(guile.info.gz) Closure

 
 14.4.5 Closure
 --------------
 
 Consider a `let' expression that doesn't contain any `lambda's:
 
      (let ((s (/ (+ a b c) 2)))
        (sqrt (* s (- s a) (- s b) (- s c))))
 
 When the Scheme interpreter evaluates this, it
 
    * creates a new environment with a reference to the environment that
      was current when it encountered the `let'
 
    * creates a variable binding for `s' in the new environment, with
      value given by `(/ (+ a b c) 2)'
 
    * evaluates the expression in the body of the `let' in the context of
      the new local environment, and remembers the value `V'
 
    * forgets the local environment
 
    * continues evaluating the expression that contained the `let', using
      the value `V' as the value of the `let' expression, in the context
      of the containing environment.
 
    After the `let' expression has been evaluated, the local environment
 that was created is simply forgotten, and there is no longer any way to
 access the binding that was created in this environment.  If the same
 code is evaluated again, it will follow the same steps again, creating
 a second new local environment that has no connection with the first,
 and then forgetting this one as well.
 
    If the `let' body contains a `lambda' expression, however, the local
 environment is _not_ forgotten.  Instead, it becomes associated with
 the procedure that is created by the `lambda' expression, and is
 reinstated every time that that procedure is called.  In detail, this
 works as follows.
 
    * When the Scheme interpreter evaluates a `lambda' expression, to
      create a procedure object, it stores the current environment as
      part of the procedure definition.
 
    * Then, whenever that procedure is called, the interpreter
      reinstates the environment that is stored in the procedure
      definition and evaluates the procedure body within the context of
      that environment.
 
    The result is that the procedure body is always evaluated in the
 context of the environment that was current when the procedure was
 created.
 
    This is what is meant by "closure".  The next few subsections
 present examples that explore the usefulness of this concept.
Info Catalog
(guile.info.gz) Lexical Scope
(guile.info.gz) About Closure
(guile.info.gz) Serial Number
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