Syntax Terminology (MIT/GNU Scheme 10.1.6)

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#### 2.11.3.1 Syntax Terminology

This section defines the concepts and data types used by the syntactic closures facility.

- Forms are the syntactic entities out of which programs are recursively constructed. A form is any expression, any definition, any syntactic keyword, or any syntactic closure. The variable name that appears in a `set!` special form is also a form. Examples of forms:

      17
      #t
      car
      (+ x 4)
      (lambda (x) x)
      (define pi 3.14159)
      if
      define

- An alias is an alternate name for a given symbol. It can appear anywhere in a form that the symbol could be used, and when quoted it is replaced by the symbol; however, it does not satisfy the predicate `symbol?`. Macro transformers rarely distinguish symbols from aliases, referring to both as identifiers. Another name for an alias is synthetic identifier; this document uses both names.
- A syntactic environment maps identifiers to their meanings. More precisely, it determines whether an identifier is a syntactic keyword or a variable. If it is a keyword, the meaning is an interpretation for the form in which that keyword appears. If it is a variable, the meaning identifies which binding of that variable is referenced. In short, syntactic environments contain all of the contextual information necessary for interpreting the meaning of a particular form.
- A syntactic closure consists of a form, a syntactic environment, and a list of identifiers. All identifiers in the form take their meaning from the syntactic environment, except those in the given list. The identifiers in the list are to have their meanings determined later.

  A syntactic closure may be used in any context in which its form could have been used. Since a syntactic closure is also a form, it may not be used in contexts where a form would be illegal. For example, a form may not appear as a clause in the `cond` special form.

  A syntactic closure appearing in a quoted structure is replaced by its form.

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