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 14.3.1.3 Evaluating a Procedure Invocation Expression
 .....................................................
 
 This is where evaluation starts getting interesting!  As already noted,
 a procedure invocation expression has the form
 
      (PROCEDURE [ARG1 [ARG2 ...]])
 
 where PROCEDURE must be an expression whose value, when evaluated, is a
 procedure.
 
    The evaluation of a procedure invocation expression like this
 proceeds by
 
    * evaluating individually the expressions PROCEDURE, ARG1, ARG2, and
      so on
 
    * calling the procedure that is the value of the PROCEDURE
      expression with the list of values obtained from the evaluations of
      ARG1, ARG2 etc. as its parameters.
 
    For a procedure defined in Scheme, "calling the procedure with the
 list of values as its parameters" means binding the values to the
 procedure's formal parameters and then evaluating the sequence of
 expressions that make up the body of the procedure definition.  The
 value of the procedure invocation expression is the value of the last
 evaluated expression in the procedure body.  The side effects of calling
 the procedure are the combination of the side effects of the sequence of
 evaluations of expressions in the procedure body.
 
    For a built-in procedure, the value and side-effects of calling the
 procedure are best described by that procedure's documentation.
 
    Note that the complete side effects of evaluating a procedure
 invocation expression consist not only of the side effects of the
 procedure call, but also of any side effects of the preceding
 evaluation of the expressions PROCEDURE, ARG1, ARG2, and so on.
 
    To illustrate this, let's look again at the procedure invocation
 expression:
 
      (string-length (string-append "/home" "/" "andrew"))
 
    In the outermost expression, PROCEDURE is `string-length' and ARG1
 is `(string-append "/home" "/" "andrew")'.
 
    * Evaluation of `string-length', which is a variable, gives a
      procedure value that implements the expected behaviour for
      "string-length".
 
    * Evaluation of `(string-append "/home" "/" "andrew")', which is
      another procedure invocation expression, means evaluating each of
 
         * `string-append', which gives a procedure value that
           implements the expected behaviour for "string-append"
 
         * `"/home"', which gives the string value `"/home"'
 
         * `"/"', which gives the string value `"/"'
 
         * `"andrew"', which gives the string value `"andrew"'
 
      and then invoking the procedure value with this list of string
      values as its arguments.  The resulting value is a single string
      value that is the concatenation of all the arguments, namely
      `"/home/andrew"'.
 
    In the evaluation of the outermost expression, the interpreter can
 now invoke the procedure value obtained from PROCEDURE with the value
 obtained from ARG1 as its arguments.  The resulting value is a numeric
 value that is the length of the argument string, which is 12.
 

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