6.2 Invoking Units

When no tagged-sig-specs are provided, unit-expr must produce a unit that expect no imports. To invoke the unit, all bindings are first initialized to the #<undefined> value. Next, the unit’s body definitions and expressions are evaluated in order; in the case of a definition, evaluation sets the value of the corresponding variable(s). Finally, the result of the last expression in the unit is the result of the invoke-unit expression.

Each supplied tagged-sig-spec takes bindings from the surrounding context and turns them into imports for the invoked unit. The unit need not declare an imports for evey provided tagged-sig-spec, but one tagged-sig-spec must be provided for each declared import of the unit. For each variable identifier in each provided tagged-sig-spec, the value of the identifier’s binding in the surrounding context is used for the corresponding import in the invoked unit.

The unit produced by unit-expr is linked and invoked as for invoke-unit. In addition, the export clause is treated as a kind of import into the local definition context. That is, for every binding that would be available in a unit that used the export clauses’s tagged-sig-spec as an import, a definition is generated for the context of the define-values/invoke-unit form.