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elisp: Closures

 
 12.10 Closures
 ==============
 
 As explained in Variable Scoping, Emacs can optionally enable
 lexical binding of variables.  When lexical binding is enabled, any
 named function that you create (e.g., with ‘defun’), as well as any
 anonymous function that you create using the ‘lambda’ macro or the
 ‘function’ special form or the ‘#'’ syntax (Anonymous
 Functions), is automatically converted into a “closure”.
 
    A closure is a function that also carries a record of the lexical
 environment that existed when the function was defined.  When it is
 invoked, any lexical variable references within its definition use the
 retained lexical environment.  In all other respects, closures behave
 much like ordinary functions; in particular, they can be called in the
 same way as ordinary functions.
 
    Lexical Binding, for an example of using a closure.
 
    Currently, an Emacs Lisp closure object is represented by a list with
 the symbol ‘closure’ as the first element, a list representing the
 lexical environment as the second element, and the argument list and
 body forms as the remaining elements:
 
      ;; lexical binding is enabled.
      (lambda (x) (* x x))
           ⇒ (closure (t) (x) (* x x))
 
 However, the fact that the internal structure of a closure is exposed to
 the rest of the Lisp world is considered an internal implementation
 detail.  For this reason, we recommend against directly examining or
 altering the structure of closure objects.
 

Info Catalog elisp: Function Cells elisp: Functions elisp: Advising Functions