I’ve written a few posts about features of the Lisp language that a newcomer arriving from C++ might not have encountered.  I’m going through them alphabetically, so here is a summary page of the functions beginning with the letter R:

random

read-preserving-whitespace

read-delimited-list

read-from-string

read-line

readtable-case

reduce

reinitialize-instance

remove-method

remprop

restart-bind

restart-case

rotatef

row-major-aref

Next on our list is the shadow function.  This function affects symbols in packages, so if those concepts are unclear to you you might want to read this earlier article.  This function interns a new symbol with the supplied name in the given package if that symbol doesn’t already exist in that package directly (i.e. not through inheritance).  If the symbol exists through inheritance alone, the symbol interned by shadow has the effect of shadowing the inherited symbol in the package.

The function set-syntax-from-char allows the programmer to copy a read-macro from one character in a particular readtable and duplicate it on possibly a different character in another readtable.  To understand what this means, you may want to review the earlier discussion of readtables.

Here is an example.  It’s not something I would recommend doing in real code, it’s just here for demonstration purposes.  If I want to write some code which contains a lot of strings with embedded double-quote marks, I might be tempted to come up with a new string delimiter to use in that case.  In this code, I copy the meaning of ” into the underscore.  I can then write my code using double-quotes in some places, and underscores in others.  A string that begins with an underscore can only be ended with another underscore, not the double-quote, so this allows me to embed double-quote characters easily in a string.

Naturally, this novel string syntax would break at load time, so we have to use eval-when to modify the readtable before forms using this syntax are read.  Also, note that the colorization is wrong in the code, because SLIME doesn’t know about this new string format, and thinks that I have an unterminated string in my code.  Here’s the code:

(eval-when (:compile-toplevel :load-toplevel :execute)
  (set-syntax-from-char #\_ #\"))

(defun demonstrate ()
  (format t "Here is a string: ~S~%" "abcd")
  (format t "Here is another string: ~S~%" _ab"cd_))

CL-USER> (demonstrate)
Here is a string: "abcd"
Here is another string: "ab\"cd"
NIL

Next in our review is set-pprint-dispatch.  We’ve already covered this in two earlier articles, pprint-dispatch and pprint-logical-block.  Please refer to those articles for explanations and examples.