Evaluates the contents of a file, if it hasn't been seen before.

This function is equivalent to (load path), except that if the specified file has previously been passed to a require call in the current Runtime, it's a silent no-op.

When you have a complicated source hierarchy, this function can be useful to avoid accidentally loading the same source file twice.

Deduplication is based on the file which is being referred to, rather than the literal text of the path argument. For example, these three require calls would only load example.glsp once:

(require "src/example.glsp")
(require "./src/example.glsp")
(require "src/../src/example.glsp")

In a sandboxed Runtime, this macro is not defined.