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A syntax object is a representation of a Scheme form that contains contextual information about the form in addition to its structure. This contextual information is used by the expander to maintain lexical scoping and may also be used by an implementation to maintain source–object correlation.
A syntax object may be wrapped. It may also be unwrapped, fully or partially, i.e., consist of list and vector structure with wrapped syntax objects or nonsymbol values at the leaves. More formally, a syntax object is:
The distinction between the terms “syntax object” and “wrapped syntax object” is important. For example, when invoked by the expander, a transformer must accept a wrapped syntax object but may return any syntax object, including an unwrapped syntax object.
Syntax objects representing identifiers are always wrapped and are distinct from other types of values. Wrapped syntax objects that are not identifiers may or may not be distinct from other types of values.