What is the difference between an "interned" and an "uninterned" symbol

Question

What is the difference between an "interned" and an "uninterned" symbol. Is it only Racket that has uninterned symbols or do other dialects of scheme or lisp have them?

Answer 1

Interned symbols are eq? if and only if they have the same name. Uninterned symbols are not eq? to any other symbol, so they are a kind of unique token with an attached string. Interned symbols are the kind that are produced by the default reader. Uninterned symbols can be used as identifiers when generating code in a macro, such an identifier cannot be shadowed by any other identifier. Most Lisp dialects have this concepts, in Scheme it is rarer, since hygienic macros are supposed to reduce its usefulness.

Answer 2

Common Lisp has uninterned symbols. As Juho's answer says, an uninterned symbol is guaranteed not to be equal to any other value.

Common Lisp-style requires uninterned symbols in order to write many macros correctly (particularly macros whose expansion requires introducing and binding new variables), because any interned symbol you use in a macro expansion might capture or shadow a binding in its expansion site.

Scheme's hygienic macro systems, on the other hand, do not have this problem, so a Scheme system does not need to provide uninterned symbols. Still, many of them do. Why? Several reasons:

• Some Scheme systems offer a Common Lisp-style defmacro capability.
• In others, the hygienic macro system's implementation may use uninterned symbols internally, but the concept of an uninterned symbol may be exposed.
• Uninterned symbols can be useful in many programs that use s-expressions to represent a language, and transform this language into another s-expr language. These sorts of tasks often benefit from an ability to generate an identifier guaranteed to be new.