The mapping functions in this group differ in how they construct a return value. Instead, it processes the inputs just for effect, and returns the first input list as the result of MAPC . Instead, it processes the inputs just for effect, and returns the first input list as the result of MAPL . MAP-INTO is a destructive version of MAP . The first argument is a sequence that receives the results of the mapping.
Mapping stops upon reaching the end of the result sequence or any of the input sequences.
The input sequences are not modified. Your Lisp implementation may print different initial values for the unmodified sequences in the above examples. A filter passes some of its inputs through to its output, and drops others. This is the key observation that we need to construct a filter. WHEN returns NIL if the condition is NIL .