# Lists: Lists and higher-order functions

module plfa.part1.Lists where

This chapter discusses the list data type. It gives further examples of many of the techniques we have developed so far, and provides examples of polymorphic types and higher-order functions.

## Imports

import Relation.Binary.PropositionalEquality as Eq open Eq using (_≡_; refl; sym; trans; cong) open Eq.≡-Reasoning open import Data.Bool using (Bool; true; false; T; _∧_; _∨_; not) open import Data.Nat using (ℕ; zero; suc; _+_; _*_; _∸_; _≤_; s≤s; z≤n) open import Data.Nat.Properties using (+-assoc; +-identityˡ; +-identityʳ; *-assoc; *-identityˡ; *-identityʳ) open import Relation.Nullary using (¬_; Dec; yes; no) open import Data.Product using (_×_; ∃; ∃-syntax) renaming (_,_ to ⟨_,_⟩) open import Function using (_∘_) open import Level using (Level) open import plfa.part1.Isomorphism using (_≃_; _⇔_)

## Lists

Lists are defined in Agda as follows:

data List (A : Set) : Set where [] : List A _∷_ : A → List A → List A infixr 5 _∷_

Let’s unpack this definition. If `A`

is a set, then `List A`

is a set.
The next two lines tell us that `[]`

(pronounced *nil*) is a list of
type `A`

(often called the *empty* list), and that `_∷_`

(pronounced
*cons*, short for *constructor*) takes a value of type `A`

and a value
of type `List A`

and returns a value of type `List A`

. Operator `_∷_`

has precedence level 5 and associates to the right.

For example,

_ : List ℕ _ = 0 ∷ 1 ∷ 2 ∷ []

denotes the list of the first three natural numbers. Since `_∷_`

associates to the right, the term parses as `0 ∷ (1 ∷ (2 ∷ []))`

.
Here `0`

is the first element of the list, called the *head*,
and `1 ∷ (2 ∷ [])`

is a list of the remaining elements, called the
*tail*. A list is a strange beast: it has a head and a tail,
nothing in between, and the tail is itself another list!

As we’ve seen, parameterised types can be translated to indexed types. The definition above is equivalent to the following:

data List′ : Set → Set where []′ : ∀ {A : Set} → List′ A _∷′_ : ∀ {A : Set} → A → List′ A → List′ A

Each constructor takes the parameter as an implicit argument. Thus, our example list could also be written:

_ : List ℕ _ = _∷_ {ℕ} 0 (_∷_ {ℕ} 1 (_∷_ {ℕ} 2 ([] {ℕ})))

where here we have provided the implicit parameters explicitly.

Including the pragma:

```
{-# BUILTIN LIST List #-}
```

tells Agda that the type `List`

corresponds to the Haskell type
list, and the constructors `[]`

and `_∷_`

correspond to nil and
cons respectively, allowing a more efficient representation of lists.

## List syntax

We can write lists more conveniently by introducing the following definitions:

pattern [_] z = z ∷ [] pattern [_,_] y z = y ∷ z ∷ [] pattern [_,_,_] x y z = x ∷ y ∷ z ∷ [] pattern [_,_,_,_] w x y z = w ∷ x ∷ y ∷ z ∷ [] pattern [_,_,_,_,_] v w x y z = v ∷ w ∷ x ∷ y ∷ z ∷ [] pattern [_,_,_,_,_,_] u v w x y z = u ∷ v ∷ w ∷ x ∷ y ∷ z ∷ []

This is our first use of pattern declarations. For instance,
the third line tells us that `[ x , y , z ]`

is equivalent to
`x ∷ y ∷ z ∷ []`

, and permits the former to appear either in
a pattern on the left-hand side of an equation, or a term
on the right-hand side of an equation.

## Append

Our first function on lists is written `_++_`

and pronounced
*append*:

infixr 5 _++_ _++_ : ∀ {A : Set} → List A → List A → List A [] ++ ys = ys (x ∷ xs) ++ ys = x ∷ (xs ++ ys)

The type `A`

is an implicit argument to append, making it a
*polymorphic* function (one that can be used at many types). The
empty list appended to another list yields the other list. A
non-empty list appended to another list yields a list with head the
same as the head of the first list and tail the same as the tail of
the first list appended to the second list.

Here is an example, showing how to compute the result of appending two lists:

_ : [ 0 , 1 , 2 ] ++ [ 3 , 4 ] ≡ [ 0 , 1 , 2 , 3 , 4 ] _ = begin 0 ∷ 1 ∷ 2 ∷ [] ++ 3 ∷ 4 ∷ [] ≡⟨⟩ 0 ∷ (1 ∷ 2 ∷ [] ++ 3 ∷ 4 ∷ []) ≡⟨⟩ 0 ∷ 1 ∷ (2 ∷ [] ++ 3 ∷ 4 ∷ []) ≡⟨⟩ 0 ∷ 1 ∷ 2 ∷ ([] ++ 3 ∷ 4 ∷ []) ≡⟨⟩ 0 ∷ 1 ∷ 2 ∷ 3 ∷ 4 ∷ [] ∎

Appending two lists requires time linear in the number of elements in the first list.

## Reasoning about append

We can reason about lists in much the same way that we reason about numbers. Here is the proof that append is associative:

++-assoc : ∀ {A : Set} (xs ys zs : List A) → (xs ++ ys) ++ zs ≡ xs ++ (ys ++ zs) ++-assoc [] ys zs = begin ([] ++ ys) ++ zs ≡⟨⟩ ys ++ zs ≡⟨⟩ [] ++ (ys ++ zs) ∎ ++-assoc (x ∷ xs) ys zs = begin (x ∷ xs ++ ys) ++ zs ≡⟨⟩ x ∷ (xs ++ ys) ++ zs ≡⟨⟩ x ∷ ((xs ++ ys) ++ zs) ≡⟨ cong (x ∷_) (++-assoc xs ys zs) ⟩ x ∷ (xs ++ (ys ++ zs)) ≡⟨⟩ x ∷ xs ++ (ys ++ zs) ∎

The proof is by induction on the first argument. The base case instantiates
to `[]`

, and follows by straightforward computation.
The inductive case instantiates to `x ∷ xs`

,
and follows by straightforward computation combined with the
inductive hypothesis. As usual, the inductive hypothesis is indicated by a recursive
invocation of the proof, in this case `++-assoc xs ys zs`

.

Recall that Agda supports sections.
Applying `cong (x ∷_)`

promotes the inductive hypothesis:

```
(xs ++ ys) ++ zs ≡ xs ++ (ys ++ zs)
```

to the equality:

```
x ∷ ((xs ++ ys) ++ zs) ≡ x ∷ (xs ++ (ys ++ zs))
```

which is needed in the proof.

It is also easy to show that `[]`

is a left and right identity for `_++_`

.
That it is a left identity is immediate from the definition:

++-identityˡ : ∀ {A : Set} (xs : List A) → [] ++ xs ≡ xs ++-identityˡ xs = begin [] ++ xs ≡⟨⟩ xs ∎

That it is a right identity follows by simple induction:

++-identityʳ : ∀ {A : Set} (xs : List A) → xs ++ [] ≡ xs ++-identityʳ [] = begin [] ++ [] ≡⟨⟩ [] ∎ ++-identityʳ (x ∷ xs) = begin (x ∷ xs) ++ [] ≡⟨⟩ x ∷ (xs ++ []) ≡⟨ cong (x ∷_) (++-identityʳ xs) ⟩ x ∷ xs ∎

As we will see later,
these three properties establish that `_++_`

and `[]`

form
a *monoid* over lists.

## Length

Our next function finds the length of a list:

length : ∀ {A : Set} → List A → ℕ length [] = zero length (x ∷ xs) = suc (length xs)

Again, it takes an implicit parameter `A`

.
The length of the empty list is zero.
The length of a non-empty list
is one greater than the length of the tail of the list.

Here is an example showing how to compute the length of a list:

_ : length [ 0 , 1 , 2 ] ≡ 3 _ = begin length (0 ∷ 1 ∷ 2 ∷ []) ≡⟨⟩ suc (length (1 ∷ 2 ∷ [])) ≡⟨⟩ suc (suc (length (2 ∷ []))) ≡⟨⟩ suc (suc (suc (length {ℕ} []))) ≡⟨⟩ suc (suc (suc zero)) ∎

Computing the length of a list requires time linear in the number of elements in the list.

In the second-to-last line, we cannot write simply `length []`

but
must instead write `length {ℕ} []`

. Since `[]`

has no elements, Agda
has insufficient information to infer the implicit parameter.

## Reasoning about length

The length of one list appended to another is the sum of the lengths of the lists:

length-++ : ∀ {A : Set} (xs ys : List A) → length (xs ++ ys) ≡ length xs + length ys length-++ {A} [] ys = begin length ([] ++ ys) ≡⟨⟩ length ys ≡⟨⟩ length {A} [] + length ys ∎ length-++ (x ∷ xs) ys = begin length ((x ∷ xs) ++ ys) ≡⟨⟩ suc (length (xs ++ ys)) ≡⟨ cong suc (length-++ xs ys) ⟩ suc (length xs + length ys) ≡⟨⟩ length (x ∷ xs) + length ys ∎

The proof is by induction on the first argument. The base case
instantiates to `[]`

, and follows by straightforward computation. As
before, Agda cannot infer the implicit type parameter to `length`

, and
it must be given explicitly. The inductive case instantiates to
`x ∷ xs`

, and follows by straightforward computation combined with the
inductive hypothesis. As usual, the inductive hypothesis is indicated
by a recursive invocation of the proof, in this case `length-++ xs ys`

,
and it is promoted by the congruence `cong suc`

.

## Reverse

Using append, it is easy to formulate a function to reverse a list:

reverse : ∀ {A : Set} → List A → List A reverse [] = [] reverse (x ∷ xs) = reverse xs ++ [ x ]

The reverse of the empty list is the empty list. The reverse of a non-empty list is the reverse of its tail appended to a unit list containing its head.

Here is an example showing how to reverse a list:

_ : reverse [ 0 , 1 , 2 ] ≡ [ 2 , 1 , 0 ] _ = begin reverse (0 ∷ 1 ∷ 2 ∷ []) ≡⟨⟩ reverse (1 ∷ 2 ∷ []) ++ [ 0 ] ≡⟨⟩ (reverse (2 ∷ []) ++ [ 1 ]) ++ [ 0 ] ≡⟨⟩ ((reverse [] ++ [ 2 ]) ++ [ 1 ]) ++ [ 0 ] ≡⟨⟩ (([] ++ [ 2 ]) ++ [ 1 ]) ++ [ 0 ] ≡⟨⟩ (([] ++ 2 ∷ []) ++ 1 ∷ []) ++ 0 ∷ [] ≡⟨⟩ (2 ∷ [] ++ 1 ∷ []) ++ 0 ∷ [] ≡⟨⟩ 2 ∷ ([] ++ 1 ∷ []) ++ 0 ∷ [] ≡⟨⟩ (2 ∷ 1 ∷ []) ++ 0 ∷ [] ≡⟨⟩ 2 ∷ (1 ∷ [] ++ 0 ∷ []) ≡⟨⟩ 2 ∷ 1 ∷ ([] ++ 0 ∷ []) ≡⟨⟩ 2 ∷ 1 ∷ 0 ∷ [] ≡⟨⟩ [ 2 , 1 , 0 ] ∎

Reversing a list in this way takes time *quadratic* in the length of
the list. This is because reverse ends up appending lists of lengths
`1`

, `2`

, up to `n - 1`

, where `n`

is the length of the list being
reversed, append takes time linear in the length of the first
list, and the sum of the numbers up to `n - 1`

is `n * (n - 1) / 2`

.
(We will validate that last fact in an exercise later in this chapter.)

#### Exercise `reverse-++-distrib`

(recommended)

Show that the reverse of one list appended to another is the reverse of the second appended to the reverse of the first:

```
reverse (xs ++ ys) ≡ reverse ys ++ reverse xs
```

#### Exercise `reverse-involutive`

(recommended)

A function is an *involution* if when applied twice it acts
as the identity function. Show that reverse is an involution:

```
reverse (reverse xs) ≡ xs
```

## Faster reverse

The definition above, while easy to reason about, is less efficient than one might expect since it takes time quadratic in the length of the list. The idea is that we generalise reverse to take an additional argument:

shunt : ∀ {A : Set} → List A → List A → List A shunt [] ys = ys shunt (x ∷ xs) ys = shunt xs (x ∷ ys)

The definition is by recursion on the first argument. The second argument
actually becomes *larger*, but this is not a problem because the argument
on which we recurse becomes *smaller*.

Shunt is related to reverse as follows:

shunt-reverse : ∀ {A : Set} (xs ys : List A) → shunt xs ys ≡ reverse xs ++ ys shunt-reverse [] ys = begin shunt [] ys ≡⟨⟩ ys ≡⟨⟩ reverse [] ++ ys ∎ shunt-reverse (x ∷ xs) ys = begin shunt (x ∷ xs) ys ≡⟨⟩ shunt xs (x ∷ ys) ≡⟨ shunt-reverse xs (x ∷ ys) ⟩ reverse xs ++ (x ∷ ys) ≡⟨⟩ reverse xs ++ ([ x ] ++ ys) ≡⟨ sym (++-assoc (reverse xs) [ x ] ys) ⟩ (reverse xs ++ [ x ]) ++ ys ≡⟨⟩ reverse (x ∷ xs) ++ ys ∎

The proof is by induction on the first argument.
The base case instantiates to `[]`

, and follows by straightforward computation.
The inductive case instantiates to `x ∷ xs`

and follows by the inductive
hypothesis and associativity of append. When we invoke the inductive hypothesis,
the second argument actually becomes *larger*, but this is not a problem because
the argument on which we induct becomes *smaller*.

Generalising on an auxiliary argument, which becomes larger as the argument on which we recurse or induct becomes smaller, is a common trick. It belongs in your quiver of arrows, ready to slay the right problem.

Having defined shunt be generalisation, it is now easy to respecialise to give a more efficient definition of reverse:

reverse′ : ∀ {A : Set} → List A → List A reverse′ xs = shunt xs []

Given our previous lemma, it is straightforward to show the two definitions equivalent:

reverses : ∀ {A : Set} (xs : List A) → reverse′ xs ≡ reverse xs reverses xs = begin reverse′ xs ≡⟨⟩ shunt xs [] ≡⟨ shunt-reverse xs [] ⟩ reverse xs ++ [] ≡⟨ ++-identityʳ (reverse xs) ⟩ reverse xs ∎

Here is an example showing fast reverse of the list `[ 0 , 1 , 2 ]`

:

_ : reverse′ [ 0 , 1 , 2 ] ≡ [ 2 , 1 , 0 ] _ = begin reverse′ (0 ∷ 1 ∷ 2 ∷ []) ≡⟨⟩ shunt (0 ∷ 1 ∷ 2 ∷ []) [] ≡⟨⟩ shunt (1 ∷ 2 ∷ []) (0 ∷ []) ≡⟨⟩ shunt (2 ∷ []) (1 ∷ 0 ∷ []) ≡⟨⟩ shunt [] (2 ∷ 1 ∷ 0 ∷ []) ≡⟨⟩ 2 ∷ 1 ∷ 0 ∷ [] ∎

Now the time to reverse a list is linear in the length of the list.

## Map

Map applies a function to every element of a list to generate a corresponding list.
Map is an example of a *higher-order function*, one which takes a function as an
argument or returns a function as a result:

map : ∀ {A B : Set} → (A → B) → List A → List B map f [] = [] map f (x ∷ xs) = f x ∷ map f xs

Map of the empty list is the empty list. Map of a non-empty list yields a list with head the same as the function applied to the head of the given list, and tail the same as map of the function applied to the tail of the given list.

Here is an example showing how to use map to increment every element of a list:

_ : map suc [ 0 , 1 , 2 ] ≡ [ 1 , 2 , 3 ] _ = begin map suc (0 ∷ 1 ∷ 2 ∷ []) ≡⟨⟩ suc 0 ∷ map suc (1 ∷ 2 ∷ []) ≡⟨⟩ suc 0 ∷ suc 1 ∷ map suc (2 ∷ []) ≡⟨⟩ suc 0 ∷ suc 1 ∷ suc 2 ∷ map suc [] ≡⟨⟩ suc 0 ∷ suc 1 ∷ suc 2 ∷ [] ≡⟨⟩ 1 ∷ 2 ∷ 3 ∷ [] ∎

Map requires time linear in the length of the list.

It is often convenient to exploit currying by applying map to a function to yield a new function, and at a later point applying the resulting function:

sucs : List ℕ → List ℕ sucs = map suc _ : sucs [ 0 , 1 , 2 ] ≡ [ 1 , 2 , 3 ] _ = begin sucs [ 0 , 1 , 2 ] ≡⟨⟩ map suc [ 0 , 1 , 2 ] ≡⟨⟩ [ 1 , 2 , 3 ] ∎

Any type that is parameterised on another type, such as lists, has a
corresponding map, which accepts a function and returns a function
from the type parameterised on the domain of the function to the type
parameterised on the range of the function. Further, a type that is
parameterised on *n* types will have a map that is parameterised on
*n* functions.

#### Exercise `map-compose`

(practice)

Prove that the map of a composition is equal to the composition of two maps:

```
map (g ∘ f) ≡ map g ∘ map f
```

The last step of the proof requires extensionality.

-- Your code goes here

#### Exercise `map-++-distribute`

(practice)

Prove the following relationship between map and append:

```
map f (xs ++ ys) ≡ map f xs ++ map f ys
```

-- Your code goes here

#### Exercise `map-Tree`

(practice)

Define a type of trees with leaves of type `A`

and internal
nodes of type `B`

:

data Tree (A B : Set) : Set where leaf : A → Tree A B node : Tree A B → B → Tree A B → Tree A B

Define a suitable map operator over trees:

```
map-Tree : ∀ {A B C D : Set} → (A → C) → (B → D) → Tree A B → Tree C D
```

-- Your code goes here

## Fold

Fold takes an operator and a value, and uses the operator to combine each of the elements of the list, taking the given value as the result for the empty list:

foldr : ∀ {A B : Set} → (A → B → B) → B → List A → B foldr _⊗_ e [] = e foldr _⊗_ e (x ∷ xs) = x ⊗ foldr _⊗_ e xs

Fold of the empty list is the given value. Fold of a non-empty list uses the operator to combine the head of the list and the fold of the tail of the list.

Here is an example showing how to use fold to find the sum of a list:

_ : foldr _+_ 0 [ 1 , 2 , 3 , 4 ] ≡ 10 _ = begin foldr _+_ 0 (1 ∷ 2 ∷ 3 ∷ 4 ∷ []) ≡⟨⟩ 1 + foldr _+_ 0 (2 ∷ 3 ∷ 4 ∷ []) ≡⟨⟩ 1 + (2 + foldr _+_ 0 (3 ∷ 4 ∷ [])) ≡⟨⟩ 1 + (2 + (3 + foldr _+_ 0 (4 ∷ []))) ≡⟨⟩ 1 + (2 + (3 + (4 + foldr _+_ 0 []))) ≡⟨⟩ 1 + (2 + (3 + (4 + 0))) ∎

Here we have an instance of `foldr`

where `A`

and `B`

are both `ℕ`

.
Fold requires time linear in the length of the list.

It is often convenient to exploit currying by applying fold to an operator and a value to yield a new function, and at a later point applying the resulting function:

sum : List ℕ → ℕ sum = foldr _+_ 0 _ : sum [ 1 , 2 , 3 , 4 ] ≡ 10 _ = begin sum [ 1 , 2 , 3 , 4 ] ≡⟨⟩ foldr _+_ 0 [ 1 , 2 , 3 , 4 ] ≡⟨⟩ 10 ∎

Just as the list type has two constructors, `[]`

and `_∷_`

,
so the fold function takes two arguments, `e`

and `_⊗_`

(in addition to the list argument).
In general, a data type with *n* constructors will have
a corresponding fold function that takes *n* arguments.

As another example, observe that

```
foldr _∷_ [] xs ≡ xs
```

Here, if `xs`

is of type `List A`

, then we see we have an instance of
`foldr`

where `A`

is `A`

and `B`

is `List A`

. It follows that

```
xs ++ ys ≡ foldr _∷_ ys xs
```

Demonstrating both these equations is left as an exercise.

#### Exercise `product`

(recommended)

Use fold to define a function to find the product of a list of numbers. For example:

```
product [ 1 , 2 , 3 , 4 ] ≡ 24
```

-- Your code goes here

#### Exercise `foldr-++`

(recommended)

Show that fold and append are related as follows:

```
foldr _⊗_ e (xs ++ ys) ≡ foldr _⊗_ (foldr _⊗_ e ys) xs
```

-- Your code goes here

#### Exercise `foldr-∷`

(practice)

Show

```
foldr _∷_ [] xs ≡ xs
```

Show as a consequence of `foldr-++ above that

```
xs ++ ys ≡ foldr _∷_ ys xs
```

-- Your code goes here

#### Exercise `map-is-foldr`

(practice)

Show that map can be defined using fold:

```
map f ≡ foldr (λ x xs → f x ∷ xs) []
```

The proof requires extensionality.

-- Your code goes here

#### Exercise `fold-Tree`

(practice)

Define a suitable fold function for the type of trees given earlier:

```
fold-Tree : ∀ {A B C : Set} → (A → C) → (C → B → C → C) → Tree A B → C
```

-- Your code goes here

#### Exercise `map-is-fold-Tree`

(practice)

Demonstrate an analogue of `map-is-foldr`

for the type of trees.

-- Your code goes here

#### Exercise `sum-downFrom`

(stretch)

Define a function that counts down as follows:

downFrom : ℕ → List ℕ downFrom zero = [] downFrom (suc n) = n ∷ downFrom n

For example:

_ : downFrom 3 ≡ [ 2 , 1 , 0 ] _ = refl

Prove that the sum of the numbers `(n - 1) + ⋯ + 0`

is
equal to `n * (n ∸ 1) / 2`

:

```
sum (downFrom n) * 2 ≡ n * (n ∸ 1)
```

## Monoids

Typically when we use a fold the operator is associative and the
value is a left and right identity for the operator, meaning that the
operator and the value form a *monoid*.

We can define a monoid as a suitable record type:

record IsMonoid {A : Set} (_⊗_ : A → A → A) (e : A) : Set where field assoc : ∀ (x y z : A) → (x ⊗ y) ⊗ z ≡ x ⊗ (y ⊗ z) identityˡ : ∀ (x : A) → e ⊗ x ≡ x identityʳ : ∀ (x : A) → x ⊗ e ≡ x open IsMonoid

As examples, sum and zero, multiplication and one, and append and the empty list, are all examples of monoids:

+-monoid : IsMonoid _+_ 0 +-monoid = record { assoc = +-assoc ; identityˡ = +-identityˡ ; identityʳ = +-identityʳ } *-monoid : IsMonoid _*_ 1 *-monoid = record { assoc = *-assoc ; identityˡ = *-identityˡ ; identityʳ = *-identityʳ } ++-monoid : ∀ {A : Set} → IsMonoid {List A} _++_ [] ++-monoid = record { assoc = ++-assoc ; identityˡ = ++-identityˡ ; identityʳ = ++-identityʳ }

If `_⊗_`

and `e`

form a monoid, then we can re-express fold on the
same operator and an arbitrary value:

foldr-monoid : ∀ {A : Set} (_⊗_ : A → A → A) (e : A) → IsMonoid _⊗_ e → ∀ (xs : List A) (y : A) → foldr _⊗_ y xs ≡ foldr _⊗_ e xs ⊗ y foldr-monoid _⊗_ e ⊗-monoid [] y = begin foldr _⊗_ y [] ≡⟨⟩ y ≡⟨ sym (identityˡ ⊗-monoid y) ⟩ (e ⊗ y) ≡⟨⟩ foldr _⊗_ e [] ⊗ y ∎ foldr-monoid _⊗_ e ⊗-monoid (x ∷ xs) y = begin foldr _⊗_ y (x ∷ xs) ≡⟨⟩ x ⊗ (foldr _⊗_ y xs) ≡⟨ cong (x ⊗_) (foldr-monoid _⊗_ e ⊗-monoid xs y) ⟩ x ⊗ (foldr _⊗_ e xs ⊗ y) ≡⟨ sym (assoc ⊗-monoid x (foldr _⊗_ e xs) y) ⟩ (x ⊗ foldr _⊗_ e xs) ⊗ y ≡⟨⟩ foldr _⊗_ e (x ∷ xs) ⊗ y ∎

In a previous exercise we showed the following.

postulate foldr-++ : ∀ {A : Set} (_⊗_ : A → A → A) (e : A) (xs ys : List A) → foldr _⊗_ e (xs ++ ys) ≡ foldr _⊗_ (foldr _⊗_ e ys) xs

As a consequence, using a previous exercise, we have the following:

foldr-monoid-++ : ∀ {A : Set} (_⊗_ : A → A → A) (e : A) → IsMonoid _⊗_ e → ∀ (xs ys : List A) → foldr _⊗_ e (xs ++ ys) ≡ foldr _⊗_ e xs ⊗ foldr _⊗_ e ys foldr-monoid-++ _⊗_ e monoid-⊗ xs ys = begin foldr _⊗_ e (xs ++ ys) ≡⟨ foldr-++ _⊗_ e xs ys ⟩ foldr _⊗_ (foldr _⊗_ e ys) xs ≡⟨ foldr-monoid _⊗_ e monoid-⊗ xs (foldr _⊗_ e ys) ⟩ foldr _⊗_ e xs ⊗ foldr _⊗_ e ys ∎

#### Exercise `foldl`

(practice)

Define a function `foldl`

which is analogous to `foldr`

, but where
operations associate to the left rather than the right. For example:

```
foldr _⊗_ e [ x , y , z ] = x ⊗ (y ⊗ (z ⊗ e))
foldl _⊗_ e [ x , y , z ] = ((e ⊗ x) ⊗ y) ⊗ z
```

-- Your code goes here

#### Exercise `foldr-monoid-foldl`

(practice)

Show that if `_⊗_`

and `e`

form a monoid, then `foldr _⊗_ e`

and
`foldl _⊗_ e`

always compute the same result.

-- Your code goes here

## All

We can also define predicates over lists. Two of the most important
are `All`

and `Any`

.

Predicate `All P`

holds if predicate `P`

is satisfied by every element of a list:

data All {A : Set} (P : A → Set) : List A → Set where [] : All P [] _∷_ : ∀ {x : A} {xs : List A} → P x → All P xs → All P (x ∷ xs)

The type has two constructors, reusing the names of the same constructors for lists.
The first asserts that `P`

holds for every element of the empty list.
The second asserts that if `P`

holds of the head of a list and for every
element of the tail of a list, then `P`

holds for every element of the list.
Agda uses types to disambiguate whether the constructor is building
a list or evidence that `All P`

holds.

For example, `All (_≤ 2)`

holds of a list where every element is less
than or equal to two. Recall that `z≤n`

proves `zero ≤ n`

for any
`n`

, and that if `m≤n`

proves `m ≤ n`

then `s≤s m≤n`

proves ```
suc m ≤
suc n
```

, for any `m`

and `n`

:

_ : All (_≤ 2) [ 0 , 1 , 2 ] _ = z≤n ∷ s≤s z≤n ∷ s≤s (s≤s z≤n) ∷ []

Here `_∷_`

and `[]`

are the constructors of `All P`

rather than of `List A`

.
The three items are proofs of `0 ≤ 2`

, `1 ≤ 2`

, and `2 ≤ 2`

, respectively.

(One might wonder whether a pattern such as `[_,_,_]`

can be used to
construct values of type `All`

as well as type `List`

, since both use
the same constructors. Indeed it can, so long as both types are in
scope when the pattern is declared. That’s not the case here, since
`List`

is defined before `[_,_,_]`

, but `All`

is defined later.)

## Any

Predicate `Any P`

holds if predicate `P`

is satisfied by some element of a list:

data Any {A : Set} (P : A → Set) : List A → Set where here : ∀ {x : A} {xs : List A} → P x → Any P (x ∷ xs) there : ∀ {x : A} {xs : List A} → Any P xs → Any P (x ∷ xs)

The first constructor provides evidence that the head of the list
satisfies `P`

, while the second provides evidence that some element of
the tail of the list satisfies `P`

. For example, we can define list
membership as follows:

infix 4 _∈_ _∉_ _∈_ : ∀ {A : Set} (x : A) (xs : List A) → Set x ∈ xs = Any (x ≡_) xs _∉_ : ∀ {A : Set} (x : A) (xs : List A) → Set x ∉ xs = ¬ (x ∈ xs)

For example, zero is an element of the list `[ 0 , 1 , 0 , 2 ]`

. Indeed, we can demonstrate
this fact in two different ways, corresponding to the two different
occurrences of zero in the list, as the first element and as the third element:

_ : 0 ∈ [ 0 , 1 , 0 , 2 ] _ = here refl _ : 0 ∈ [ 0 , 1 , 0 , 2 ] _ = there (there (here refl))

Further, we can demonstrate that three is not in the list, because any possible proof that it is in the list leads to contradiction:

not-in : 3 ∉ [ 0 , 1 , 0 , 2 ] not-in (here ()) not-in (there (here ())) not-in (there (there (here ()))) not-in (there (there (there (here ())))) not-in (there (there (there (there ()))))

The five occurrences of `()`

attest to the fact that there is no
possible evidence for `3 ≡ 0`

, `3 ≡ 1`

, `3 ≡ 0`

, `3 ≡ 2`

, and
`3 ∈ []`

, respectively.

## All and append

A predicate holds for every element of one list appended to another if and only if it holds for every element of both lists:

All-++-⇔ : ∀ {A : Set} {P : A → Set} (xs ys : List A) → All P (xs ++ ys) ⇔ (All P xs × All P ys) All-++-⇔ xs ys = record { to = to xs ys ; from = from xs ys } where to : ∀ {A : Set} {P : A → Set} (xs ys : List A) → All P (xs ++ ys) → (All P xs × All P ys) to [] ys Pys = ⟨ [] , Pys ⟩ to (x ∷ xs) ys (Px ∷ Pxs++ys) with to xs ys Pxs++ys ... | ⟨ Pxs , Pys ⟩ = ⟨ Px ∷ Pxs , Pys ⟩ from : ∀ { A : Set} {P : A → Set} (xs ys : List A) → All P xs × All P ys → All P (xs ++ ys) from [] ys ⟨ [] , Pys ⟩ = Pys from (x ∷ xs) ys ⟨ Px ∷ Pxs , Pys ⟩ = Px ∷ from xs ys ⟨ Pxs , Pys ⟩

#### Exercise `Any-++-⇔`

(recommended)

Prove a result similar to `All-++-⇔`

, but with `Any`

in place of `All`

, and a suitable
replacement for `_×_`

. As a consequence, demonstrate an equivalence relating
`_∈_`

and `_++_`

.

-- Your code goes here

#### Exercise `All-++-≃`

(stretch)

Show that the equivalence `All-++-⇔`

can be extended to an isomorphism.

-- Your code goes here

#### Exercise `¬Any≃All¬`

(recommended)

Show that `Any`

and `All`

satisfy a version of De Morgan’s Law:

```
(¬_ ∘ Any P) xs ≃ All (¬_ ∘ P) xs
```

(Can you see why it is important that here `_∘_`

is generalised
to arbitrary levels, as described in the section on
universe polymorphism?)

Do we also have the following?

```
(¬_ ∘ All P) xs ≃ Any (¬_ ∘ P) xs
```

If so, prove; if not, explain why.

#### Exercise `All-∀`

(practice)

Show that `All P xs`

is isomorphic to `∀ {x} → x ∈ xs → P x`

.

-- You code goes here

#### Exercise `Any-∃`

(practice)

Show that `Any P xs`

is isomorphic to `∃[ x ] (x ∈ xs × P x)`

.

-- You code goes here

## Decidability of All

If we consider a predicate as a function that yields a boolean,
it is easy to define an analogue of `All`

, which returns true if
a given predicate returns true for every element of a list:

all : ∀ {A : Set} → (A → Bool) → List A → Bool all p = foldr _∧_ true ∘ map p

The function can be written in a particularly compact style by
using the higher-order functions `map`

and `foldr`

.

As one would hope, if we replace booleans by decidables there is again
an analogue of `All`

. First, return to the notion of a predicate `P`

as
a function of type `A → Set`

, taking a value `x`

of type `A`

into evidence
`P x`

that a property holds for `x`

. Say that a predicate `P`

is *decidable*
if we have a function that for a given `x`

can decide `P x`

:

Decidable : ∀ {A : Set} → (A → Set) → Set Decidable {A} P = ∀ (x : A) → Dec (P x)

Then if predicate `P`

is decidable, it is also decidable whether every
element of a list satisfies the predicate:

All? : ∀ {A : Set} {P : A → Set} → Decidable P → Decidable (All P) All? P? [] = yes [] All? P? (x ∷ xs) with P? x | All? P? xs ... | yes Px | yes Pxs = yes (Px ∷ Pxs) ... | no ¬Px | _ = no λ{ (Px ∷ Pxs) → ¬Px Px } ... | _ | no ¬Pxs = no λ{ (Px ∷ Pxs) → ¬Pxs Pxs }

If the list is empty, then trivially `P`

holds for every element of
the list. Otherwise, the structure of the proof is similar to that
showing that the conjunction of two decidable propositions is itself
decidable, using `_∷_`

rather than `⟨_,_⟩`

to combine the evidence for
the head and tail of the list.

#### Exercise `any?`

(stretch)

Just as `All`

has analogues `all`

and `All?`

which determine whether a
predicate holds for every element of a list, so does `Any`

have
analogues `any`

and `Any?`

which determine whether a predicate holds
for some element of a list. Give their definitions.

-- Your code goes here

#### Exercise `split`

(stretch)

The relation `merge`

holds when two lists merge to give a third list.

data merge {A : Set} : (xs ys zs : List A) → Set where [] : -------------- merge [] [] [] left-∷ : ∀ {x xs ys zs} → merge xs ys zs -------------------------- → merge (x ∷ xs) ys (x ∷ zs) right-∷ : ∀ {y xs ys zs} → merge xs ys zs -------------------------- → merge xs (y ∷ ys) (y ∷ zs)

For example,

_ : merge [ 1 , 4 ] [ 2 , 3 ] [ 1 , 2 , 3 , 4 ] _ = left-∷ (right-∷ (right-∷ (left-∷ [])))

Given a decidable predicate and a list, we can split the list into two lists that merge to give the original list, where all elements of one list satisfy the predicate, and all elements of the other do not satisfy the predicate.

Define the following variant of the traditional `filter`

function on lists,
which given a decidable predicate and a list returns all elements of the
list satisfying the predicate:

```
split : ∀ {A : Set} {P : A → Set} (P? : Decidable P) (xs : List A)
→ ∃[ ys ] ∃[ zs ] ( merge xs ys zs × All P ys × All (¬_ ∘ P) zs )
```

-- Your code goes here

## Standard Library

Definitions similar to those in this chapter can be found in the standard library:

import Data.List using (List; _++_; length; reverse; map; foldr; downFrom) import Data.List.All using (All; []; _∷_) import Data.List.Any using (Any; here; there) import Data.List.Membership.Propositional using (_∈_) import Data.List.Properties using (reverse-++-commute; map-compose; map-++-commute; foldr-++) renaming (mapIsFold to map-is-foldr) import Algebra.Structures using (IsMonoid) import Relation.Unary using (Decidable) import Relation.Binary using (Decidable)

The standard library version of `IsMonoid`

differs from the
one given here, in that it is also parameterised on an equivalence relation.

Both `Relation.Unary`

and `Relation.Binary`

define a version of `Decidable`

,
one for unary relations (as used in this chapter where `P`

ranges over
unary predicates) and one for binary relations (as used earlier, where `_≤_`

ranges over a binary relation).

## Unicode

This chapter uses the following unicode:

```
∷ U+2237 PROPORTION (\::)
⊗ U+2297 CIRCLED TIMES (\otimes, \ox)
∈ U+2208 ELEMENT OF (\in)
∉ U+2209 NOT AN ELEMENT OF (\inn, \notin)
```