\section{s:compilation-units}{Compilation units}
%HEVEA\cutname{compunit.html}

\begin{syntax}
unit-interface: { specification [';;'] }
;
unit-implementation: [ module-items ]
\end{syntax}

Compilation units bridge the module system and the separate
compilation system. A compilation unit is composed of two parts: an
interface and an implementation. The interface contains a sequence of
specifications, just as the inside of a @'sig' \ldots 'end'@
signature expression. The implementation contains a sequence of
definitions and expressions, just as the inside of a
@'struct' \ldots 'end'@ module
expression. A compilation unit also has a name @unit-name@, derived
from the names of the files containing the interface and the
implementation (see chapter~\ref{c:camlc} for more details). A
compilation unit behaves roughly as the module definition
\begin{center}
@'module' unit-name ':' 'sig' unit-interface 'end' '='
 'struct' unit-implementation 'end'@
\end{center}

A compilation unit can refer to other compilation units by their
names, as if they were regular modules. For instance, if "U" is a
compilation unit that defines a type "t", other compilation units can
refer to that type under the name "U.t"; they can also refer to "U" as
a whole structure. Except for names of other compilation units, a unit
interface or unit implementation must not have any other free variables.
In other terms, the type-checking and compilation of an interface or
implementation proceeds in the initial environment
\begin{center}
@name_1 ':' 'sig' specification_1 'end' \ldots
 name_n ':' 'sig' specification_n 'end'@
\end{center}
where @name_1 \ldots name_n@ are the names of the other
compilation units available in the search path (see
chapter~\ref{c:camlc} for more details) and @specification_1 \ldots
specification_n@ are their respective interfaces.