Defines a class of item with component fields. Multiple items enumerate their fields in parallel, regardless of what type of pattern the multiple-item appears in. Fields may contain constant values or item streams. name is the name for the multiple item. Each field declares a component of the multiple item. Following the field specification comes zero or more option lists:

(:element-period {field | :all {field}* | :any {field}*})

sets which field or combination of fields controls the period length of the multiple item. If the value is a single field then that field controls the period length. Otherwise the value should be :all or :any followed by zero or more qualifying fields. :all means than each qualified field (or all the fields if none are qualified) must be at end-of-period before the multiple item is considered to be at end-of-period. :any specifies that the multiple-item will be at end-of-period when any of the qualifying fields (or all the fields if none are qualified) are at end-of-period. If only a single field is tracked, then :all and :any have the same effect.

(:element-parser lambda)

Defines a parser for the multiple item. The parser function is passed as many parameters as there are fields declared, and it must return the list of parsed field values. For example, a parser for a multiple item with two fields, the second of which generates scale degrees, would look like:

    (lambda (x y) (list x (degree y))

(:element-state field)

Names the field whose state is returned as the state of the multiple item.

(:element-filter lambda-expression)

Defines an element filter for the multiple item. The filter is passed the current elements in the multiple item and returns the actual list of elements to return.

Example:

? (defmultiple-item gesture (pit rhy)
    (:element-period :all)
    (:element-parser (lambda (n r) (list (note n) (rhythm r)))))
#<Standard-Class GESTURE 137535711>

? (setf x (items (gesture c4 q)
                 (gesture (notes c5 c6 in random) w)
                 (gesture b3 (rhythms s e for 1))
                 in heap for 10))
#<HEAP-ITEM-STREAM 137443141>

? (read-items x)
((C4 1.0) (C5 4.0) (C5 4.0) (B3 0.25) (B3 0.5) (C6 4.0) (C5 4.0) 
 (C4 1.0) (C4 1.0) (B3 0.25) (C5 4.0) (C5 4.0) (C4 1.0))

See Also: