Scheme 48 Manual | Contents | In Chapter: Module system
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## Configuration packages

It is possible to set up multiple configuration packages. The default configuration package opens the following structures:

- `module-system`, which exports `define-structure` and the other configuration language keywords, as well as standard types and type constructors (`:syntax`, `:value`, `proc`, etc.).
- `built-in-structures`, which exports structures that are built into the initial Scheme 48 image; these include `scheme`, `threads`, `tables`, and `records`.
- `more-structures`, which exports additional structures that are available in the development environment. A complete listing can be found in the definition of `more-structures-interface` at the end of the file `scheme/packages.scm`.

Note that it does not open `scheme`.

You can define additional configuration packages by making a package that opens `module-system` and, optionally, `built-in-structures`, `more-structures`, or other structures that export structures and interfaces.

For example:

>     > ,config (define-structure foo (export)
>                 (open module-system
>                       built-in-structures
>                       more-structures))
>     > ,in foo
>     foo> (define-structure x (export a b)
>            (open scheme)
>            (files x))
>     foo>

Unfortunately, the above example does not work. The problem is that every environment in which `define-structure` is defined must also have a way to create "reflective towers" (a misnomer; a better name would be "syntactic towers"). A new reflective tower is required whenever a new environment is created for compiling the source code in the package associated with a new structure. The environment's tower is used at compile time for evaluating the `macro-source` in

>     (define-syntax name macro-source)
>     (let-syntax ((name macro-source) ...) body)

and so forth. It is a "tower" because that environment, in turn, has to say what environment to use if `macro-source` itself contains a use of `let-syntax`.

The simplest way to provide a tower maker is to pass on the one used by an existing configuration package. The special form `export-reflective-tower` creates an interface that exports a configuration package's tower. The following example uses `export-reflective-tower` and the `,structure` command to obtain a tower maker and create a new configuration environment.

>     > ,config ,structure t (export-reflective-tower-maker)
>     > ,config (define-structure foo (export)
>                 (open module-system
>                       t
>                       built-in-structures
>                       more-structures))

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