## 6.12 Compilation units

unit-interface | ::= | { specification [`;;`] }
unit-implementation | ::= | { definition [`;;`] }

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` … `end` signature expression. The implementation contains a sequence of definitions, just as the inside of a `struct` … `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 8 for more details). A compilation unit behaves roughly as the module definition

`module` unit-name `:` `sig` unit-interface `end` `=` `struct` unit-implementation `end`

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

name1 `:` `sig` specification1 `end` … namen `:` `sig` specificationn `end`

where name1 … namen are the names of the other compilation units available in the search path (see chapter 8 for more details) and specification1 … specificationn are their respective interfaces.