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General accumulator

A general-purpose, extensible value accumulator library for Common Lisp

Author and license

Author: Teemu Likonen <>

License: Public domain

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


General accumulator is a general-purpose, extensible value accumulator library for the Common Lisp language. Its main interface is with-accumulator macro which sets an environment for easy accumulation. The library provides several built-in accumulators which should cover the most common use-cases but any kind of accumulators can be added because the accumulator back-end is implemented through generic functions.

(with-accumulator (NAME OBJECT &key KEYWORD-ARGUMENTS ...)
  BODY ...)

The with-accumulator macro creates an accumulation environment in which the local function name handles the accumulation. Accumulator's type is defined by the object argument. Then all body forms are executed normally and the return value of the last form is returned.

The local function name can optionally take one argument which is an object to be accumulated. If the function is called without arguments it returns the currently accumulated value. The accumulation process is handled by generic functions initialize, accumulate and value.

For more information see the documentation of with-accumulator in the next section.

Interface (API)

Function: accumulate

The lambda list:

 (accumulator object)

Accumulate object to accumulator instance. Methods of this generic function should specialize at least on the first argument (_accumulator) and they should accumulate the second argument (_object) to the accumulator object.

Function: initialize

The lambda list:

 (object &allow-other-keys)

Return an accumulator object which is used to keep the information of an accumulation process.

The object argument can anything and its primary purpose is a method dispatching: different classes of the object establish different kind of accumulators. Methods can use the object value too, as well as any keyword arguments passed to the generic function.

Methods should return an object, usually an instance of some class. That object can later be used with generic functions accumulate and value.

Function: value

The lambda list:


Return the accumulated value of accumulator object.

Macro: with-accumulator

The lambda list:

 ((name object &rest keyword-arguments) &body body)

Create a local function name for handling an accumulation of type object. Execute body forms and return the value of the last form.

This macro uses generic functions to handle the accumulation. There are some built-in methods defined for common use-cases (see below) but user can add more methods and therefore any kind of accumulation is possible.

First a new accumulator object is created with the generic function initialize. The object argument (evaluated) and optional keyword-arguments (evaluated) are passed to initialize and it should return an accumulator object that stores the state of the accumulation.

Then a local function name is created for simple accumulation. The function can optionally take one argument which is an object to be accumulated. The generic function accumulate is used to handle the accumulation. The return value of the local function comes from the generic function accumulate. The built-in accumulators return the input argument.

If the local function is called without arguments then the generic function value is called. It should return the currently accumulated value.

Built-in accumulators

The object argument is used to define the type of accumulation process. There are several built-in types:

Adding a custom accumulator

The whole accumulation process is handled by three generic functions: initialize, accumulate and value. Writing new methods for those functions allow adding any kind of accumulators. The following example adds an accumulator which calculates the arithmetic mean of accumulated numbers.

First we define a class whose instances will keep the state of the accumulator. In this case we need to store the sum and the count of accumulated numbers so we create slots for them.

(defclass mean-accumulator ()
  ((sum :initform 0)
   (count :initform 0)))

Then we add a method for initializing an instance of the class. The generic function initialize is used for that. It is called with the object argument of with-accumulator macro and with optional keyword-arguments. In this example we use an eql specializer for symbol :mean. We don't use any keyword arguments so there's just empty &key at the end of the lambda list.

(defmethod genacc:initialize ((type (eql :mean)) &key)
  (make-instance 'mean-accumulator))

Now we create a method for generic function accumulate. The function is called with two arguments: (1) the accumulator object created by initialize and (2) the object that is meant to be accumulated. This method specializes on our mean-accumulator class as well as on number class. The number is added to the previous value and the count is increased by one.

(defmethod genacc:accumulate ((object mean-accumulator)
                              (number number))
  (with-slots (sum count) object
    (incf sum number)
    (incf count 1)))

For returning the accumulated mean value we create a method for the generic function value. This method, too, must specialize on the mean-accumulator class. We get the current accumulated mean value by dividing the value of sum slot with the value of count slot.

(defmethod genacc:value ((object mean-accumulator))
  (with-slots (sum count) object
    (/ sum count)))

Now the custom accumulator is ready and it can be used with the with-accumulator macro. Example:

GENACC> (with-accumulator (mean :mean)
          (loop repeat 10 do (mean (random 1000)))
          (format t "The mean so far: ~A~%" (mean))
          (loop repeat 10 do (mean (random 1000)))
          (format t "The final mean:  ~A~%" (mean)))
The mean so far: 2512/5
The final mean:  2704/5

The source code

GitHub repository: