clojure has a handy (into to-coll from-coll) function, adding elements from from-coll to to-coll, retaining to-coll's type.
How can this one be implemented in common lisp? The first attempt would be
(defun into (seq1 seq2)
(concatenate (type-of seq1) seq1 seq2))
but this one obviously fails, since type-of includes the vector's length in it's result, disallowing adding more elements (as of sbcl), though it still works for list as a first arg
(while still failing for empty list).
the question is: is it possible to make up this kind of function without using generic methods and/or complex type-of result processing (e.g. removing length for vectors/arrays etc) ?
i'm okay with into acting as append (in contrast with clojure, where into result depends on target collection type) Let's call it concat-into
In Clojure, you have a concrete idea (most of the time) of what kind that first collection is when you use into, because it changes the semantics: if it is a list, additional elements will be conjed onto the front, if it is a vector, they will be conjed to the back, if it is a map, you need to supply map entry designators (i. e. actual map entries or two-element vectors), sets are more flexible but also carry their own semantics. That's why I'd guess that using concatenate directly, explicitly supplying the type, is probably a good enough fit for many use cases.
Other than that, I think that it could be useful to extend this functionality (Common Lisp only has a closed set of sequence types), but for that, it seems too obviously convenient to use generic functions to ignore. It is not trivial to provide a solution that is extensible, generic, and performant.
EDIT: To summarize: no, you can't get that behaviour with clever application of one or two “built-ins”, but you can certainly write an extensible and generic solution using generic functions.