6.8 | Type and exception definitions

6.8.1 | Type definitions

Type definitions bind type constructors to data types: either variant types, record types, type abbreviations, or abstract data types. They also bind the value constructors and record fields associated with the definition.

`type-definition` | ::= | `type` `typedef` { `and` `typedef` }
`typedef` | ::= | [`type-params`] `typeconstr-name` [`type-information`]
`type-information` | ::= | [`type-equation`] [`type-representation`] { `type-constraint` }
`type-equation` | ::= | `=` `typexpr`
`type-representation` | ::= | `=` `constr-decl` { `|` `constr-decl` }
| | | `=` `{` `field-decl` { `;` `field-decl` } `}`
`type-params` | ::= | `'` `ident`
| | | `(` `'` `ident` { `,` `'` `ident` } `)`
`constr-decl` | ::= | `cconstr-name`
| | | `ncconstr-name` `of` `typexpr`
`field-decl` | ::= | `field-name` `:` `typexpr`
| | | `mutable` `field-name` `:` `typexpr`
`type-constraint` | ::= | `constraint` `'` `ident` `=` `typexpr`

Type definitions are introduced by the `type` keyword, and consist in one or several simple definitions, possibly mutually recursive, separated by the `and` keyword. Each simple definition defines one type constructor.
A simple definition consists in a lowercase identifier, possibly preceded by one or several type parameters, and followed by an optional type equation, then an optional type representation, and then a constraint clause. The identifier is the name of the type constructor being defined.
The optional type parameters are either one type variable `'` `ident`, for type constructors with one parameter, or a list of type variables (`'``ident`1,...,`'` `ident`n), for type constructors with several parameters. These type parameters can appear in the type expressions of the right-hand side of the definition.
The optional type equation `=` `typexpr` makes the defined type equivalent to the type expression `typexpr` on the right of the `=` sign: one can be substituted for the other during typing. If no type equation is given, a new type is generated: the defined type is incompatible with any other type.
The optional type representation describes the data structure representing the defined type, by giving the list of associated constructors (if it is a variant type) or associated fields (if it is a record type). If no type representation is given, nothing is assumed on the structure of the type besides what is stated in the optional type equation.
The type representation `=` `constr-decl` { `|` `constr-decl` } describes a variant type. The constructor declarations `constr-decl`1, ..., `constr-decl`n describe the constructors associated to this variant type. The constructor declaration `ncconstr-name` `of` `typexpr` declares the name `ncconstr-name` as a non-constant constructor, whose argument has type `typexpr`. The constructor declaration `cconstr-name` declares the name `cconstr-name` as a constant constructor. Constructor names must be capitalized.
The type representation `=` `{` `field-decl` { `;` `field-decl` } `}` describes a record type. The field declarations `field-decl`1, ..., `field-decl`n describe the fields associated to this record type. The field declaration `field-name` `:` `typexpr` declares `field-name` as a field whose argument has type `typexpr`. The field declaration `mutable` `field-name` `:` `typexpr` behaves similarly; in addition, it allows physical modification over the argument to this field.
The two components of a type definition, the optional equation and the optional representation, can be combined independently, giving rise to four typical situations:

Abstract type: no equation, no representation.
  When appearing in a module signature, this definition specifies nothing on the type constructor, besides its number of parameters: its representation is hidden and it is assumed incompatible with any other type.
Type abbreviation: an equation, no representation.
  This defines the type constructor as an abbreviation for the type expression on the right of the `=` sign.
New variant type or record type: no equation, a representation.
  This generates a new type constructor and defines associated constructors or fields, through which values of that type can be directly built or inspected.
Re-exported variant type or record type: an equation, a representation.
  In this case, the type constructor is defined as an abbreviation for the type expression given in the equation, but in addition the constructors or fields given in the representation remain attached to the defined type constructor. The type expression in the equation part must agree with the representation: it must be of the same kind (record or variant) and have exactly the same constructors or fields, in the same order, with the same arguments.

The construct `constraint` `'` `ident` `=` `typexpr` allows to specify type parameters. Any actual type argument corresponding to the type parameter `ident` have to be an instance of `typexpr` (more precisely, `ident` and `typexpr` are unified). Type variables of `typexpr` can appear in the type equation and the type declaration.

6.8.2 | Exception definitions

`exception-definition` | ::= | `exception` `constr-decl`
| | | `exception` `cconstr-name` `=` `cconstr`
| | | `exception` `ncconstr-name` `=` `ncconstr`

Exception definitions add new constructors to the built-in variant type `exn` of exception values. The constructors are declared as for a definition of a variant type.
The form `exception` `constr-decl` generates a new exception, distinct from all other exceptions in the system. The form `exception` `name` `=` `constr` gives an alternate name to an existing exception.