mapcan

(mapcan function list &rest more-lists)

Applies function to successive elements of the given lists -- each call should return a list -- and concatenates the results into a single list. rontolisp joins the pieces with non-destructive append (it does not splice with nconc as standard Common Lisp does). It is a convenient way to map and filter at once: return nil for elements to drop. When several lists are supplied, the function is called with one element from each list in parallel, and iteration stops at the end of the shortest list.

Each argument must be a list (nil, the empty list, is accepted); passing a non-list such as a string signals an error rather than silently returning nil. Use map to map over a string or vector.