\section{s:module-expr}{Module expressions (module implementations)}
%HEVEA\cutname{modules.html}

Module expressions are the module-level equivalent of value
expressions: they evaluate to modules, thus providing implementations
for the specifications expressed in module types.

\ikwd{struct\@\texttt{struct}}
\ikwd{end\@\texttt{end}}
\ikwd{functor\@\texttt{functor}}
\ikwd{let\@\texttt{let}}
\ikwd{and\@\texttt{and}}
\ikwd{external\@\texttt{external}}
\ikwd{type\@\texttt{type}}
\ikwd{exception\@\texttt{exception}}
\ikwd{class\@\texttt{class}}
\ikwd{module\@\texttt{module}}
\ikwd{open\@\texttt{open}}
\ikwd{include\@\texttt{include}}

\begin{syntax}
module-expr:
          module-path
        | 'struct' [ module-items ] 'end'
        | 'functor' {{ '(' module-name ':' module-type ')' }} '->' module-expr
        | module-expr '(' module-expr ')'
        | '(' module-expr ')'
        | '(' module-expr ':' module-type ')'
;
module-items:
        { ';;' } ( definition || expr ) { { ';;' } ( definition || ';;' expr) } { ';;' }
;
%\end{syntax} \begin{syntax}
definition:
          'let' ['rec'] let-binding { 'and' let-binding }
        | 'external' value-name ':' typexpr '=' external-declaration
        | type-definition
        | exception-definition
        | class-definition
        | classtype-definition
        | 'module' module-name { '(' module-name ':' module-type ')' }
                   [ ':' module-type ] \\ '=' module-expr
        | 'module' 'type' modtype-name '=' module-type
        | 'open' module-path
        | 'include' module-expr
\end{syntax}
See also the following language extensions:
\hyperref[s:recursive-modules]{recursive modules},
\hyperref[s:first-class-modules]{first-class modules},
\hyperref[s:explicit-overriding-open]{overriding in open statements},
\hyperref[s:attributes]{attributes},
\hyperref[s:extension-nodes]{extension nodes} and
\hyperref[s:generative-functors]{generative functors}.

\subsection{ss:mexpr-simple}{Simple module expressions}

The expression @module-path@ evaluates to the module bound to the name
@module-path@.

The expression @'(' module-expr ')'@ evaluates to the same module as
@module-expr@.

The expression @'(' module-expr ':' module-type ')'@ checks that the
type of @module-expr@ is a subtype of @module-type@, that is, that all
components specified in @module-type@ are implemented in
@module-expr@, and their implementation meets the requirements given
in @module-type@. In other terms, it checks that the implementation
@module-expr@ meets the type specification @module-type@. The whole
expression evaluates to the same module as @module-expr@, except that
all components not specified in @module-type@ are hidden and can no
longer be accessed.

\subsection{ss:mexpr-structures}{Structures}

\ikwd{struct\@\texttt{struct}}
\ikwd{end\@\texttt{end}}

Structures @'struct' \ldots 'end'@ are collections of definitions for
value names, type names, exceptions, module names and module type
names. The definitions are evaluated in the order in which they appear
in the structure. The scopes of the bindings performed by the
definitions extend to the end of the structure. As a consequence, a
definition may refer to names bound by earlier definitions in the same
structure.

For compatibility with toplevel phrases (chapter~\ref{c:camllight}),
optional @";;"@ are allowed after and before each definition in a structure. These
@";;"@ have no semantic meanings. Similarly, an @expr@ preceded by ";;" is allowed as
a component of a structure. It is equivalent to @'let' '_' '=' expr@, i.e. @expr@ is
evaluated for its side-effects but is not bound to any identifier. If @expr@ is
the first component of a structure, the preceding ";;" can be omitted.

\subsubsection*{sss:mexpr-value-defs}{Value definitions}

\ikwd{let\@\texttt{let}}

A value definition @'let' ['rec'] let-binding  { 'and' let-binding }@
bind value names in the same way as a @'let' \ldots 'in' \ldots@ expression
(see section~\ref{sss:expr-localdef}). The value names appearing in the
left-hand sides of the bindings are bound to the corresponding values
in the right-hand sides.

\ikwd{external\@\texttt{external}}

A value definition @'external' value-name ':' typexpr '=' external-declaration@
implements @value-name@ as the external function specified in
@external-declaration@ (see chapter~\ref{c:intf-c}).

\subsubsection*{sss:mexpr-type-defs}{Type definitions}

\ikwd{type\@\texttt{type}}

A definition of one or several type components is written
@'type' typedef { 'and' typedef }@ and consists of a sequence
of mutually recursive definitions of type names.

\subsubsection*{sss:mexpr-exn-defs}{Exception definitions}

\ikwd{exception\@\texttt{exception}}

Exceptions are defined with the syntax @'exception' constr-decl@
or @'exception' constr-name '=' constr@.

\subsubsection*{sss:mexpr-class-defs}{Class definitions}

\ikwd{class\@\texttt{class}}

A definition of one or several classes is written @'class'
class-binding { 'and' class-binding }@ and consists of a sequence of
mutually recursive definitions of class names. Class definitions are
described more precisely in section~\ref{ss:class-def}.

\subsubsection*{sss:mexpr-classtype-defs}{Class type definitions}

\ikwd{class\@\texttt{class}}
\ikwd{type\@\texttt{type}}

A definition of one or several classes is written
@'class' 'type' classtype-def { 'and' classtype-def }@ and consists of
a sequence of mutually recursive definitions of class type names.
Class type definitions are described more precisely in
section~\ref{ss:classtype}.

\subsubsection*{sss:mexpr-module-defs}{Module definitions}

\ikwd{module\@\texttt{module}}

The basic form for defining a module component is
@'module' module-name '=' module-expr@, which evaluates @module-expr@ and binds
the result to the name @module-name@.

One can write
\begin{center}
@'module' module-name ':' module-type '=' module-expr@
\end{center}
instead of
\begin{center}
@'module' module-name '=' '(' module-expr ':' module-type ')'@.
\end{center}
Another derived form is
\begin{center}
@'module' module-name '(' name_1 ':' module-type_1 ')' \ldots
                      '(' name_n ':' module-type_n ')' '=' module-expr@
\end{center}
which is equivalent to
\begin{center}
@'module' module-name '='
 'functor' '(' name_1 ':' module-type_1 ')' '->' \ldots
                                            '->' module-expr@
\end{center}

\subsubsection*{sss:mexpr-modtype-defs}{Module type definitions}

\ikwd{type\@\texttt{type}}
\ikwd{module\@\texttt{module}}

A definition for a module type is written
@'module' 'type' modtype-name '=' module-type@.
It binds the name @modtype-name@ to the module type denoted by the
expression @module-type@.

\subsubsection*{sss:mexpr-open}{Opening a module path}

\ikwd{open\@\texttt{open}}

The expression @'open' module-path@ in a structure does not define any
components nor perform any bindings. It simply affects the parsing of
the following items of the structure, allowing components of the
module denoted by @module-path@ to be referred to by their simple names
@name@ instead of path accesses @module-path '.' name@.  The scope of
the @"open"@ stops at the end of the structure expression.

\subsubsection*{sss:mexpr-include}{Including the components of another structure}

\ikwd{include\@\texttt{include}}

The expression @'include' module-expr@ in a structure re-exports in
the current structure all definitions of the structure denoted by
@module-expr@.  For instance, if you define a module "S" as below
\begin{caml_example*}{verbatim}
module S = struct type t = int  let x = 2 end
\end{caml_example}
defining the module "B" as
\begin{caml_example*}{verbatim}
module B = struct include S  let y = (x + 1 : t) end
\end{caml_example}
is equivalent to defining it as
\begin{caml_example*}{verbatim}
module B = struct type t = S.t  let x = S.x  let y = (x + 1 : t) end
\end{caml_example}
The difference between @'open'@ and @'include'@ is that @'open'@
simply provides short names for the components of the opened
structure, without defining any components of the current structure,
while @'include'@ also adds definitions for the components of the
included structure.

\subsection{ss:mexpr-functors}{Functors}

\subsubsection*{sss:mexpr-functor-defs}{Functor definition}

\ikwd{functor\@\texttt{functor}}

The expression @'functor' '(' module-name ':' module-type ')' '->'
module-expr@ evaluates to a functor that takes as argument modules of
the type @module-type_1@, binds @module-name@ to these modules,
evaluates @module-expr@ in the extended environment, and returns the
resulting modules as results. No restrictions are placed on the type of the
functor argument; in particular, a functor may take another functor as
argument (``higher-order'' functor).

When the result module expression is itself a functor,
\begin{center}
@'functor' '(' name_1 ':' module-type_1 ')' '->' \ldots '->'
 'functor' '(' name_n ':' module-type_n ')' '->' module-expr@
\end{center}
one may use the abbreviated form
\begin{center}
@'functor' '(' name_1 ':' module-type_1 ')' \ldots
           '(' name_n ':' module-type_n ')' '->' module-expr@
\end{center}

\subsubsection*{sss:mexpr-functor-app}{Functor application}

The expression @module-expr_1 '(' module-expr_2 ')'@ evaluates
@module-expr_1@ to a functor and @module-expr_2@ to a module, and
applies the former to the latter. The type of @module-expr_2@ must
match the type expected for the arguments of the functor @module-expr_1@.