Applicative programming and common lisp types.

Question

I've just started learning Common Lisp--and rapidly falling in love with it--and I've just moved onto the type system. I seem to be developing a particular fondness for applicative programming.

As I understand it, in CL strings and lists are both sequences, but there don't seem to be any standard functions for mapping over a sequence, only lists. I can see why they would be supplied for lists, what with them being the fundamental datatype and all, but why was it not designed to work with sequences? As they are a more general type, it would seem more useful to target applicative functions at them rather than lists. Or am I completely misunderstandimatifying how it works?

Edit:

What I was feeling particularly confused about was the way that sequences -- the abstraction -- and lists -- an implementation -- seem to be muddled up in CL. The consensus seems to be that this is for historical reasons; lisp has been around so long that you can pretty much map out the development of software engineering practices through its functions and macros; which functions apply to sequences and which to lists seems arbitrary at first glance because CL has a mixture of pre-sequence-abstraction functions that operate only on lists, and functions that do the same thing in a more general way on sequences. As someone who is just learning CL at the moment, I think it would be useful if authors introduced sequences first as the cleaner abstraction, and then bought in lists as the most fundamental implementation of that abstraction. Lists would still be needed as syntax of course, but by the time it is necessary to state this explicitly many readers would have worked this out by themselves, which would be quite an ego boost when starting out.

Answer 1

Well, you are generally correct. Most functions do indeed focus on lists (mapcar, find, count, remove, append etc.) For a few of these there are equivalent functions for sequences (concatenate, some and every come to mind), and some, where the list-equivalent is outdated (eg. nth for lists only vs. elt for all sequences). Some functions simply work on sequences (length, for example).

CL is a bit of a mess. It's a big language, as in huge. Over 700 functions, AFAIK. And it's old. Some of these functions are deprecated by convention, and others are rarely, if ever, used.

Yes, it would be more useful to have mapping functions be methods, that applied as intended on all sequences. CL was simply not built that way. If it were to be built again today, I'm sure this would be considered, and it would look very different.

That said, you are not left completely in the cold. The loop macro works on sequences, as does iterate (a separate looping macro, which i happen to like more). This will get you far. For most practical purposes you will be using lists, and this won't be more than a pragmatic problem. If you do happen to lack a mapping function for vectors (or sequences in general), who's to stop you from writing it?

Answer 2

Why, there are a lot of functions working on sequences. Mapping over a sequence is done with MAP or MAP-INTO.

Look at the sequences section of the CLHS to find out more.

There is also a quick reference that is nicely organized.