This is the general-accumulator Reference Manual, generated automatically by Declt version 4.0 beta 2 "William Riker" on Tue Jul 15 03:53:20 2025 GMT+0.
The main system appears first, followed by any subsystem dependency.
general-accumulatorA general-purpose, extensible value accumulator
Teemu Likonen <tlikonen@iki.fi>
Creative Commons CC0 (public domain dedication)
accumulator.lisp (file).
Files are sorted by type and then listed depth-first from the systems components trees.
general-accumulator/general-accumulator.asdgeneral-accumulator (system).
general-accumulator/accumulator.lispgeneral-accumulator (system).
accumulate (generic function).
initialize (generic function).
value (generic reader).
with-accumulator (macro).
accumulator (class).
list-accumulator (class).
reduce-accumulator (class).
special-vector-accumulator (class).
vector-accumulator (class).
Packages are listed by definition order.
general-accumulatorgenacc
common-lisp.
accumulate (generic function).
initialize (generic function).
value (generic reader).
with-accumulator (macro).
accumulator (class).
list-accumulator (class).
reduce-accumulator (class).
special-vector-accumulator (class).
vector-accumulator (class).
Definitions are sorted by export status, category, package, and then by lexicographic order.
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:
* ‘:list‘
Creates a list collector. Each accumulated object is collected to a
list. Example:
GENACC> (with-accumulator (collect :list)
(collect 1) (collect 2) (collect 3)
(collect))
(1 2 3)
The collecting is done destructively. The applicable ‘accumulate‘
method maintains a pointer to the last cons cell of the list and
each time modifies its cdr value to point to a new cons cell.
* [a list]
If _object_ is of type ‘list‘ then new elements are collected at the
end. Example:
GENACC> (with-accumulator (collect (list 1 2 3))
(collect 4) (collect 5)
(collect))
(1 2 3 4 5)
This is a destructive operation. The cdr value of the last cons cell
of the original list is modified and linked to a new cons cell.
* ‘:vector‘
Creates a general vector collector. It creates an adjustable vector
with a fill pointer 0 and element type T. New elements are pushed to
that vector with ‘cl:vector-push-extend‘ function. Example:
GENACC> (with-accumulator (collect :vector)
(collect "first") (collect "second")
(collect))
#("first" "second")
* ‘:string‘
This is similar to ‘:vector‘ but the element type is ‘character‘.
The underlying ‘accumulate‘ methods can take a single character or a
sequence of characters as the argument. Example:
GENACC> (with-accumulator (collect :string)
(collect #\a)
(collect "bcd")
(collect #(#\e #\f))
(collect ’(#\g #\h #\i))
(collect))
"abcdefghi"
* ‘:bit-vector‘
This is similar to ‘:string‘ but the element type is ‘bit‘. The
argument for the accumulator function can a bit or a sequence of
bits.
* [a vector]
If _object_ is of type ‘vector‘ which satisfies the test
‘cl:array-has-fill-pointer-p‘ then that vector is appended starting
from its current fill pointer.
GENACC> (with-accumulator
(collect (make-array 2 :fill-pointer 2 :adjustable t
:initial-contents (vector 1 2)))
(collect 3)
(collect 4)
(collect))
#(1 2 3 4)
Note that if the vector is not adjustable then the accumulator may reach vector’s limits and ‘cl:vector-push-extend‘ signals an error.
* [a function]
If _object_ is of type ‘function‘ then the accumulator behaves like
the ‘cl:reduce‘ function: all accumulated objects are combined into
one by calling the given reducer function. Examples:
GENACC> (with-accumulator (summing #’+)
(summing 5) (summing 7) (summing 11)
(summing))
23
GENACC> (with-accumulator (nc #’nconc)
(nc (list 1 2 3))
(nc (list 4 5 6))
(nc (list 7 8 9))
(nc))
(1 2 3 4 5 6 7 8 9)
GENACC> (with-accumulator (early-char (lambda (a b)
(if (char< a b) a b)))
(early-char #\o)
(early-char #\b)
(early-char #\s)
(early-char))
#\b
#### 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
NIL
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.
reduce-accumulator) item) ¶list-accumulator) item) ¶special-vector-accumulator) (item cons)) ¶special-vector-accumulator) (item vector)) ¶vector-accumulator) item) ¶accumulator) item) ¶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) &key) ¶(eql :list)) &key) ¶list) &key) ¶(eql :bit-vector)) &key) ¶(eql :string)) &key) ¶(eql :vector)) &key) ¶vector) &key) ¶Return the accumulated value of _accumulator_ object.
accumulator)) ¶automatically generated reader method
:last-cons
common-lisp.
:function
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