# Chapter 1  The Gallina specification language

This chapter describes Gallina, the specification language of Coq. It allows to develop mathematical theories and to prove specifications of programs. The theories are built from axioms, hypotheses, parameters, lemmas, theorems and definitions of constants, functions, predicates and sets. The syntax of logical objects involved in theories is described in section 1.2. The language of commands, called The Vernacular is described in section 1.3.

In Coq, logical objects are typed to ensure their logical correctness. The rules implemented by the typing algorithm are described in chapter 4.

### About the grammars in the manual

Grammars are presented in Backus-Naur form (BNF). Terminal symbols are set in typewriter font. In addition, there are special notations for regular expressions.

An expression enclosed in square brackets [...] means at most one occurrence of this expression (this corresponds to an optional component).

The notation ``entry sep ... sep entry'' stands for a non empty sequence of expressions parsed by entry and separated by the literal ``sep''1.

Similarly, the notation ``entry ... entry'' stands for a non empty sequence of expressions parsed by the ``entry'' entry, without any separator between.

At the end, the notation ``[entry sep ... sep entry]'' stands for a possibly empty sequence of expressions parsed by the ``entry'' entry, separated by the literal ``sep''.

## 1.1  Lexical conventions

##### Blanks
Space, newline and horizontal tabulation are considered as blanks. Blanks are ignored but they separate tokens.

Comments in Coq are enclosed between (* and *), and can be nested. They can contain any character. However, string literals must be correctly closed. Comments are treated as blanks.

##### Identifiers and access identifiers
Identifiers, written ident, are sequences of letters, digits, `_` and `'`, that do not start with a digit or `'`. That is, they are recognized by the following lexical class:

 first_letter ::= a..z | A..Z | _ subsequent_letter ::= a..z | A..Z | 0..9 | _ | ' ident ::= first_letter [subsequent_letter...subsequent_letter]
All characters are meaningful. In particular, identifiers are case-sensitive. Access identifiers, written access_ident, are identifiers prefixed by `.` (dot) without blank. They are used in the syntax of qualified identifiers.

##### Natural numbers and integers
Numerals are sequences of digits. Integers are numerals optionally preceded by a minus sign.

 digit ::= 0..9 num ::= digit...digit integer ::= [-]num

##### Strings
Strings are delimited by `"` (double quote), and enclose a sequence of any characters different from `"` or the sequence `""` to denote the double quote character. In grammars, the entry for quoted strings is string.

##### Keywords
The following identifiers are reserved keywords, and cannot be employed otherwise:
 `_` `as` `at` `cofix` `else` `end` `exists` `exists2` `fix` `for` `forall` `fun` `if` `IF` `in` `let` `match` `mod` `Prop` `return` `Set` `then` `Type` `using` `where` `with`

##### Special tokens
The following sequences of characters are special tokens:
 `!` `%` `&` `&&` `(` `()` `)` `*` `+` `++` `,` `-` `->` `.` `.(` `..` `/` `/\` `:` `::` `:<` `:=` `:>` `;` `<` `<-` `<->` `<:` `<=` `<>` `=` `=>` `=_D` `>` `>->` `>=` `?` `?=` `@` `[` `\/` `]` `^` `{` `|` `|-` `||` `}` `~`

Lexical ambiguities are resolved according to the ``longest match'' rule: when a sequence of non alphanumerical characters can be decomposed into several different ways, then the first token is the longest possible one (among all tokens defined at this moment), and so on.

## 1.2  Terms

### 1.2.1  Syntax of terms

Figures 1.1 and 1.2 describe the basic set of terms which form the Calculus of Inductive Constructions (also called pCic). The formal presentation of pCic is given in chapter 4. Extensions of this syntax are given in chapter 2. How to customize the syntax is described in chapter 11.

 term ::= forall binderlist , term (1.2.8) | fun binderlist => term (1.2.7) | fix fix_bodies (1.2.14) | cofix cofix_bodies (1.2.14) | let ident_with_params := term in term (1.2.12) | let fix fix_body in term (1.2.14) | let cofix cofix_body in term (1.2.14) | let ( [name , ... , name] ) [dep_ret_type] := term in term (1.2.13, 2.2.1) | if term [dep_ret_type] then term else term (1.2.13, 2.2.1) | term : term (1.2.10) | term -> term (1.2.8) | term arg ... arg (1.2.9) | @ qualid [term ... term] (2.6.7) | term % ident (11.2.2) | match match_item , ... , match_item [return_type] with [[|] equation | ... | equation] end (1.2.13) | qualid (1.2.3) | sort (1.2.5) | num (1.2.4) | _ (1.2.11) arg ::= term | ( ident := term ) (2.6.7) binderlist ::= name ... name [: term] 1.2.6 | binder binderlet ... binderlet binder ::= name 1.2.6 | ( name ... name : term ) binderlet ::= binder 1.2.6 | ( name [: term] := term ) name ::= ident | _ qualid ::= ident | qualid access_ident sort ::= Prop  |  Set  |  Type

Figure 1.1: Syntax of terms

 ident_with_params ::= ident [binderlet ... binderlet] [: term] fix_bodies ::= fix_body | fix_body with fix_body with ... with fix_body for ident cofix_bodies ::= cofix_body | cofix_body with cofix_body with ... with cofix_body for ident fix_body ::= ident binderlet ... binderlet [annotation] [: term] := term cofix_body ::= ident_with_params := term annotation ::= { struct ident } match_item ::= term [as name] [in term] dep_ret_type ::= [as name] return_type return_type ::= return term equation ::= pattern , ... , pattern => term pattern ::= qualid pattern ... pattern | pattern as ident | pattern % ident | qualid | _ | num | ( pattern , ... , pattern )

Figure 1.2: Syntax of terms (continued)

### 1.2.2  Types

Coq terms are typed. Coq types are recognized by the same syntactic class as term. We denote by type the semantic subclass of types inside the syntactic class term.

### 1.2.3  Qualified identifiers and simple identifiers

Qualified identifiers (qualid) denote global constants (definitions, lemmas, theorems, remarks or facts), global variables (parameters or axioms), inductive types or constructors of inductive types. Simple identifiers (or shortly ident) are a syntactic subset of qualified identifiers. Identifiers may also denote local variables, what qualified identifiers do not.

### 1.2.4  Numerals

Numerals have no definite semantics in the calculus. They are mere notations that can be bound to objects through the notation mechanism (see chapter 11 for details). Initially, numerals are bound to Peano's representation of natural numbers (see 3.1.3).

Note: negative integers are not at the same level as num, for this would make precedence unnatural.

### 1.2.5  Sorts

There are three sorts Set, Prop and Type.
• Prop is the universe of logical propositions. The logical propositions themselves are typing the proofs. We denote propositions by form. This constitutes a semantic subclass of the syntactic class term.
• Set is is the universe of program types or specifications. The specifications themselves are typing the programs. We denote specifications by specif. This constitutes a semantic subclass of the syntactic class term.
• Type is the type of Set and Prop
More on sorts can be found in section 4.1.1.

### 1.2.6  Binders

Various constructions introduce variables which scope is some of its sub-expressions. There is a uniform syntax for this. A binder may be an (unqualified) identifier: the name to use to refer to this variable. If the variable is not to be used, its name can be _. When its type cannot be synthesized by the system, it can be specified with notation ( ident : type ). There is a notation for several variables sharing the same type: ( ident1...identn : type ).

Some constructions allow ``let-binders'', that is either a binder as defined above, or a variable with a value. The notation is ( ident := term ). Only one variable can be introduced at the same time. It is also possible to give the type of the variable before the symbol :=.

The last kind of binders is the ``binder list''. It is either a list of let-binders (the first one not being a variable with value), or ident1...identn : type if all variables share the same type.

Coq terms are typed. Coq types are recognized by the same syntactic class as term. We denote by type the semantic subclass of types inside the syntactic class term.

### 1.2.7  Abstractions

The expression ``fun ident : type=> term'' denotes the abstraction of the variable ident of type type, over the term term. Put in another way, it is function of formal parameter ident of type type returning term.

Keyword fun is followed by a ``binder list'', so any of the binders of Section 1.2.6 apply. Internally, abstractions are only over one variable. Multiple variable binders are an iteration of the single variable abstraction: notation fun ident1 ... identn : type => term stands for fun ident1 : type => ... fun identn : type => term. Variables with a value expand to a local definition (see Section 1.2.12).

### 1.2.8  Products

The expression ``forall ident : type , term'' denotes the product of the variable ident of type type, over the term term. As for abstractions, forall is followed by a binder list, and it is represented by an iteration of single variable products.

Non dependent product types have a special notation ``A -> B'' stands for ``forall _:A, B''. This is to stress on the fact that non dependent product types are usual functional types.

### 1.2.9  Applications

The expression term0 term1 denotes the application of term term0 to term1.

The expression term0 term1 ... termn denotes the application of the term term0 to the arguments term1 ... then termn. It is equivalent to ... ( term0 term1 ) ... termn : associativity is to the left.

When using implicit arguments mechanism, implicit positions can be forced a value with notation ( ident := term ) or ( num := term ). See Section 2.6.7 for details.

### 1.2.10  Type cast

The expression ``term : type'' is a type cast expression. It enforces the type of term to be type.

### 1.2.11  Inferable subterms

Since there are redundancies, a term can be type-checked without giving it in totality. Subterms that are left to guess by the type-checker are replaced by ``_''.

### 1.2.12  Local definitions (let-in)

let ident := term1 in term2 denotes the local binding of term1 to the variable ident in term2.

There is a syntactic sugar for local definition of functions: let ident binder1 ...bindern := term1 in term2 stands for let ident := fun binder1 ...bindern in term2.

### 1.2.13  Definition by case analysis

This paragraph only shows simple variants of case analysis. See Section 2.2.1 and Chapter 15 for explanations of the general form.

Objects of inductive types can be destructurated by a case-analysis construction, also called pattern-matching in functional languages. In its simple form, a case analysis expression is used to analyze the structure of an inductive objects (upon which constructor it is built).

The expression match term0 return_type with pattern1 => term1 | ... | patternn => termn end, denotes a pattern-matching over the term term0 (expected to be of an inductive type I). term1...termn are called branches. In a simple pattern qualid ident ... ident, the qualified identifier qualid is intended to be a constructor. There should be a branch for every constructor of I.

The return_type is used to compute the resulting type of the whole match expression and the type of the branches. Most of the time, when this type is the same as the types of all the termi, the annotation is not needed2. This annotation has to be given when the resulting type of the whole match depends on the actual term0 matched.

There are specific notations for case analysis on types with one or two constructors: if / then / else and let (...) := ...in....

### 1.2.14  Recursive functions

Expression ``fix ident1 binder1 : type1 := term1 with ... with identn bindern : typen := termn for identi'' denotes the ith component of a block of functions defined by mutual well-founded recursion. It is the local counterpart of the Fixpoint command. See Section 1.3.4 for more details. When n=1, the for identi is omitted.

The expression ``cofix ident1 binder1 : type1 with ... with identn bindern : typen for identi'' denotes the ith component of a block of terms defined by a mutual guarded co-recursion. It is the local counterpart of the CoFixpoint command. See Section 1.3.4 for more details. When n=1, the for identi is omitted.

The association of a single fixpoint and a local definition have a special syntax: ``let fix f ... := ... in ...'' stands for ``let f := fix f ... := ... in ...''. The same applies for co-fixpoints.

## 1.3  The Vernacular

 sentence ::= declaration | definition | inductive | fixpoint | statement [proof] declaration ::= declaration_keyword assums . declaration_keyword ::= Axiom | Conjecture | Parameter | Parameters | Variable | Variables | Hypothesis | Hypotheses assums ::= ident ... ident : term | binder ... binder definition ::= Definition ident_with_params := term . | Let ident_with_params := term . inductive ::= Inductive ind_body with ... with ind_body . | CoInductive ind_body with ... with ind_body . ind_body ::= ident [binderlet ... binderlet] : term := [[|] ident_with_params | ... | ident_with_params] fixpoint ::= Fixpoint fix_body with ... with fix_body . | CoFixpoint cofix_body with ... with cofix_body . statement ::= statement_keyword ident [binderlet ... binderlet] : term . statement_keyword ::= Theorem | Lemma | Definition proof ::= Proof . ... Qed . | Proof . ... Defined . | Proof . ... Admitted .

Figure 1.3: Syntax of sentences

Figure 1.3 describes The Vernacular which is the language of commands of Gallina. A sentence of the vernacular language, like in many natural languages, begins with a capital letter and ends with a dot.

The different kinds of command are described hereafter. They all suppose that the terms occurring in the sentences are well-typed.

### 1.3.1  Declarations

The declaration mechanism allows the user to specify his own basic objects. Declared objects play the role of axioms or parameters in mathematics. A declared object is an ident associated to a term. A declaration is accepted by Coq if and only if this term is a correct type in the current context of the declaration and ident was not previously defined in the same module. This term is considered to be the type, or specification, of the ident.

#### Axiom ident :term .

This command links term to the name ident as its specification in the global context. The fact asserted by term is thus assumed as a postulate.

Error messages:

Variants:
1. Parameter ident :term.
Is equivalent to Axiom ident : term

2. Parameter ident1...identn :term.
Adds n parameters with specification term

3. Parameter ( ident1,1...ident1,k1 : term1 ) ... ( identn,1...identn,kn : termn ).
Adds n blocks of parameters with different specifications.

4. Conjecture ident :term.
Is equivalent to Axiom ident : term.
Remark: It is possible to replace Parameter by Parameters.

#### Variable ident :term.

This command links term to the name ident in the context of the current section (see Section 2.3 for a description of the section mechanism). When the current section is closed, name ident will be unknown and every object using this variable will be explicitly parameterized (the variable is discharged). Using the Variable command out of any section is equivalent to Axiom.

Error messages:

Variants:
1. Variable ident1...identn :term.
2. Variable ( ident1,1...ident1,k1 : term1 ) ... ( identn,1...identn,kn : termn ).
Adds n blocks of variables with different specifications.
3. Hypothesis ident :term.
Hypothesis is a synonymous of Variable
Remark: It is possible to replace Variable by Variables and Hypothesis by Hypotheses.

It is advised to use the keywords `Axiom` and `Hypothesis` for logical postulates (i.e. when the assertion term is of sort `Prop`), and to use the keywords `Parameter` and `Variable` in other cases (corresponding to the declaration of an abstract mathematical entity).

### 1.3.2  Definitions

Definitions differ from declarations in allowing to give a name to a term whereas declarations were just giving a type to a name. That is to say that the name of a defined object can be replaced at any time by its definition. This replacement is called delta-conversion (see Section 4.3). A defined object is accepted by the system if and only if the defining term is well-typed in the current context of the definition. Then the type of the name is the type of term. The defined name is called a constant and one says that the constant is added to the environment.

A formal presentation of constants and environments is given in Section 4.2.

#### Definition ident := term.

This command binds the value term to the name ident in the environment, provided that term is well-typed.

Error messages:

Variants:
1. Definition ident :term1 := term2.
It checks that the type of term2 is definitionally equal to term1, and registers ident as being of type term1, and bound to value term2.
2. Definition ident binder1...bindern :term1 := term2.
This is equivalent to
Definition ident : forall binder1...bindernterm1 := fun binder1...bindern => term2 .

Error messages:
1. In environment ... the term: term2 does not have type term1.
Actually, it has type term3.

#### Let ident := term.

This command binds the value term to the name ident in the environment of the current section. The name ident disappears when the current section is eventually closed, and, all persistent objects (such as theorems) defined within the section and depending on ident are prefixed by the local definition let ident := term in.

Error messages:

Variants:
1. Let ident : term1 := term2.

### 1.3.3  Inductive definitions

We gradually explain simple inductive types, simple annotated inductive types, simple parametric inductive types, mutually inductive types. We explain also co-inductive types.

#### Simple inductive types

The definition of a simple inductive type has the following form:

Inductive ident : sort :=
 ident1 : type1 | ... | identn : typen

The name ident is the name of the inductively defined type and sort is the universes where it lives. The names ident1, ..., identn are the names of its constructors and type1, ..., typen their respective types. The types of the constructors have to satisfy a positivity condition (see Section 4.5.3) for ident. This condition ensures the soundness of the inductive definition. If this is the case, the constants ident, ident1, ..., identn are added to the environment with their respective types. Accordingly to the universe where the inductive type lives (e.g. its type sort), Coq provides a number of destructors for ident. Destructors are named ident_ind, ident_rec or ident_rect which respectively correspond to elimination principles on Prop, Set and Type. The type of the destructors expresses structural induction/recursion principles over objects of ident. We give below two examples of the use of the Inductive definitions.

The set of natural numbers is defined as:
Coq < Inductive nat : Set :=
Coq <   | O : nat
Coq <   | S : nat -> nat.
nat is defined
nat_rect is defined
nat_ind is defined
nat_rec is defined

The type nat is defined as the least `Set` containing O and closed by the S constructor. The constants nat, O and S are added to the environment.

Now let us have a look at the elimination principles. They are three : nat_ind, nat_rec and nat_rect. The type of nat_ind is:
Coq < Check nat_ind.
nat_ind
: forall P : nat -> Prop,
P O -> (forall n : nat, P n -> P (S n)) -> forall n : nat, P n

This is the well known structural induction principle over natural numbers, i.e. the second-order form of Peano's induction principle. It allows to prove some universal property of natural numbers (forall n:nat, P n) by induction on n.

The types of nat_rec and nat_rect are similar, except that they pertain to (P:nat->Set) and (P:nat->Type) respectively . They correspond to primitive induction principles (allowing dependent types) respectively over sorts `Set` and `Type`. The constant ident_ind is always provided, whereas ident_rec and ident_rect can be impossible to derive (for example, when ident is a proposition).

Variants:
1. Coq < Inductive nat : Set := O | S (_:nat).
In the case where inductive types have no annotations (next section gives an example of such annotations), the positivity condition implies that a constructor can be defined by only giving the type of its arguments.

#### Simple annotated inductive types

In an annotated inductive types, the universe where the inductive type is defined is no longer a simple sort, but what is called an arity, which is a type whose conclusion is a sort.

As an example of annotated inductive types, let us define the even predicate:

Coq < Inductive even : nat -> Prop :=
Coq <   | even_0 : even O
Coq <   | even_SS : forall n:nat, even n -> even (S (S n)).
even is defined
even_ind is defined

The type nat->Prop means that even is a unary predicate (inductively defined) over natural numbers. The type of its two constructors are the defining clauses of the predicate even. The type of even_ind is:

Coq < Check even_ind.
even_ind
: forall P : nat -> Prop,
P O ->
(forall n : nat, even n -> P n -> P (S (S n))) ->
forall n : nat, even n -> P n

From a mathematical point of view it asserts that the natural numbers satisfying the predicate even are exactly the naturals satisfying the clauses even_0 or even_SS. This is why, when we want to prove any predicate P over elements of even, it is enough to prove it for O and to prove that if any natural number n satisfies P its double successor (S (S n)) satisfies also P. This is indeed analogous to the structural induction principle we got for nat.

Error messages:
1. Non strictly positive occurrence of ident in type
2. The conclusion of type is not valid; it must be built from ident

#### Parameterized inductive types

Inductive types may be parameterized. Parameters differ from inductive type annotations in the fact that recursive invokations of inductive types must always be done with the same values of parameters as its specification.

The general scheme is:
Inductive ident binder1...binderk : term := ident1: term1 | ... | identn: termn .

A typical example is the definition of polymorphic lists:
Coq < Inductive list (A:Set) : Set :=
Coq <   | nil : list A
Coq <   | cons : A -> list A -> list A.

Note that in the type of nil and cons, we write (list A) and not just list.
The constants nil and cons will have respectively types:

Coq < Check nil.
nil
: forall A : Set, list A

Coq < Check cons.
cons
: forall A : Set, A -> list A -> list A

Types of destructors are also quantified with (A:Set).

Variants:
1. Coq < Inductive list (A:Set) : Set := nil | cons (_:A) (_:list A).
This is an alternative definition of lists where we specify the arguments of the constructors rather than their full type.

Error messages:
1. The numth argument of ident must be ident' in type

#### Mutually defined inductive types

The definition of a block of mutually inductive types has the form:

Inductive ident1 : type1 :=
 ident11 : type11 | ... | identn11 : typen11
with
...
with identm : typem :=
 ident1m : type1m | ... | identnmm : typenmm.

It has the same semantics as the above Inductive definition for each ident1, ..., identm. All names ident1, ..., identm and ident11, ..., identnmm are simultaneously added to the environment. Then well-typing of constructors can be checked. Each one of the ident1, ..., identm can be used on its own.

It is also possible to parameterize these inductive definitions. However, parameters correspond to a local context in which the whole set of inductive declarations is done. For this reason, the parameters must be strictly the same for each inductive types The extended syntax is:

Inductive ident1 params : type1 :=

ident11 : type11
| ..
|
identn11 : typen11
with
..
with
identm params : typem :=

ident1m : type1m
| ..
|
identnmm : typenmm.

Example: The typical example of a mutual inductive data type is the one for trees and forests. We assume given two types A and B as variables. It can be declared the following way.

Coq < Variables A B : Set.

Coq < Inductive tree : Set :=
Coq <     node : A -> forest -> tree
Coq < with forest : Set :=
Coq <   | leaf : B -> forest
Coq <   | cons : tree -> forest -> forest.

This declaration generates automatically six induction principles. They are respectively called tree_rec, tree_ind, tree_rect, forest_rec, forest_ind, forest_rect. These ones are not the most general ones but are just the induction principles corresponding to each inductive part seen as a single inductive definition.

To illustrate this point on our example, we give the types of tree_rec and forest_rec.

Coq < Check tree_rec.
tree_rec
: forall P : tree -> Set,
(forall (a : A) (f : forest), P (node a f)) -> forall t : tree, P t

Coq < Check forest_rec.
forest_rec
: forall P : forest -> Set,
(forall b : B, P (leaf b)) ->
(forall (t : tree) (f : forest), P f -> P (cons t f)) ->
forall f1 : forest, P f1

Assume we want to parameterize our mutual inductive definitions with the two type variables A and B, the declaration should be done the following way:

Coq < Inductive tree (A B:Set) : Set :=
Coq <     node : A -> forest A B -> tree A B
Coq < with forest (A B:Set) : Set :=
Coq <   | leaf : B -> forest A B
Coq <   | cons : tree A B -> forest A B -> forest A B.

Assume we define an inductive definition inside a section. When the section is closed, the variables declared in the section and occurring free in the declaration are added as parameters to the inductive definition.

#### Co-inductive types

The objects of an inductive type are well-founded with respect to the constructors of the type. In other words, such objects contain only a finite number constructors. Co-inductive types arise from relaxing this condition, and admitting types whose objects contain an infinity of constructors. Infinite objects are introduced by a non-ending (but effective) process of construction, defined in terms of the constructors of the type.

An example of a co-inductive type is the type of infinite sequences of natural numbers, usually called streams. It can be introduced in Coq using the CoInductive command:
Coq < CoInductive Stream : Set :=
Coq <     Seq : nat -> Stream -> Stream.
Stream is defined

The syntax of this command is the same as the command Inductive (cf. Section 1.3.3). Notice that no principle of induction is derived from the definition of a co-inductive type, since such principles only make sense for inductive ones. For co-inductive ones, the only elimination principle is case analysis. For example, the usual destructors on streams hd:Stream->nat and tl:Str->Str can be defined as follows:
Coq < Definition hd (x:Stream) := let (a,s) := x in a.
hd is defined

Coq < Definition tl (x:Stream) := let (a,s) := x in s.
tl is defined

Definition of co-inductive predicates and blocks of mutually co-inductive definitions are also allowed. An example of a co-inductive predicate is the extensional equality on streams:

Coq < CoInductive EqSt : Stream -> Stream -> Prop :=
Coq <     eqst :
Coq <       forall s1 s2:Stream,
Coq <         hd s1 = hd s2 -> EqSt (tl s1) (tl s2) -> EqSt s1 s2.
EqSt is defined

In order to prove the extensionally equality of two streams s1 and s2 we have to construct and infinite proof of equality, that is, an infinite object of type (EqSt s1 s2). We will see how to introduce infinite objects in Section 1.3.4.

### 1.3.4  Definition of recursive functions

#### Fixpoint identparams {struct ident0 } : type0 := term0

This command allows to define inductive objects using a fixed point construction. The meaning of this declaration is to define ident a recursive function with arguments specified by binder1...bindern such that ident applied to arguments corresponding to these binders has type type0, and is equivalent to the expression term0. The type of the ident is consequently forall params , type0 and the value is equivalent to fun params => term0.

To be accepted, a Fixpoint definition has to satisfy some syntactical constraints on a special argument called the decreasing argument. They are needed to ensure that the Fixpoint definition always terminates. The point of the {struct ident} annotation is to let the user tell the system which argument decreases along the recursive calls. This annotation may be left implicit for fixpoints with one argument. For instance, one can define the addition function as :

Coq < Fixpoint add (n m:nat) {struct n} : nat :=
Coq <   match n with
Coq <   | O => m
Coq <   | S p => S (add p m)
Coq <   end.

The match operator matches a value (here `n`) with the various constructors of its (inductive) type. The remaining arguments give the respective values to be returned, as functions of the parameters of the corresponding constructor. Thus here when `n` equals `O` we return `m`, and when `n` equals `(S p)` we return `(S (add p m))`.

The match operator is formally described in detail in Section 4.5.4. The system recognizes that in the inductive call (add p m) the first argument actually decreases because it is a pattern variable coming from match n with.

Example: The following definition is not correct and generates an error message:

Coq < Fixpoint wrongplus (n m:nat) {struct n} : nat :=
Coq <   match m with
Coq <   | O => n
Coq <   | S p => S (wrongplus n p)
Coq <   end.
Coq < Coq < Error:
Recursive definition of wrongplus is ill-formed.
In environment
n : nat
m : nat
p : nat
Recursive call to wrongplus has principal argument equal to
"n"
instead of a subterm of n

because the declared decreasing argument n actually does not decrease in the recursive call. The function computing the addition over the second argument should rather be written:

Coq < Fixpoint plus (n m:nat) {struct m} : nat :=
Coq <   match m with
Coq <   | O => n
Coq <   | S p => S (plus n p)
Coq <   end.

The ordinary match operation on natural numbers can be mimicked in the following way.
Coq < Fixpoint nat_match
Coq <   (C:Set) (f0:C) (fS:nat -> C -> C) (n:nat) {struct n} : C :=
Coq <   match n with
Coq <   | O => f0
Coq <   | S p => fS p (nat_match C f0 fS p)
Coq <   end.
The recursive call may not only be on direct subterms of the recursive variable n but also on a deeper subterm and we can directly write the function mod2 which gives the remainder modulo 2 of a natural number.
Coq < Fixpoint mod2 (n:nat) : nat :=
Coq <   match n with
Coq <   | O => O
Coq <   | S p => match p with
Coq <            | O => S O
Coq <            | S q => mod2 q
Coq <            end
Coq <   end.
In order to keep the strong normalisation property, the fixed point reduction will only be performed when the argument in position of the decreasing argument (which type should be in an inductive definition) starts with a constructor.

The Fixpoint construction enjoys also the with extension to define functions over mutually defined inductive types or more generally any mutually recursive definitions.

Variants:
1. Fixpoint ident1 params1 :type1 := term1
with ...
with
identm paramsm :typem := typem
Allows to define simultaneously ident1, ..., identm.

Example: The size of trees and forests can be defined the following way:
Coq < Fixpoint tree_size (t:tree) : nat :=
Coq <   match t with
Coq <   | node a f => S (forest_size f)
Coq <   end
Coq <  with forest_size (f:forest) : nat :=
Coq <   match f with
Coq <   | leaf b => 1
Coq <   | cons t f' => (tree_size t + forest_size f')
Coq <   end.
A generic command Scheme is useful to build automatically various mutual induction principles. It is described in Section 8.13.

#### CoFixpoint ident : type0 := term0.

The CoFixpoint command introduces a method for constructing an infinite object of a coinductive type. For example, the stream containing all natural numbers can be introduced applying the following method to the number O (see Section 1.3.3 for the definition of Stream, hd and tl):
Coq < CoFixpoint from (n:nat) : Stream := Seq n (from (S n)).
from is corecursively defined

Oppositely to recursive ones, there is no decreasing argument in a co-recursive definition. To be admissible, a method of construction must provide at least one extra constructor of the infinite object for each iteration. A syntactical guard condition is imposed on co-recursive definitions in order to ensure this: each recursive call in the definition must be protected by at least one constructor, and only by constructors. That is the case in the former definition, where the single recursive call of from is guarded by an application of Seq. On the contrary, the following recursive function does not satisfy the guard condition:

Coq < CoFixpoint filter (p:nat -> bool) (s:Stream) : Stream :=
Coq <   if p (hd s) then Seq (hd s) (filter p (tl s)) else filter p (tl s).
Coq < Coq < Error:
Recursive definition of filter is ill-formed.
In environment
filter : (nat -> bool) -> Stream -> Stream
p : nat -> bool
s : Stream
unguarded recursive call in "filter p (tl s)"

The elimination of co-recursive definition is done lazily, i.e. the definition is expanded only when it occurs at the head of an application which is the argument of a case analysis expression. In any other context, it is considered as a canonical expression which is completely evaluated. We can test this using the command Eval, which computes the normal forms of a term:

Coq < Eval compute in (from 0).
= (cofix from (n : nat) : Stream := Seq n (from (S n))) 0
: Stream

Coq < Eval compute in (hd (from 0)).
= 0
: nat

Coq < Eval compute in (tl (from 0)).
= (cofix from (n : nat) : Stream := Seq n (from (S n))) 1
: Stream

Variants:
1. CoFixpoint ident1 params :type1 := term1
As for most constructions, arguments of co-fixpoints expressions can be introduced before the := sign.
2. CoFixpoint ident1 :type1 := term1
with
...
with
identm : typem := termm
As in the Fixpoint command (cf. Section 1.3.4), it is possible to introduce a block of mutually dependent methods.

### 1.3.5  Statement and proofs

A statement claims a goal of which the proof is then interactively done using tactics. More on the proof editing mode, statements and proofs can be found in chapter 7.

#### Theorem ident : type.

This command binds type to the name ident in the environment, provided that a proof of type is next given.

After a statement, Coq needs a proof.

Variants:
1. Lemma ident : type.
It is a synonymous of Theorem
2. Remark ident : type.
It is a synonymous of Theorem
3. Fact ident : type.
It is a synonymous of Theorem
4. Definition ident : type.
Allow to define a term of type type using the proof editing mode. It behaves as Theorem but is intended for the interactive definition of expression which computational behaviour will be used by further commands.

#### Proof. ...Qed.

A proof starts by the keyword Proof. Then Coq enters the proof editing mode until the proof is completed. The proof editing mode essentially contains tactics that are described in chapter 8. Besides tactics, there are commands to manage the proof editing mode. They are described in chapter 7. When the proof is completed it should be validated and put in the environment using the keyword Qed.

Error message:

Remarks:
1. Several statements can be simultaneously opened.
2. Not only other statements but any vernacular command can be given within the proof editing mode. In this case, the command is understood as if it would have been given before the statements still to be proved.
3. Proof is recommended but can currently be omitted. On the opposite, Qed (or Defined, see below) is mandatory to validate a proof.
4. Proofs ended by Qed are declared opaque (see 6.2.4) and cannot be unfolded by conversion tactics (see 8.5). To be able to unfold a proof, you should end the proof by Defined (see below).

Variants:
1. Proof . ...Defined .
Same as Proof . ...Qed . but the proof is then declared transparent (see 6.2.5), which means it can be unfolded in conversion tactics (see 8.5).
2. Proof . ...Save.
Same as Proof . ...Qed .
3. Goal type...Save ident
Same as Lemma ident: type...Save. This is intended to be used in the interactive mode. Conversely to named lemmas, anonymous goals cannot be nested.