The Fortress Language Specification - CiteSeerX

An import statement either imports an API and allows the specified names (separated by commas) declared in the API
to be referred to with their unqualified names:
import {name + } from apiName
or imports an API as another name:
import apiName as anotherAPIName
For convenience, an import statement can import an API and allow all elements declared in that API to be referred to
with unqualified names:
import * from apiName
or can import an API and allow all elements except the specified names (separated by commas) declared in that API
to be referred to with unqualified names:
import * except {name + } from apiName
If multiple elements with conflicting names are imported from separate APIs, all references to those elements within
the component definition must be fully qualified. An export statement specifies the APIs that the component exports.
Every component implicitly imports the Fortress core APIs; every fortress has at least one component implementing
all of these APIs. A preferred component exporting these APIs (configurable by the user) is implicitly linked to every
component installed in the fortress.
An API (described in Section 22.3) serves as an interface of a component. For every API A exported by a component
C, C must provide a definition for every program construct declared in A. These definitions must match the declarations
in A exactly; the modifiers on constructs, the types of variables, the headers of functions and methods, and the
headers of traits must be identical. There is one exception: A trait declaration with an empty comprises clause in A
can be implemented by an object declaration in C. However, it is permissible for a trait or object definition to include
additional methods and fields that are not declared in A. Also, a component is allowed to include top-level definitions
that do not correspond to declarations in any of its exported APIs. The additional definitions that are not declared in
A are not visible from outside the component.
When a component is compiled, the APIs it refers to must be present in the fortress. The import statements in a
component are not a way to abbreviate unqualified names of objects or functions. In our system, an import statement
merely allows references to the imported API to appear in the component definition. References to elements of an
imported API must be fully qualified unless they are imported by an import statement with a from clause. When a
component imports a functional f (either a top-level function or a functional method) by an import statement with a
from clause, the imported f may be overloaded with a functional f declared by the component. When a component
imports a top-level declaration f from an API A, all the relevant types to type check the uses of f are implicitly
imported from A. However, these implicitly imported types for type checking are not expressible by programmers;
programmers must import the types explicitly by import statements to use them.
A key design choice we make is to require that components never refer to other components directly; all external
references are to APIs. This requirement allows programmers to extend and test existing components more easily,
swapping new implementations of libraries in and out of programs at will.
One important restriction on components is that no API may be both imported and exported by the same component.
This restriction is required throughout to ground the semantics of operations on components, as discussed in
Section 22.7.
Every component has a unique name, used for the purposes of component linking. This name includes a user-provided
identifier. In the case of a simple component, the identifier is determined by a component name given at the top of the
source file from which it is compiled. A build script may keep a tally on version numbers and append them to the first
line of a component, incrementing its tally on each compilation. The name of a compound component is specified as
an argument to the link operation (described in Section 22.7) that defines it.
164