.
simpl. (* Actually, this simpl is unnecessary, since
apply will do a simpl step first. *)

apply H. Qed.

#### Exercise: 3 stars, recommended (apply_exercise1)

Theorem rev_exercise1 : (l l' : natlist),
l = rev l'
l' = rev l.
Proof.
(* Hint: you can use apply with previously defined lemmas, not
just hypotheses in the context.  Remember that SearchAbout is
your friend. *)

(* FILL IN HERE *) Admitted.

#### Exercise: 1 star (apply_rewrite)

Briefly explain the difference between the tactics apply and rewrite. Are there situations where both can usefully be applied?
(* FILL IN HERE *)

# Varying the Induction Hypothesis

One subtlety in these inductive proofs is worth noticing here. For example, look back at the proof of the app_ass theorem. The induction hypothesis (in the second subgoal generated by the induction tactic) is
(l1' ++ l2) ++ l3 = l1' ++ l2 ++ l3 .
(Note that, because we've defined ++ to be right associative, the expression on the right of the = is the same as writing l1' ++ (l2 ++ l3).)
This hypothesis makes a statement about l1' together with the particular lists l2 and l3. The lists l2 and l3, which were introduced into the context by the intros at the top of the proof, are "held constant" in the induction hypothesis. If we set up the proof slightly differently by introducing just n into the context at the top, then we get an induction hypothesis that makes a stronger claim:
l2 l3, (l1' ++ l2) ++ l3 = l1' ++ l2 ++ l3
Use Coq to see the difference for yourself.
In the present case, the difference between the two proofs is minor, since the definition of the ++ function just examines its first argument and doesn't do anything interesting with its second argument. But we'll soon come to situations where setting up the induction hypothesis one way or the other can make the difference between a proof working and failing.

#### Exercise: 2 stars, optional (app_ass')

Give an alternate proof of the associativity of ++ with a more general induction hypothesis. Complete the following (leaving the first line unchanged).

Theorem app_ass' : l1 l2 l3 : natlist,
(l1 ++ l2) ++ l3 = l1 ++ (l2 ++ l3).
Proof.
intros l1. induction l1 as [ | n l1'].
(* FILL IN HERE *) Admitted.

#### Exercise: 3 stars (apply_exercise2)

Notice that we don't introduce m before performing induction. This leaves it general, so that the IH doesn't specify a particular m, but lets us pick.

Theorem beq_nat_sym : (n m : nat),
beq_nat n m = beq_nat m n.
Proof.
intros n. induction n as [| n'].
(* FILL IN HERE *) Admitted.

#### Exercise: 3 stars, recommended (beq_nat_sym_informal)

Provide an informal proof of this lemma that corresponds to your formal proof above:
Theorem: For any nats n m, beq_nat n m = beq_nat m n.
Proof: (* FILL IN HERE *)

End NatList.

# Exercise: Dictionaries

Module Dictionary.

Inductive dictionary : Type :=
| empty : dictionary
| record : nat nat dictionary dictionary.

This declaration can be read: "There are two ways to construct a dictionary: either using the constructor empty to represent an empty dictionary, or by applying the constructor record to a key, a value, and an existing dictionary to construct a dictionary with an additional key to value mapping."

Definition insert (key value : nat) (d : dictionary) : dictionary :=
(record key value d).

Below is a function find that searches a dictionary for a given key. It evaluates evaluates to None if the key was not found and Some val if the key was mapped to val in the dictionary. If the same key is mapped to multiple values, find will return the first one it finds.

Fixpoint find (key : nat) (d : dictionary) : option nat :=
match d with
| empty => None
| record k v d' => if (beq_nat key k) then (Some v) else (find key d')
end.

#### Exercise: 1 star (dictionary_invariant1)

(* Complete the following proof. *)
Theorem dictionary_invariant1 : (d : dictionary) (k v: nat),
(find k (insert k v d)) = Some v.
Proof.
(* FILL IN HERE *) Admitted.

#### Exercise: 1 star (dictionary_invariant2)

(* Complete the following proof. *)
Theorem dictionary_invariant2 : (d : dictionary) (m n o: nat),
(beq_nat m n) = false (find m d) = (find m (insert n o d)).
Proof.
(* FILL IN HERE *) Admitted.

End Dictionary.

The following declaration puts beq_nat_sym into the top-level namespace, so that we can use it later without having to write NatList.beq_nat_sym.