## 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
`name`1 `:` `sig` `interface`1 `end` … `name`n `:` `sig` `interface`n `end`
where `name`1 … `name`n are the names of the other compilation units available in the search path (see chapter 8 for more details) and `interface`1 … `interface`n are their respective interfaces.