# Declarative Programming

Most of the languages we’ve considered so far in this text have followed the imperative programming model, where a computation is decomposed into individual statements that modify the state of the program. These languages have also been procedural, grouping statements into subroutines that are then called explicitly.

We have also seen the functional programming model, which decomposes a computation into functions that are closely related to those in mathematics. In such a model, programming is done with expressions that avoid side effects. We have also considered specific languages that provide a mix of the functional and imperative paradigms.

Functional programs are *declarative*, since they declare a
relationship between the inputs and outputs of a function. We turn our
attention to other models that are declarative, including those that
express computation using logical relations, constraints, and
dependencies.

# Logic Programming

Whereas functional programming is based on the theoretical foundations
of \(\lambda\)-calculus, logic programming is based on the
foundation of formal logic. More specifically, it is based on
*first-order predicate calculus*, which expresses quantified
statements such as:

This states that for every value `X`

, over some implicit universe of
values, there is some value `Y`

such that either `P(X)`

is true or
`Q(Y)`

is false or both. This specific statement can also be written
in the form of an implication:

The implication \(a ~\Longrightarrow~ b\) is equivalent to \(\neg a ~\vee~ b\).

In most logic languages, a program is specified in terms of *axioms*
that are assumed to be true, and a programmer specifies a *goal* that
the system should attempt to prove from the set of axioms. An axiom is
usually written in the form of a *Horn clause*, which has the
following structure:

```
H :- B1, B2, ..., BN
```

The `:-`

symbol specifies a reverse implication, and the comma is
used for conjunction. The equivalent form in predicate calculus is:

In the Horn clause above, `H`

is the *head* of the clause, while
`B1, B2, ..., BN`

is the *body*. In natural language, the Horn
clause is stating that if `B1`

is true, and `B2`

is true, …, and
`BN`

is true, then it must be that `H`

is also true. (Quantifiers
are implicit in a Horn clause, though we will not discuss the details
here.)

The individual elements of a Horn clause, such as `H`

or `B2`

above, are called *terms*. A term may be a variable, an *atom* in the
form of a symbol, or a compound term, such as a *predicate* applied to
some arguments which are themselves terms.

A set of Horn clauses establishes *relations* among data, which we can
then use to query whether a relation holds or what pieces of data
satisfy a particular relation.

As a concrete example, consider the following clauses that represent familial relationships:

```
parent(P, C) :- mother(P, C). % rule 1
parent(P, C) :- father(P, C). % rule 2
sibling(A, B) :- parent(P, A), parent(P, B). % rule 3
```

Here, we have stated three *rules*. The first establishes that if
`P`

is the mother of `C`

, then `P`

is also a parent of `C`

.
The second states that if `P`

is the father of `C`

, then `P`

is
also a parent of `C`

. The last rule states that if `P`

is a parent
of `A`

, and `P`

is also a parent of `B`

, then `A`

and `B`

are siblings.

We can state some specific relationships as *facts*, which are Horn
clauses without a body and thus are unconditionally true:

```
mother(molly, bill). % fact 1
mother(molly, charlie). % fact 2
```

We can give the logic interpreter a query of the form ```
sibling(bill,
S)
```

. The interpreter will then attempt to solve this query using a
process known as *resolution*, which applies rules to existing
information. Part of this process is *unification*, which connects
terms that match. One possible resolution sequence for the query
above is:

```
sibling(bill, S)
-> parent(P, bill), parent(P, S) (rule 3)
-> mother(P, bill), parent(P, S) (rule 1)
-> mother(molly, bill), parent(molly, S) (fact 1)
-> mother(molly, bill), mother(molly, S) (rule 1)
-> mother(molly, bill), mother(molly, charlie) (fact 2)
```

The end result in this sequence would be that `S = charlie`

.

In the process above, the third step unifies the term ```
mother(P,
bill)
```

with `mother(molly, bill)`

, which in turn unifies `P`

with
`molly`

. This unification is reflected in all occurrences of `P`

,
resulting in the second term becoming `parent(molly, S)`

.
Unification is a generalized form of variable binding, except that
full terms can be unified with each other rather than just binding
variables to values.

In our formulation of familial relationships, however, there is
nothing preventing the resolution engine from applying ```
mother(molly,
bill)
```

in resolving `mother(molly, S)`

, so another perfectly valid
solution is that `S = bill`

. We will see later how to fix this
specific problem in Prolog.

## Prolog

Before we proceed further in our exploration of logic programming, let us introduce a concrete programming language to work with. Among logic languages, Prolog is by far the most popular, and many implementations are available. For the purposes of this text, we will use a specific interpreter called SWI-Prolog, of which there is also a web-based version.

The syntax we used above is actually that of Prolog. A Prolog program
consists of a set of Horn clauses that are assumed to be true. A
clause is composed of a head term and zero or more body terms, and a
term may be atomic, compound, or a variable. An atomic term may be an
*atom*, which is either a Scheme-like symbol or a quoted string. The
following are all atoms:

```
hello =< + 'logic programming'
```

If an atom starts with a letter, then that letter must be lowercase.
Thus, `hEllo`

is an atom, but `Hello`

is not. Numbers, which can
be integer or floating-point, are also atomic terms.

Variables are identifiers that begin with a capital letter. Thus,
`Hello`

is a variable, as are `A`

and `X`

.

Compound terms consist of a *functor*, which is itself an atom,
followed by a list of one or more argument terms. The following are
compound terms:

```
pair(1, 2) wizard(harry) writeln(hello(world))
```

A compound term is interpreted as a *predicate*, meaning that it has a
truth value, when it occurs as the head term or one of the body terms
of a clause, as well as when it is the goal query. Otherwise, it is
generally interpreted as data, as in `hello(world)`

in
`writeln(hello(world))`

.

While the syntax of a compound term resembles that of a function call in many imperative or functional languages, Prolog does not have functions, so a compound term is never interpreted as such.

A Horn clause with no body is a fact, since it is always true. Thus, the following are facts:

```
mother(molly, bill).
mother(molly, charlie).
```

Notice the period that signifies the end of a clause.

A Horn clause with a body is called a rule, and it consists of a head
term, the reverse implication symbol (`:-`

), and one or more body
terms, separated by commas. The comma signifies conjunction so that
the head is true when all the body terms are true. The following are
rules:

```
parent(P, C) :- mother(P, C).
sibling(A, B) :- parent(P, A), parent(P, B).
```

The first rule states that if `mother(P, C)`

is true, then
`parent(P, C)`

is also true. The second rule states that if both
`parent(P, A)`

and `parent(P, B)`

are true, then `sibling(A, B)`

is true.

A program is composed of a set of facts and rules. Once these have been established, we can query the Prolog interpreter with a a goal predicate. The interpreter will attempt to establish that the goal is true, and if it contains variables, instantiate them with terms that result in the satisfaction of the goal. If the query succeeds, the interpreter reports success, along with the terms that the variables unified with in order to establish the result. If more than one solution may exist, we can ask for the next one using a semicolon in most interpreters. If we ask for a solution and no more exist, the interpreter reports failure.

As an example, consider the query `sibling(bill, S)`

. Loading a file
containing the two facts and rules above will result in the
interactive prompt `?-`

(For now, we have elided the `parent`

rule
that depended on `father`

, since we haven’t established any
`father`

facts and our Prolog interpreter will report an error as a
result.):

```
?- sibling(bill, S).
S = bill ;
S = charlie.
```

At the prompt, we’ve entered the query followed by a period to signify
the end of the query. The interpreter reports `S = bill`

as the
first result, and we have the option of entering a semicolon to search
for another or a period to end the query. We enter a semicolon, and
the interpreter finds and reports `S = charlie`

, as well as a period
to indicate its certainty that no more solutions exist.

The actual order in which Prolog searches for a result is
deterministic, as we will see shortly. Thus, the query will always
find `S = bill`

as its first result and `S = charlie`

as its
second.

### Lists

Compound terms, which can relate multiple individual terms, allow us
to represent data structures. For example, we can use the compound
term `pair(First, Second)`

to represent a pair composed of `First`

and `Second`

. The term will not appear on its own as a head or body
term, so it will be treated as data. We can then define relations for
lists as follows:

```
cons(First, Second, pair(First, Second)).
car(pair(First, _), First).
cdr(pair(_, Second), Second).
is_null(nil).
```

In the clauses above, an underscore represents an anonymous variable.
Many Prolog implementations will raise a warning if a variable is used
only once in a clause – such a variable is called a *singleton*, and
it can be introduced inadvertently due to a typo, as in the
following:

```
cons(First, Second, pair(Frist, Second)).
```

Here, we misspelled `First`

as `Frist`

, so that both are singleton
variables. The warning from the Prolog interpreter directs our
attention to this, so that we can fix it. If, on the other hand, we do
intend a variable to be used only once, we can start it with an
underscore to inform the implementation of that intent. We can also
use a lone underscore, and each occurrence of a solitary underscore is
considered a separate, anonymous variable.

We’ve set `nil`

as our representation for an empty list. We can then
make queries on lists as follows:

```
?- cons(1, nil, X).
X = pair(1, nil).
?- car(pair(1, pair(2, nil)), X).
X = 1.
?- cdr(pair(1, pair(2, nil)), X).
X = pair(2, nil).
?- cdr(pair(1, pair(2, nil)), X), car(X, Y), cdr(X, Z).
X = pair(2, nil),
Y = 2,
Z = nil.
?- is_null(nil).
true.
?- is_null(pair(1, pair(2, nil))).
false.
```

In the fourth example, we’ve used conjunction to obtain the cdr of the original list, as well as the car and the cdr of the result.

As in Scheme, lists are a fundamental data structure in Prolog, so Prolog provides its own syntax for lists. A list can be specified by placing elements in square brackets, separated by commas:

```
[]
[1, a]
[b, 3, foo(bar)]
```

A list can be decomposed into a number of items followed by a rest, much like the period in Scheme, using a pipe:

```
?- writeln([1, 2 | [3, 4]]). % similar to (1 2 . (3 4)) in Scheme
[1,2,3,4]
true.
```

We can use this syntax to write rules on lists. For example, a contains predicate is as follows:

```
contains([Item|_Rest], Item).
contains([_First|Rest], Item) :-
contains(Rest, Item).
```

The first clause asserts that a list whose first element is `Item`

contains `Item`

. The second clause states that a list contains
`Item`

if the remaining list, excluding the first item, contains
`Item`

. Thus:

```
?- contains([], a).
false.
?- contains([a], a).
true .
?- contains([b, c, a, d], a).
true .
```

The built-in `member`

predicate works similarly to our definition of
`contains`

, except that it takes the arguments in reverse order:

```
?- member(a, []).
false.
?- member(a, [a]).
true.
```

### Arithmetic

Prolog provides numbers, as well as comparison predicates on numbers. For convenience, these predicates may be written in infix order:

```
?- 3 =< 4. % less than or equal
true.
?- 4 =< 3.
false.
?- 3 =:= 3. % equal (for arithmetic)
true.
?- 3 =\= 3. % not equal (for arithmetic)
false.
```

Prolog also provides arithmetic operators, but they merely represent
compound terms. Thus, `3 + 4`

is another means of writing the
compound term `+(3, 4)`

. If we attempt to unify this with 7 using
the explicit unification operator `=`

, it will fail:

```
?- 7 = 3 + 4.
false.
```

Similarly, if we attempt to unify a variable with an arithmetic expression, it will be unified with the compound term itself:

```
?- X = 3 + 4.
X = 3+4.
```

Comparison operators, however, do evaluate the arithmetic expressions in their operands::

```
?- 7 =:= 3 + 4.
true.
?- 2 + 5 =:= 3 + 4.
true.
?- 4 < 3 + 2.
true.
```

In order for the operands to be evaluated, variables in the operands
must be *instantiated* with numeric values. A comparison cannot be
applied to an uninstantiated variable:

```
?- X =:= 3 + 4.
ERROR: Arguments are not sufficiently instantiated
```

Instead, the `is`

operator is defined to unify its first argument
with the arithmetic result of its second argument, allowing the first
argument to be an uninstantiated variable:

```
?- 7 is 3 + 4.
true.
?- X is 3 + 4.
X = 7.
?- X is 3 + 4, X =:= 7.
X = 7.
?- X is 3 + 4, X = 7.
X = 7.
```

In the third example, `X`

is unified with 7, the result of adding 3
and 4. Since `X`

is now instantiated with 7, it can be compared
to 7. In the fourth example, `X`

is 7 so it unifies with the
number 7.

We can use this to define a length predicate on our list representation above:

```
len(nil, 0).
len(pair(_First, Second), Length) :-
len(Second, SecondLength), Length is SecondLength + 1.
```

Here, `Length`

is unified with the arithmetic result of adding 1 to
`SecondLength`

. This must occur after the recursive application of
`len`

, so that `SecondLength`

is sufficiently instantiated to be
able to perform arithmetic on it. Then:

```
?- len(nil, X).
X = 0.
?- len(pair(1, pair(b, nil)), X).
X = 2.
```

### Side Effects

Prolog provides several predicates that perform input and output.
We’ve already used the `writeln`

predicate, which writes a term to
standard out and then writes a newline. The `write`

predicate also
writes a term to standard out, but without a trailing newline:

```
?- X = 3, write('The value of X is: '), writeln(X).
The value of X is: 3
X = 3.
```

We will not discuss the remaining I/O routines here.

## Unification and Search

The core computational engine in Prolog revolves around unification and search. The search procedure takes a set of goal terms and looks for a clause that has a head that can unify with one of the terms. The unification process can recursively unify subterms, which may instantiate or unify variables. If the current term unifies with the head of a clause, then the body terms, with variables suitably instantiated, are added to the set of goal terms. The search process succeeds when no more goal terms remain.

This process of starting from goal terms and working backwards,
replacing heads with bodies, is called *backward chaining*. A logic
interpreter may use *forward chaining* instead, which starts from
facts and works forward to derive the goal. However, Prolog is defined
to use backward chaining.

The unification rules for two terms in Prolog are as follows:

An atomic term only unifies with itself.

An uninstantiated variable unifies with any term. If the other term is not a variable, then the variable is instantiated with the value of the other term. If the other term is another variable, then the two variables are bound together such that if one of them is later instantiated with a value, then so is the other.

A compound term unifies with another compound term that has the same functor and number of arguments, and only if the arguments of the two compound terms also unify.

As stated by the first rule, the atomic term 1 only unifies with 1,
and the term `abc`

only unifies with `abc`

.

The second rule states that a variable `X`

unifies with a
non-variable by instantiating it to the given value. This essentially
means that all occurrences of the variable are replaced with the given
value. Thus `X`

unifies with 3 by instantiating `X`

with 3, `Y`

unifies with `foo(1, 3)`

by instantiating it with `foo(1, 3)`

, and
`Z`

unifies with `foo(A, B)`

by instantiating it with ```
foo(A,
B)
```

.

A variable unifies with another variable by binding them together.
Thus, if `X`

unifies with `Y`

, and if `Y`

is later instantiated
with 3, then `X`

is also instantiated with 3.

The last rule states that a compound term such as `foo(1, X)`

unifies with `foo(Y, 3)`

by recursively unifying the arguments, such
that `Y`

is instantiated with 1 and `X`

with 3.

Care must be taken in the search process to treat variables that appear in independent contexts as independent, even if they have the same name. Thus, given the clause:

```
foo(X, Y) :- bar(Y, X).
```

and the goal `foo(3, X)`

, the variable `X`

should be treated as
distinct in the contexts of the goal and the clause. One way to
accomplish this is renaming before applying a rule, analogous to
\(\alpha\)-reduction in \(\lambda\)-calculus:

```
foo(X1, Y1) :- bar(Y1, X1).
```

Thus, unifying the goal `foo(3, X)`

with the head `foo(X1, Y1)`

produces `X1 = 3`

and `Y1 = X`

, resulting in the subsequent goal
`bar(X, 3)`

.

### Search Order and Backtracking

In pure logic programming, the order in which clauses are applied and body terms are resolved doesn’t matter as long as the search process terminates. However, since Prolog has side effects and non-pure operations, it specifies a well-defined order for both. In particular, clauses for a predicate are attempted to be applied in program order, and terms in a conjunction are resolved from left to right. This provides the programmer with some control over how computation proceeds, which can be used to improve efficiency as well as sequence side effects.

A search process that goes down one path may end up in a dead end,
where no clauses can be applied to a goal term. This should not
immediately result in failure, since changing a previous decision made
by the search may lead to a solution. Thus, the search process
performs *backtracking* on failure, or even on success if a user
requests more solutions. This reverts the search process to the last
choice point with remaining options, at which a different choice is
made about which clause to apply.

As an example, consider the following clauses:

```
sibling(A, B) :- mother(P, A), mother(P, B).
mother(lily, harry).
mother(molly, bill).
mother(molly, charlie).
```

Suppose the goal is `sibling(S, bill)`

. Then the search tree is as
in Figure 34.

The search will first unify `sibling(S, bill)`

with the goal term
`sibling(A, B)`

, binding `S`

and `A`

together and instantiating
`B`

with `bill`

. We use the notation `S = A`

to denote that
`S`

and `A`

are bound together, as in
Figure 35.

Prolog will then add the body terms to its set of goals, so that
`mother(P, A)`

and `mother(P, B)`

need to be satisfied. It then
searches for a solution to `mother(P, A)`

, under an environment in
which `S`

and `A`

are bound together and `B`

is instantiated
with `bill`

. There are several clauses that can be applied to
satisfy `mother(P, A)`

, introducing a choice point. Prolog attempts
to apply clauses in program order, so the first choice the search
engine will make is to unify `mother(P, A)`

with ```
mother(lily,
harry)
```

, as shown in Figure 36.

This instantiates `A`

, and therefore `S`

since `A`

and `S`

are
bound together, with `harry`

and `P`

with `lily`

. Then only the
goal term `mother(P, B)`

remains, and since multiple clauses can be
applied, another choice point is introduced. The first choice is to
unify `mother(P, B)`

with `mother(lily, harry)`

, as demonstrated
in Figure 37.

This unification fails, since it requires `B`

to be unified with
`harry`

. However, `B`

is currently instantiated with the atom
`bill`

, and two atoms only unify if they are the same, so that
`bill`

and `harry`

do not unify. The unification failure causes
the search engine to backtrack to the previous choice point, so that
it instead attempts to unify `mother(P, B)`

with ```
mother(molly,
bill)
```

. Figure 38 illustrates this.

This unification also fails, since it requires `P`

, currently
instantiated with `lily`

, to be unified with `molly`

. The search
backtracks once again, trying to unify `mother(P, B)`

with
`mother(molly, charlie)`

, as shown in
Figure 39.

Again, the unification fails, so the search backtracks. At this point,
it has exhausted all the choices for `mother(P, B)`

, so it
backtracks further to the preceding choice point. Now, it makes the
choice of unifying `mother(P, A)`

with `mother(molly, bill)`

, as
illustrated in Figure 40.

This instantiates `P`

with `molly`

and `A`

and `S`

with
`bill`

. Then, as shown in Figure 41, the
search attempts to find a solution for `mother(P, B)`

, first
attempting to unify it with `mother(lily, harry)`

.

This fails, since `P = molly`

cannot unify with `lily`

. Thus, the
search backtracks to the previous choice point, attempting to unify
`mother(P, B)`

with `mother(molly, bill)`

.
Figure 42 demonstrates this.

This succeeds. No more goal terms remain, so the query succeeds with a
solution of `S = bill`

.

We can proceed to ask the interpreter for more solutions, which
continues the search at the last choice point. One more choice
remains, to unify `mother(P, B)`

with `mother(molly, charlie)`

, as
shown in Figure 43.

However, this fails, so the search backtracks to the preceding choice
point, unifying `mother(P, A)`

with `mother(molly, charlie)`

, as
illustrated in Figure 44.

Continuing the search with `P`

instantiated with `molly`

and `A`

and `S`

with `charlie`

reaches another choice point for
`mother(P, B)`

. As Figure 45 demonstrates,
the first choice fails.

However, the second choice of unifying `mother(P, B)`

with
`mother(molly, bill)`

succeeds, as shown in
Figure 46.

Thus, we have another solution of `S = charlie`

. We can then ask for
another solution, resulting in the search engine trying the last
choice for `mother(P, B)`

, as demonstrated in
Figure 47.

This choice fails. At this point, all choice points have been exhausted, so the interpreter reports that no more solutions can be found. The full interpreter interaction is as follows, reflecting the search process above:

```
?- sibling(S, bill).
S = bill ;
S = charlie ;
false.
```

## The Cut Operator

By default, Prolog considers each possible alternative in turn when it
reaches a choice point. However, Prolog provides the *cut operator*,
written as `!`

, to eliminate choice points associated with the
current predicate. For example, recall the `contains`

predicate:

```
contains([Item|_Rest], Item).
contains([_First|Rest], Item) :-
contains(Rest, Item).
```

A query such as `contains([1, 2, 3, 4], 2)`

introduces a choice
point as to which clause to unify with the goal. The first choice
fails, since `contains([1, 2, 3, 4], 2)`

cannot unify with
`contains([Item|_Rest], Item)`

. However, the second choice succeeds,
so that we have a new goal term of `contains([2, 3, 4], 2)`

. Here
another choice point occurs, and the first choice succeeds, with
`Item`

instantiated with 2 and `_Rest`

instantiated with ```
[3,
4]
```

. Since no goal terms remain, the query as a whole succeeds.
However, the interpreter still has an unexplored choice available, so
it will report that more solutions may exist, requiring us to manually
either continue the query with a semicolon or end it with a dot:

```
?- contains([1, 2, 3, 4], 2).
true ;
false.
```

Instead, we can add the cut operator to tell the interpreter to stop
searching for alternatives once it has found a solution for
`contains([1, 2, 3, 4], 2)`

:

```
?- contains([1, 2, 3, 4], 2), !.
true.
```

We can similarly rewrite `contains`

to eliminate choice points upon
success:

```
contains([Item|_Rest], Item) :- !.
contains([_First|Rest], Item) :-
contains(Rest, Item).
```

Here, as soon as a goal term unifies with ```
contains([Item|_Rest],
Item)
```

, the choice point of which `contains`

clause to unify with
that goal term is eliminated. Thus, only one solution is found:

```
?- contains([1, 2, 3, 4], 2).
true.
```

On the other hand, the fact that the cut operator prevents other choices from being considered can result in queries that previously succeeded to now fail:

```
?- contains([1, 2, 3, 4], X), X = 3.
false.
```

Here, the first goal term succeeds by instantiating `X`

with 1, and
the cut operator prevents other choices for `X`

from being
considered. Then `X`

, now instantiated as 1, fails to unify with 3
in the second goal term.

Given the potential negative consequences of eliminating choice points, using the cut operator is often considered bad practice, so that it should be avoided in most cases. In this text, we only use the cut operator as part of a query, not as part of a rule.

## Negation

The search above for `sibling(S, bill)`

produced the undesirable
result `S = bill`

. In order to eliminate results from consideration,
Prolog provides a limited form of negation. For instance, we can
rewrite the `sibling`

rule as:

```
sibling(A, B) :- A \= B, mother(P, A), mother(P, B).
```

This states that `A`

must not be unifiable with `B`

. Unfortunately,
our query will now fail completely:

```
?- sibling(S, bill).
false.
```

This is because when the body term `A \= B`

is reached, `A`

is
uninstantiated while `B`

is instantiated with `bill`

. The
unification rules above allow an uninstantiated variable to unify with
anything: `A`

can unify with `B`

by instantiating `A`

with
`bill`

. Since `A`

is unifiable with `B`

, the goal term ```
A \=
B
```

fails.

On the other hand, if we write the rule as follows:

```
sibling(A, B) :- mother(P, A), mother(P, B), A \= B.
```

then our query succeeds:

```
?- sibling(S, bill).
S = charlie .
```

This is because `A`

and `B`

are instantiated as atoms by the time
they get to the last term, and we can assert that two atoms not unify.

Prolog also provides an explicit negation predicate, `\+`

. We can
therefore query whether `harry`

and `bill`

are not siblings:

```
?- \+(sibling(harry, bill)).
true.
```

Unfortunately, we cannot obtain a more general result from the search
engine, such as asking it to find someone who is not a sibling of
`bill`

:

```
?- \+(sibling(S, bill)).
false.
```

This is because negation in Prolog is handled by attempting to prove
the term being negated, and only if the proof fails is the negation
true. However, the query `sibling(S, bill)`

does indeed succeed with
`S = charlie`

, so negation results in false.

Thus, while Prolog does provide negation, it is of limited use. This is not a deficiency in Prolog itself, but rather follows from the limits of the logic-programming paradigm as a whole, which cannot provide the full expressiveness of first-order predicate calculus.

## Examples

We conclude with some more interesting examples expressed in the logic paradigm.

Suppose we wish to find a seven digit number such that the first digit is the count of zeroes in the digits of the number, the second digit is the count of ones, and so on. Using Prolog, we can express this computation as follows. We will represent our results as a list of digits. First, we define a predicate to count the occurrences of a particular numerical value in a list:

count(_Item, [], 0). count(Item, [Item|Rest], Count) :- count(Item, Rest, RestCount), Count is RestCount + 1. count(Item, [Other|Rest], Count) :- Item =\= Other, count(Item, Rest, Count).

The first rule states that an arbitrary item occurs zero times in an empty list. The second states that if a value is the first item in a list, then the number times it occurs in the list is one more than the number of times it appears in the rest of the list. The last rule states that if a value is not equal to the first item, then its number of occurrences is that same as the number of times it appears in the rest of the lest.

Next, we define facts to restrict the values of a digit:

is_digit(0). is_digit(1). is_digit(2). is_digit(3). is_digit(4). is_digit(5). is_digit(6).

Alternatively, we can define the

`is_digit`

predicate using`member`

:is_digit(Digit) :- member(Digit, [0, 1, 2, 3, 4, 5, 6]).

Finally, we define a predicate to compute our result:

digits(List) :- List = [Digit0, Digit1, Digit2, Digit3, Digit4, Digit5, Digit6], is_digit(Digit0), is_digit(Digit1), is_digit(Digit2), is_digit(Digit3), is_digit(Digit4), is_digit(Digit5), is_digit(Digit6), count(0, List, Digit0), count(1, List, Digit1), count(2, List, Digit2), count(3, List, Digit3), count(4, List, Digit4), count(5, List, Digit5), count(6, List, Digit6).

We start by unifying the argument

`List`

with a list of seven items. We then specify that each item must be a digit. Finally, we require that the the first item be the count of zeroes in the list, the second the count of ones, and so on.Entering our query, we get the sole result:

?- digits(List). List = [3, 2, 1, 1, 0, 0, 0] ; false.

We can proceed to write a predicate that relates a list of digits to an actual number:

digits_number([], 0). digits_number([First|Rest], Number) :- digits_number(Rest, RestNumber), length(Rest, RestLength), Number is First * 10 ^ RestLength + RestNumber.

An empty list is related to zero. Otherwise, we compute the number represented by the list excluding its first item, as well of the length of that list. Then the number representing the total list is the number of the smaller list plus the multiple of the power of 10 represented by the first digit. Then:

?- digits(List), digits_number(List, Number), !. List = [3, 2, 1, 1, 0, 0, 0], Number = 3211000.

The

*Tower of Hanoi*is a classic puzzle that consists of three rods and a set of \(N\) discs of different sizes that slide onto a rod. The puzzle starts with discs in ascending order from top to bottom on a single rod, and the goal is to move the entire stack to another rod by moving one disc at a time. It is prohibited to place a larger disc on top of a smaller one. The solution is to recursively move the \(N-1\) smaller discs to the third rod, move the remaining, largest disc to the second rod, and then to recursively move the other \(N-1\) discs to the second rod.We can express this computation in Prolog as follows, using the

`write`

and`writeln`

predicates to print a move to standard output:move(Disc, Source, Target) :- write('Move disc '), write(Disc), write(' from '), write(Source), write(' to '), writeln(Target). hanoi(1, Source, Target, _Temporary) :- move(1, Source, Target). hanoi(NumDiscs, Source, Target, Temporary) :- Previous is NumDiscs - 1, hanoi(Previous, Source, Temporary, Target), move(NumDiscs, Source, Target), hanoi(Previous, Temporary, Target, Source).

The

`move`

predicate, given a disc and source and target rods, merely writes out the move to standard output. The`hanoi`

predicate relates a number of discs and three rods, a source rod, a target rod, and a temporary rod. The base case is when there is one disc, and that disc can be moved directly from source to target. The second`hanoi`

rule is the recursive case, which requires is recursively move all but the largest disc to the temporary rod, move the largest disc to the target rod, and then move the remaining discs from the temporary to the target rod. Since Prolog solves the body terms in order, the moves will occur in the right order.The follows is the result of a query with \(N = 4\):

?- hanoi(4, a, b, c). Move disc 1 from a to c Move disc 2 from a to b Move disc 1 from c to b Move disc 3 from a to c Move disc 1 from b to a Move disc 2 from b to c Move disc 1 from a to c Move disc 4 from a to b Move disc 1 from c to b Move disc 2 from c to a Move disc 1 from b to a Move disc 3 from c to b Move disc 1 from a to c Move disc 2 from a to b Move disc 1 from c to b true .

The

*quicksort*algorithm sorts a list by choosing a pivot, often the first item, partitioning the remaining list into elements that are less than and greater than or equal to the pivot, recursively sorting the partitions, and then appending them. The following Prolog code expresses this:quicksort([], []). quicksort([Pivot|Rest], Sorted) :- partition(Pivot, Rest, Smaller, GreaterOrEqual), quicksort(Smaller, SortedSmaller), quicksort(GreaterOrEqual, SortedGreaterOrEqual), append(SortedSmaller, [Pivot|SortedGreaterOrEqual], Sorted).

The first item is chosen as the pivot, and the remaining items are then partitioned into the smaller items and those that are greater than or equal to the pivot. The two smaller lists are recursively sorted, and then the results are appended, with the pivot placed in front of the items that are greater than or equal to it, to produce the sorted result.

The

`partition`

predicate is as follows:partition(_Pivot, [], [], []). partition(Pivot, [Item|Rest], [Item|Smaller], GreaterOrEqual) :- Item < Pivot, partition(Pivot, Rest, Smaller, GreaterOrEqual). partition(Pivot, [Item|Rest], Smaller, [Item|GreaterOrEqual]) :- Item >= Pivot, partition(Pivot, Rest, Smaller, GreaterOrEqual).

The first item in the list is either less than the pivot or greater than or equal to it. In the first case, the item should be the first one in the smaller partition, and the rest of the list is partitioned to produce the rest of the smaller and greater-than-or-equal partitions. In the second case, the item should be the first one in the greater-than-or-equal partition, and recursion handles the rest of the list.

Entering a query for a specific list produces:

?- quicksort([4, 8, 5, 3, 1, 2, 6, 9, 7], X). X = [1, 2, 3, 4, 5, 6, 7, 8, 9] .

The

*sieve of Eratosthenes*is an algorithm for computing prime numbers up to some limit \(N\). We start by constructing a list of integers in order from 2 to \(N\). Then we repeat the following process, until no numbers remain:The first item in the list is prime.

Filter out all multiples of the first item from the remaining list.

Go to step 1.

We can write this algorithm in Prolog as follows. First, we construct a list with the integers from 2 to the limit \(N\):

numbers(2, [2]). numbers(Limit, Numbers) :- LimitMinusOne is Limit - 1, numbers(LimitMinusOne, NumbersToM), append(NumbersToM, [Limit], Numbers).

We do so by recursively computing a list of integers from 2 to \(N-1\) and then appending \(N\) to the result.

We can then use write a predicate to filter out the multiples of a factor from a list:

filter_not_multiple(_Factor, [], []). filter_not_multiple(Factor, [Number|Rest], [Number|FilteredRest]) :- Number mod Factor =\= 0, filter_not_multiple(Factor, Rest, FilteredRest). filter_not_multiple(Factor, [Number|Rest], FilteredRest) :- Number mod Factor =:= 0, filter_not_multiple(Factor, Rest, FilteredRest).

The

`filter_not_multiple`

predicate relates a factor and a list of numbers to a list with the multiples of the factor filtered out. The second rule retains`Number`

in the resulting list if it is not a multiple of`Factor`

. The third rule discards`Number`

from the filtered list if it is a multiple of`Factor`

.We can proceed to define a

`sieve`

predicate that relates a list of numbers to the result of applying the prime-sieve algorithm to the list:sieve([], []). sieve([Number|Rest], [Number|SievedRest]) :- filter_not_multiple(Number, Rest, FilteredRest), sieve(FilteredRest, SievedRest).

The first number is retained in the result. All multiples of the first number are filtered out of the rest of the list. The sieve algorithm is then recursively applied to the filtered list to obtain the rest of the result list.

Finally, we write a

`primes`

predicate that relates an integer limit to a list of primes up to and including that limit:primes(Limit, Primes) :- numbers(Limit, Numbers), sieve(Numbers, Primes).

This rule constructs a list of numbers from 2 up to the limit and then applies the sieve algorithm to the list. We can then use the sieve to compute prime numbers up to 100:

?- primes(100, P). P = [2, 3, 5, 7, 11, 13, 17, 19, 23|...] [write] P = [2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47, 53, 59, 61, 67, 71, 73, 79, 83, 89, 97] .

Pressing the

`w`

key when the solution is displayed in truncated form causes the interpreter to print out the non-truncated form in the second line above.We can also use the built-in

`numlist`

predicate rather than the`numbers`

predicate we wrote:primes(Limit, Primes) :- numlist(2, Limit, Numbers), sieve(Numbers, Primes).

# Constraints and Dependencies

In addition to functional and logic programming, the declarative
paradigm includes programs that express constraints among variables
and constants as well those that describe dependency graphs. We will
look at the former in constraint logic programming and an instance of
the latter in the `make`

build automation tool.

## Constraint Logic Programming

*Constraint logic programming* is an extension of logic programming to
include constraints on variables. While logic programming allows a
limited form of constraints, languages such as Prolog only allow
arithmetic constraints to be applied to variables that have been
instantiated. For example, suppose we wanted to find a number less
than 1000 that is both a square and the sum of two squares. The
following is an attempt to specify this in Prolog:

```
square_sum([N, X, Y, Z]) :-
N =:= Z * Z, N =:= X * X + Y * Y,
X > 0, Y > 0, Z > 0, X < Y, N < 1000.
```

We can attempt a query:

```
?- square_sum(S).
ERROR: =:=/2: Arguments are not sufficiently instantiated
```

Unfortunately, since `N`

and `Z`

are not instantiated in the
comparison `N =:= Z`

, we get an error.

On the other hand, using the CLP(FD) library for Prolog, which allows constraint logic programming over finite domains, we can specify the solution as follows:

```
:- use_module(library(clpfd)). % load the clpfd library
square_sum_c([N, X, Y, Z]) :-
N #= Z * Z, N #= X * X + Y * Y,
X #> 0, Y #> 0, Z #> 0, X #< Y, N #< 1000,
label([N, X, Y, Z]).
```

The first clause loads the library for use. We can then specify
arithmetic constraints using operators that begin with a pound symbol.
For instance, the `#=`

operator constrains the two arguments to be
equal, while the `#<`

operator constraints the first argument to be
smaller than the second. Finally, the `label`

predicate forces the
solver to *ground* the given variables, computing actual values for
them rather than specifying their results as constraints. Entering a
query, we can now obtain all solutions:

```
?- square_sum_c(S).
S = [25, 3, 4, 5] ;
S = [100, 6, 8, 10] ;
S = [169, 5, 12, 13] ;
S = [225, 9, 12, 15] ;
S = [289, 8, 15, 17] ;
S = [400, 12, 16, 20] ;
S = [625, 7, 24, 25] ;
S = [625, 15, 20, 25] ;
S = [676, 10, 24, 26] ;
S = [841, 20, 21, 29] ;
S = [900, 18, 24, 30] ;
false.
```

### Search

In constraint logic programming, resolution follows the same basic
process as in plain logic programming. For a solver that uses backward
chaining, a set of goal terms is maintained, and the solver searches
for a clause whose head can be unified with the first goal term. If
unification succeeds, then the body terms that are not constraints are
added to the set of goals. Terms that are constraints are added to a
separate set called the *constraint store*. When a new constraint is
added to the store, in principle, the store is checked to make sure
that the constraints are satisfiable, and if not, backtracking is done
as in standard search. In practice, however, more limited checking is
performed in order to obtain better efficiency from the solver. A
solution is obtained when no more goal terms remain, and the set of
constraints in the store is satisfiable.

### Examples

As another example of using constraints, consider the canonical verbal arithmetic puzzle of finding a solution to the following equation:

```
S E N D
+ M O R E
-----------
= M O N E Y
```

Requirements are that each letter be a distinct digit, and that the leading digit of a number not be zero. We can express this problem in plain Prolog as the following:

```
is_digit(Digit) :-
numlist(0, 9, AllDigits),
member(Digit, AllDigits).
money([S, E, N, D, M, O, R, Y]) :-
is_digit(S), is_digit(E), is_digit(N), is_digit(D),
is_digit(M), is_digit(O), is_digit(R), is_digit(Y),
S \= 0, M \= 0,
S \= E, S \= N, S \= D, S \= M, S \= O, S \= R, S \= Y,
E \= N, E \= D, E \= M, E \= O, E \= R, E \= Y,
N \= D, N \= M, N \= O, N \= R, N \= Y,
D \= M, D \= O, D \= R, D \= Y,
M \= O, M \= R, M \= Y,
O \= R, O \= Y,
R \= Y,
1000 * S + 100 * E + 10 * N + D
+ 1000 * M + 100 * O + 10 * R + E
=:= 10000 * M + 1000 * O + 100 * N + 10 * E + Y.
```

First, we require that each variable be a digit in the range
\([0, 9]\), and we further require that `S`

and `M`

not
be zero. We then specify the pairwise uniqueness requirements.
Finally, we specify that the variables must satisfy the target
equation.

We can enter a query as follows:

```
?- money(S), !.
S = [9, 5, 6, 7, 1, 0, 8, 2].
```

Computing this solution takes close to a minute on the author’s iMac computer, since the solver has to search a large portion of the solution space, with much backtracking.

We can simplify the implementation of `money`

by using the
higher-order `maplist`

predicate, as well as the built-in
`is_set`

:

```
money(List) :-
List = [S, E, N, D, M, O, R, Y],
maplist(is_digit, List), S \= 0, M \= 0, is_set(List),
1000 * S + 100 * E + 10 * N + D
+ 1000 * M + 100 * O + 10 * R + E
=:= 10000 * M + 1000 * O + 100 * N + 10 * E + Y.
```

The term `maplist(is_digit, List)`

applies the higher-order
`maplist`

predicate to map the `is_digit`

predicate across the
elements of `List`

, and `is_set(List)`

ensures that `List`

has
no duplicates. While this solution is shorter, it runs even slower
than our original solution, taking about a minute and half on the same
machine.

On the other hand, we can use CLP(FD) to specify the problem as follows:

```
:- use_module(library(clpfd)).
money_c([S, E, N, D, M, O, R, Y]) :-
List = [S, E, N, D, M, O, R, Y],
List ins 0 .. 9, S #\= 0, M #\= 0, all_distinct(List),
1000 * S + 100 * E + 10 * N + D
+ 1000 * M + 100 * O + 10 * R + E
#= 10000 * M + 1000 * O + 100 * N + 10 * E + Y,
label(List).
```

The `ins`

predicate is defined by CLP(FD) to constrain that the
variables in the first argument each be contained in the set that is
the second argument. The `..`

operator specifies a range, so that
`0 .. 9`

is the range \([0, 9]\). The `all_distinct`

predicate
constraints the variables in the argument to take on distinct values.
Finally, we use `label`

at the end to ground the given variables
with actual values. We obtain the same result:

```
?- money_c(S), !.
S = [9, 5, 6, 7, 1, 0, 8, 2].
```

The solver can use the set of constraints to eliminate most of the
search space, and the remaining candidates are checked when the
`label`

predicate is reached. The result is that computing this
solution takes about 0.003 seconds on the author’s iMac, a speedup of
about 18000x.

As another example, consider the problem of solving a Sudoku puzzle. The following predicate takes in a nested list of lists, in row-major order, with some entries provided but others filled with anonymous variables:

```
sudoku(Rows) :-
length(Rows, 9), maplist(same_length(Rows), Rows),
append(Rows, Values), Values ins 1..9,
maplist(all_distinct, Rows),
transpose(Rows, Columns),
maplist(all_distinct, Columns),
Rows = [Row1, Row2, Row3, Row4, Row5, Row6, Row7, Row8, Row9],
blocks(Row1, Row2, Row3),
blocks(Row4, Row5, Row6),
blocks(Row7, Row8, Row9),
maplist(label, Rows).
```

The first body term requires that the number of rows be 9. The second
uses `maplist`

, which maps a predicate over the items in a list, as
we saw in the previous example. The `same_length(Rows)`

argument is
a partially applied predicate that, when applied to another argument,
requires that the two argument lists have the same length. The term as
a whole is checking that each row also has the same length as the
number of rows. The `append`

term takes a list of lists and
concatenates them into the single list `Values`

. We then constrain
that each variable be in the range \([1, 9]\). The next term
constrains each row to consist of distinct numbers, and the following
two terms constrain each of the columns to consist of distinct
numbers. The next four terms constrain each of the 9x9 squares to be
composed of distinct numbers, with the `blocks`

predicate defined
below. Finally, the last term ensures that each variable is grounded
to a value.

The `blocks`

predicate is as follows:

```
blocks([], [], []).
blocks([N1, N2, N3 | RestRow1],
[N4, N5, N6 | RestRow2],
[N7, N8, N9 | RestRow3]) :-
all_distinct([N1, N2, N3, N4, N5, N6, N7, N8, N9]),
blocks(RestRow1, RestRow2, RestRow3).
```

The predicate takes in three rows, ensures that the set consisting of the first three items from each row contains distinct values, and then recursively checks this for the remaining items in each row.

We can now provide a query to solve a specific puzzle. The following has been called the “world’s hardest Sudoku”:

```
?- S = [[8,_,_,_,_,_,_,_,_],
[_,_,3,6,_,_,_,_,_],
[_,7,_,_,9,_,2,_,_],
[_,5,_,_,_,7,_,_,_],
[_,_,_,_,4,5,7,_,_],
[_,_,_,1,_,_,_,3,_],
[_,_,1,_,_,_,_,6,8],
[_,_,8,5,_,_,_,1,_],
[_,9,_,_,_,_,4,_,_]],
sudoku(S).
S = [[8, 1, 2, 7, 5, 3, 6, 4, 9],
[9, 4, 3, 6, 8, 2, 1, 7, 5],
[6, 7, 5, 4, 9, 1, 2, 8, 3],
[1, 5, 4, 2, 3, 7, 8, 9, 6],
[3, 6, 9, 8, 4, 5, 7, 2, 1],
[2, 8, 7, 1, 6, 9, 5, 3, 4],
[5, 2, 1, 9, 7, 4, 3, 6, 8],
[4, 3, 8, 5, 2, 6, 9, 1, 7],
[7, 9, 6, 3, 1, 8, 4, 5, 2]] .
```

Solving this takes less than a tenth of a second on the author’s iMac.

## Make

Make is a family of tools used for automating the building of software
packages, as well as tracking dependencies between the various
components of a package. Make operates on programs called *makefiles*,
which contain *rules* for how to build individual *targets*. A target
may have *dependencies*, which are required to be satisfied before the
target can be built, as well as *commands* that specify how the target
should actually be built. Thus, a makefile is a combination of
declarative components relating targets to dependencies and imperative
actions specifying the actions required to build a target.

The structure of a rule in a makefile is as follows:

```
target: dependencies
commands
```

Here, `dependencies`

is a list of zero or more targets or files that
the given target depends on, and `commands`

is a list of zero or
more actions to be taken, generally each on its own line and indented
with a tab character.

As an example, consider the following simple makefile, located by
convention in a file named `Makefile`

(note the capitalization):

```
hello:
echo "Hello world!"
```

We can run this from the terminal, if we are in the same directory, as:

```
$ make hello
echo "Hello world!"
Hello world!
```

This invokes the `hello`

target, which has no dependencies and as
its sole action invokes the shell command to print `Hello world!`

to
the screen. We can leave out the explicit target when invoking
`make`

, in which case it will build the first target in the
makefile:

```
$ make
echo "Hello world!"
Hello world!
```

The target of a rule is commonly an executable file, and the
dependencies are the files needed to build the target. For example,
suppose we have a C++ project with the source files `a.cpp`

,
`b.cpp`

, and `c.cpp`

. We can structure our makefile as follows:

```
main: a.o b.o c.o
g++ -o main a.o b.o c.o
a.o: a.cpp
g++ --std=c++14 -Wall -Werror -pedantic -c a.cpp
b.o: b.cpp
g++ --std=c++14 -Wall -Werror -pedantic -c b.cpp
c.o: c.cpp
g++ --std=c++14 -Wall -Werror -pedantic -c c.cpp
```

Here, our default rule is `main`

, which depends on the targets
`a.o`

, `b.o`

, and `c.o`

. In order to build `main`

, those
targets have to be built first, so Make will attempt to build each of
those targets using their respective rules. The rule for `a.o`

depends on the file `a.cpp`

, and if it exists, the command invokes
`g++`

to build the object file `a.o`

. The rules for `b.o`

and
`c.o`

have the same structure. Once those targets have been built,
Make can then build `main`

, which links together the object files
into the final `main`

executable. Running `make`

indicates the
sequence of operations:

```
$ make
g++ --std=c++14 -Wall -Werror -pedantic -c a.cpp
g++ --std=c++14 -Wall -Werror -pedantic -c b.cpp
g++ --std=c++14 -Wall -Werror -pedantic -c c.cpp
g++ -o main a.o b.o c.o
```

Thus, we can specify complex dependency trees with rules in a makefile, and the Make tool will automatically resolve the dependencies and build the required targets. The relationship between a target and its dependencies is specified declaratively in a rule.

A key feature of Make is that it only builds a target if it has a
dependency, direct or indirect through other rules, that is newer than
the target itself. For instance, if we follow up the preceding build
by modifying the timestamp on `b.cpp`

, we can see that it is newer
than the targets `b.o`

and `main`

:

```
$ touch b.cpp
$ ls -l
-rw-r--r-- 1 kamil staff 229 Nov 17 01:01 Makefile
-rw-r--r-- 1 kamil staff 90 Nov 17 00:57 a.cpp
-rw-r--r-- 1 kamil staff 6624 Nov 17 01:01 a.o
-rw-r--r-- 1 kamil staff 31 Nov 17 01:12 b.cpp
-rw-r--r-- 1 kamil staff 640 Nov 17 01:01 b.o
-rw-r--r-- 1 kamil staff 33 Nov 17 00:58 c.cpp
-rw-r--r-- 1 kamil staff 640 Nov 17 01:01 c.o
-rwxr-xr-x 1 kamil staff 15268 Nov 17 01:01 main
```

If we then run `make`

, it will only rebuild those targets that
depend on `b.cpp`

:

```
$ make
g++ --std=c++14 -Wall -Werror -pedantic -c b.cpp
g++ -o main a.o b.o c.o
```

This is a crucial feature for working with large projects, as only the components that depend on a modification are rebuilt rather than every target in the project.

As a more complex example, consider the following makefile that was used to build a previous version of this text:

```
all: foundations functional theory data declarative
foundations: foundations.html foundations.tex
functional: functional.html functional.tex
theory: theory.html theory.tex
data: data.html data.tex
declarative: declarative.html declarative.tex
asynchronous: asynchronous.html asynchronous.tex
metaprogramming: metaprogramming.html metaprogramming.tex
%.html: %.rst
rst2html.py --stylesheet=style/style.css $< > $@
%.tex: %.rst
rst2latex.py --stylesheet=style/style.sty $< > $@
pdflatex $@
pdflatex $@
pdflatex $@
clean:
rm -vf *.html *.tex *.pdf *.aux *.log *.out
```

The default target is `all`

, which depends on the `foundations`

,
`functional`

, `theory`

, `data`

, and `declarative`

targets.
While there are also `asynchronous`

and `metaprogramming`

targets,
they were not being built since we had not reached the corresponding
units in the text.

Each of the following standard targets has two dependencies, an
`.html`

file and a `.tex`

file. In order to build an `.html`

file, Make looks for an appropriate target. We have a *pattern rule*
for `.html`

files, which depends on a corresponding `.rst`

file.
Thus, in order to build, for example, `declarative.html`

, Make
applies the pattern rule and invokes `rst2html.py`

. The special
symbol `$<`

stands for the dependencies, while `$@`

stands for the
target. Thus, the result of `rst2html.py`

is written to
`declarative.html`

, and the build for that target is complete.

We also have a pattern rule for `.tex`

files, which invokes
`rst2latex.py`

, followed by several invocations of `pdflatex`

. The
end result is that building `declarative.tex`

ends up creating
`declarative.pdf`

as well.

The last rule is to clean up target and temporary files. Thus, we can
force the `all`

target to be built from scratch with ```
make clean
all
```

. Without requesting the `clean`

target, only those targets
that depend on an `.rst`

file that has been modified will be
rebuilt.

# Pattern Matching

Many languages that are primarily functional or imperative provide a
declarative construct that does *pattern matching*, specifying
separate cases that each define a pattern against which a value can
match, and the computation to be done as a result. These separate
cases are analogous to different axioms in Prolog or different pattern
rules in Make, and the matching process is similar to
unification in Prolog.

As an example, we take a look at the `match`

statement in Python,
which has the following syntax:

```
match <expression>:
case <pattern>:
<suite>
case <pattern>:
<suite>
...
```

A controlling expression provides the value to be matched, and one or
more `case`

clauses specify a matching pattern and a suite of
statements to be executed upon a match. Only the first clause that
matches the value is executed – the remaining clauses are skipped,
even if their patterns also match the value. This is similar to the
behavior of a sequence of `if`

and `elif`

branches, or a sequence
of `except`

clauses on a `try`

statement.

There are several kinds of patterns that can be specified, a subset of which are the following:

A

*literal pattern*specifies a number, string, or boolean literal, or the`None`

literal, and it matches a value that compares equal to the literal. The following is an example:def https_error_description(code): match code: case 400: return 'Bad Request' case 401: return 'Unauthorized' case 403: return 'Forbidden' case 404: return 'Not Found' return f'Unknown code {code}'

This is similar to a switch statement, but the

`match`

construct is much more powerful in that it supports other patterns as well.A

*capture pattern*specifies an identifier, and it matches against any value, binding the identifier to that value. In the example above, we can use such a pattern to incorporate the default result in the`match`

statement:def https_error_description(code): match code: ... case 404: return 'Not Found' case unknown: return f'Unknown code {code}'

Since we don’t actually use the variable introduced in the last case, we can use a lone underscore instead as an anonymous variable:

def https_error_description(code): match code: ... case _: return f'Unknown code {code}'

A case with a pattern that consists solely of an identifier matches any value, so such a case is only permitted as the last one in a

`match`

statement.A

*class pattern*only matches a value that is an instance of the class. The simplest class pattern consists of a type name followed by empty parentheses:def https_error_description(code): match code: ... case int(): # only matches an int return f'Unknown code {code}' case _: raise ValueError(f'expected an int, got {code}')

A class pattern may also specify attributes that the object must have using syntax similar to keyword arguments:

case Point(x=0, y=0): ...

Alternatively, the class itself may define custom matching using syntax similar to positional arguments. For instance, several built-in types allow patterns of the form

`<type>(<subpattern>)`

, which matches against a value that is an instance of`<type>`

and that also matches`<subpattern>`

. Thus, the pattern`int(3)`

matches an`int`

whose value is 3, and the pattern`int(value)`

matches any`int`

and binds the name`value`

to that object.A

*sequence pattern*consists of a comma-separated list of subpatterns, which can be enclosed by either square brackets or parentheses [1]. Such a pattern can match a variety of sequence types (including user-defined ones that meet a certain set of conditions), though strings are excluded from matching a sequence pattern.The following is an example of simple sequence patterns:

def is_short(sequence): match sequence: case []: return True case [_]: return True case [_, _]: return True case _: return False

The first case matches an empty sequence, the second a one-element sequence, and the third a two-element sequence. We use anonymous variables since we are not concerned with the actual element values. Note that the pattern

`[_, _]`

has different behavior than`[var, var]`

. The latter requires both elements to be equal, since the first occurrence of`var`

binds the variable to a value and the second checks whether the corresponding element is equal to that value. On the other hand, the anonymous variable`_`

does not do any binding, so each occurrence is independent of any others.A sequence pattern may also contain a variadic subpattern, matching zero or more occurrences. The syntax is similar to variadic positional arguments:

def is_short(sequence): match sequence: case [_, _, *_]: return False case _: return True

Here, the pattern

`[_, _, *_]`

matches a sequence with at least two elements – the first occurrence of`_`

matches the first element, the second occurrence matches second element, and the`*_`

matches all remaining elements.The following is an example of recursively computing the length of a sequence using sequence patterns:

def length(sequence): match sequence: case []: return 0 case [_, *rest]: return 1 + length(rest)

The first case matches an empty sequence, the base case of the computation. The second matches a sequence with at least one element, and all but the first element match the variadic

`*rest`

subpattern. These elements are encapsulated in a list bound to the`rest`

variable, and we can recurse on this list.Compare this definition of

`length`

with an equivalent predicate in Prolog:len([], 0). len([_|Rest], Length) :- len(Rest, RestLength), Length is 1 + RestLength.

In both definitions, we specify two separate cases with patterns that are matched against an input list.

There are several other patterns, including mapping patterns, `as`

patterns, value patterns, and “or” patterns, and a case can include a
*guard* expressed as an `if`

clause to further restrict what the
case matches. More details are in the original specification as well as the tutorial.

We consider one more, complex example that illustrates the declarative nature of pattern matching. Suppose we want to compute the sum of all the elements in a nested list of numbers. The following expresses this computation in Prolog:

```
% deep_sum(NestedList, Sum).
% True if NestedList is a nested list of numbers, and Sum is the sum
% of all the numbers contained in NestedList.
deep_sum([], 0).
deep_sum([First|Rest], Sum) :-
deep_sum(First, FirstSum),
deep_sum(Rest, RestSum),
Sum is FirstSum + RestSum.
deep_sum(Item, Item). % can restrict this to numbers with :- number(Item).
```

We have three cases: an empty list whose sum is zero, a non-empty list whose sum is the recursive sum of the first item (which itself may be a list) plus the recursive sum of the rest of the list, and a non-list item whose sum is itself. We can express this same computation in Python:

```
def deep_sum(nested_list):
"""Return the sum of all the numbers in nested_list.
nested_list must be a nested list of numbers.
"""
match nested_list:
case []:
return 0
case [first, *rest]:
return deep_sum(first) + deep_sum(rest)
case item: # can do int(item) | float(item) to restrict to numbers
return item
```

There is a direct correspondence between the two implementations, reflecting the primarily declarative manner in which the computation is expressed. The fundamental difference between Prolog and Python here is that Prolog provides search and backtracking, so that other axioms can be tried if one fails (or more solutions are requested). In Python and other functional or imperative languages that have pattern matching, a value matches at most one case, and the remaining cases are never considered. Thus, while pattern matching is declarative, it does not provide the full expressiveness of logic programming.