Formal comment #126 (enhancement)

local imports
Reported by:	Abdulaziz Ghuloum

Component:	libraries
Version:	5.91

One sentence summary: A local import form, similar in syntax to
libraries' import form, should be added to the set of valid R6RS
definitions.

Description: 

The syntax of R6RS libraries contains a single library-global import
form. One import form forces programmers to dump all imported
identifiers into one flat environment that is visible throughout the
library. While some libraries (such as the r6rs library itself) are
typically used in every expression in the library, some libraries are
only used in small local regions of the library. Using a single global
import form for a library, it is impossible to express the region of
code that depends on an imported library. A library writer may import
libraries to fulfill a specific purpose within the body of the
library. In the cycle of development, other libraries may be chosen to
fulfill the same purpose, or the dependence on the original library is
no longer required. If the library imports tens of such helper
libraries, then isolating their extent may become a refactoring
nightmare. A local import form supports an elegant solution to the
problem by allowing libraries to be imported only where they are
needed.

When I program in C, I usually add the required #include lines as they
are needed to make the compiler happy. I am personally guilty of
almost never removing an #include line, even if it's no longer
needed. I really wish not to do the same in my Scheme libraries.

Recommendation: 

A local import form, with syntax identical to the library-level import
form, should be added to the set of valid definitions. An import form
makes the bindings of the exporting libraries visible in the body
where the import form appears.

RESPONSE:

While we agree with the described benefits of local import, a drawback
of local imports is that library dependencies are not readily apparent
from a library header. This drawback seems significant enough to merit
longer-term consideration. Since leaving out local `import' for now
does not preclude it from being added later, and since R6RS already
tackles so many library-related issues, we have decided to omit local
`import' from R6RS and leave it to consideration for future Scheme
standards.