\section{Type and exception definitions}
%HEVEA\cutname{typedecl.html}%
\pdfsection{Type and exception definitions}

\subsection{Type definitions}
\label{s:type-defs}

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.

\ikwd{type\@\texttt{type}}
\ikwd{and\@\texttt{and}}
\ikwd{nonrec\@\texttt{nonrec}}
\ikwd{of\@\texttt{of}}

\begin{syntax}
type-definition:
          'type' ['nonrec'] 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 }
        | '=' record-decl
;
type-params:
          type-param
        | '(' type-param { "," type-param } ')'
;
type-param:
          [variance] "'" ident
;
variance:
          '+'
        | '-'
;
record-decl:
         '{' field-decl { ';' field-decl } [';'] '}'
;
constr-decl:
          (constr-name || '[]' || '(::)') [ 'of' constr-args ]
;
constr-args:
          typexpr { '*' typexpr }
;
field-decl:
          ['mutable'] field-name ':' poly-typexpr
;
type-constraint:
    'constraint' "'" ident '=' typexpr
\end{syntax}
\ikwd{mutable\@\texttt{mutable}}
\ikwd{constraint\@\texttt{constraint}}
See also the following language extensions:
\hyperref[s:private-types]{private types},
\hyperref[s:gadts]{generalized algebraic datatypes},
\hyperref[s:attributes]{attributes},
\hyperref[s:extension-nodes]{extension nodes},
\hyperref[s:extensible-variants]{extensible variant types} and
\hyperref[s:inline-records]{inline records}.

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.

In the right-hand side of type definitions, references to one of the
type constructor name being defined are considered as recursive,
unless "type" is followed by "nonrec". The "nonrec" keyword was
introduced in OCaml 4.02.2.

The optional type parameters are either one type variable @"'" ident@,
for type constructors with one parameter, or a list of type variables
@"('"ident_1,\ldots,"'"ident_n")"@, for type constructors with several
parameters. Each type parameter may be prefixed by a variance
constraint @"+"@ (resp. @"-"@) indicating that the parameter is
covariant (resp. contravariant).  These type parameters can appear in
the type expressions of the right-hand side of the definition,
optionally restricted by a variance constraint ; {\em i.e.\/} a
covariant parameter may only appear on the right side of a functional
arrow (more precisely, follow the left branch of an even number of
arrows), and a contravariant parameter only the left side (left branch of
an odd number of arrows). If the type has a representation or
an equation, and the parameter is free ({\em i.e.\/} not bound via a
type constraint to a constructed type), its variance constraint is
checked but subtyping {\em etc.\/} will use the inferred variance of the
parameter, which may be less restrictive; otherwise ({\em i.e.\/} for abstract
types or non-free parameters), the variance must be given explicitly,
and the parameter is invariant if no variance is given.

The optional type equation @'=' typexpr@ makes the defined type
equivalent to the type expression @typexpr@:
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, \ldots, constr-decl_n@ describe the constructors
associated to this variant type. The constructor
declaration @constr-name 'of' typexpr_1 '*' \ldots '*' typexpr_n@
declares the name @constr-name@ as a non-constant constructor, whose
arguments have types @typexpr_1@ \ldots @typexpr_n@.
The constructor declaration @constr-name@
declares the name @constr-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, \ldots,
field-decl_n@ describe the fields associated to this record type.
The field declaration @field-name ':' poly-typexpr@ declares
@field-name@ as a field whose argument has type @poly-typexpr@.
The field declaration @'mutable' field-name ':' poly-typexpr@
\ikwd{mutable\@\texttt{mutable}}
behaves similarly; in addition, it allows physical modification of
this field.
Immutable fields are covariant, mutable fields are non-variant.
Both mutable and immutable fields may have explicitly polymorphic
types.  The polymorphism of the contents is statically checked whenever
a record value is created or modified.  Extracted values may have their
types instantiated.

The two components of a type definition, the optional equation and the
optional representation, can be combined independently, giving
rise to four typical situations:

\begin{description}
\item[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.

\item[Type abbreviation: an equation, no representation.] ~\\
This defines the type constructor as an abbreviation for the type
expression on the right of the @'='@ sign.

\item[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.

\item[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.
\end{description}

The type variables appearing as type parameters can optionally be
prefixed by "+" or "-" to indicate that the type constructor is
covariant or contravariant with respect to this parameter.  This
variance information is used to decide subtyping relations when
checking the validity of @":>"@ coercions (see section \ref{s:coercions}).

For instance, "type +'a t" declares "t" as an abstract type that is
covariant in its parameter; this means that if the type $\tau$ is a
subtype of the type $\sigma$, then $\tau " t"$ is a subtype of $\sigma
" t"$.  Similarly, "type -'a t" declares that the abstract type "t" is
contravariant in its parameter: if $\tau$ is a subtype of $\sigma$, then
$\sigma " t"$ is a subtype of $\tau " t"$.  If no "+" or "-" variance
annotation is given, the type constructor is assumed non-variant in the
corresponding parameter.  For instance, the abstract type declaration
"type 'a t" means that $\tau " t"$ is neither a subtype nor a
supertype of $\sigma " t"$ if $\tau$ is subtype of $\sigma$.

The variance indicated by the "+" and "-" annotations on parameters
is enforced only for abstract and private types, or when there are
type constraints.
Otherwise, for abbreviations, variant and record types without type
constraints, the variance properties of the type constructor
are inferred from its definition, and the variance annotations are
only checked for conformance with the definition.

\ikwd{constraint\@\texttt{constraint}}
The construct @ 'constraint' "'" ident '=' typexpr @ allows the
specification of
type parameters.  Any actual type argument corresponding to the type
parameter @ident@ has 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.

\subsection{Exception definitions} \label{s:excdef}
\ikwd{exception\@\texttt{exception}}

\begin{syntax}
exception-definition:
        'exception' constr-decl
      | 'exception' constr-name '=' constr
\end{syntax}

Exception definitions add new constructors to the built-in variant
type \verb"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' constr-name '=' constr@
gives an alternate name to an existing exception.
