5.2 Import declarations

An import declaration takes the following form:

(import ⟨import-set⟩ …)

An import declaration provides a way to import identifiers exported by a library. Each ⟨import set⟩ names a set of bindings from a library and possibly specifies local names for the imported bindings. It takes one of the following forms:

• ⟨library name⟩
• (only ⟨import set⟩ ⟨identifier⟩ …)
• (except ⟨import set⟩ ⟨identifier⟩ …)
• (prefix ⟨import set⟩ ⟨identifier⟩)
• (rename ⟨import set⟩ (⟨identifier₁⟩ ⟨identifier₂⟩) …)

In the first form, all of the identifiers in the named library’s export clauses are imported with the same names (or the exported names if exported with rename). The additional ⟨import set⟩ forms modify this set as follows:

• only produces a subset of the given ⟨import set⟩ including only the listed identifiers (after any renaming). It is an error if any of the listed identifiers are not found in the original set.
• except produces a subset of the given ⟨import set⟩, excluding the listed identifiers (after any renaming). It is an error if any of the listed identifiers are not found in the original set.
• rename modifies the given ⟨import set⟩, replacing each instance of ⟨identifier₁⟩ with ⟨identifier₂⟩. It is an error if any of the listed ⟨identifier₁⟩s are not found in the original set.
• prefix automatically renames all identifiers in the given ⟨import set⟩, prefixing each with the specified ⟨identifier⟩.

In a program or library declaration, it is an error to import the same identifier more than once with different bindings, or to redefine or mutate an imported binding with a definition or with set!, or to refer to an identifier before it is imported. However, a REPL should permit these actions.