Re: Beginner - Function Critique

crystal1 wrote
> These functions have been created a thousand times and are
> trivial, but I wrote these instances myself and am a
> pretty proud beginner. What do you think?

I think you are doing great. With such spirit and playfulness
your explorations will soon discover all of Common Lisp

I particularly liked the idea of using getf to extract a named
return value from amongst several. Other posters have shown
you the right way to return multiple values with values and
multiple-value-thing but in a spirit of play have a look at
this kitten-code. The idea is that

(:string "foo" :remainder "bar")

looks like a keyword argument list, so let us define a
function with keyword arguments and apply it to the list.
Then the keyword processing is doing the getf's for us.

(defun line-split (s d)
(if (= (length s) 0)
'()
(apply #'construct d (helper s d))))

(defun construct (d &key string remainder)
(cons string
(line-split remainder d)))

(defun helper (s d)
(let ((split (position d s)))
(if split
(list :string (subseq s 0 split)
:remainder (subseq s (+ split 1)))
(list :string s))))

(line-split "will florence ever get to kiss winston? " #\space)
=> ("will" "florence" "ever" "get" "to" "kiss" "winston?")

This code only works because keyword arguments default to
nil, and nil is a list of length zero, so
(= (length nil) 0) => t

The proper thing to do is to give remainder an explict
default of the string with no characters

(defun construct (d &key string (remainder "")) ... )

Then your excellent check-type's will be satisfied.

Alan Crowe
Edinburgh
Scotland
.