B.3. Gatherers

change_begin
These functions create and process gatherers.

[Function]
gatherer collector

The collector must be a function of type (function ((series )) ). Given this function gatherer returns a gatherer that accepts elements of type and returns a final result of type . The method for combining elements used by the gatherer is the same as the one used by the collector.

[Function]
next-out gatherer item

Given a gatherer and a value next-out enters the value into the gatherer.

[Function]
result-of gatherer

result-of retrieves the net result from a gatherer. result-of can be applied at any time. However it is an error to apply result-of twice to the same gatherer or to apply next-out to a gatherer once result-of has been applied.

(let ((g (gatherer #'collect-sum)))
(dolist (i '(1 2 3 4))
(next-out g i)
(if (evenp i) (next-out g (* 10 i))))
(result-of g))
=> 70

[Macro]
gathering ({(var fn)}*) {form}*

The first subform must be a list of pairs. The first element of each pair var must be a variable name. The second element of each pair fn fnd element of each pairfn)}*) {form}*

The first subform must be a list of pairs. The first by the finalml