17.3.1 Designating a #lang Language
The syntax of a language intentionally overlaps with the syntax of a module path as used in require or as a module language, so that names like racket, racket/base, slideshow, or scribble/manual can be used both as #lang languages and as module paths.
At the same time, the syntax of language is far more restricted than a module path, because only a-z, A-Z, 0-9, / (not at the start or end), _, -, and + are allowed in a language name. These restrictions keep the syntax of #lang as simple as possible. Keeping the syntax of #lang simple, in turn, is important because the syntax is inherently inflexible and non-extensible; the #lang protocol allows a language to refine and define syntax in a practically unconstrained way, but the #lang protocol itself must remain fixed so that various different tools can “boot” into the extended world.
Fortunately, the #lang protocol provides a natural way to refer to languages in ways other than the rigid language syntax: by defining a language that implements its own nested protocol. We have already seen one example (in Using #lang s-exp): the s-exp language allows a programmer to specify a module language using the general module path syntax. Meanwhile, s-exp takes care of the reader-level responsibilities of a #lang language.
Unlike racket, s-exp cannot be used as a module path with require. Although the syntax of language for #lang overlaps with the syntax of module paths, a language is not used directly as a module path. Instead, a language obtains a module path by trying two locations: first, it looks for a reader submodule of the main module for language. If this is not a valid module path, then language is suffixed with /lang/reader. (If neither is a valid module path, an error is raised.) The resulting module supplies read and read-syntax functions using a protocol that is similar to the one for #reader.
Reader Extensions introduces #reader.
A consequence of the way that a #lang language is turned into a module path is that the language must be installed in a collection, similar to the way that "racket" or "slideshow" are collections that are distributed with Racket. Again, however, there’s an escape from this restriction: the reader language lets you specify a reader-level implementation of a language using a general module path.