I'm running into an odd issue with Common Lisp's format directive that only shows itself in GNU CLISP, which leads me to wonder if it's a bug in the implementation.
Consider the following code
(defun f (s args)
(format nil "~A~{~A~%~}" s args))
The function f produces a string consisting of the s argument, followed by each argument, where each argument (but not the header) is followed by a newline. So, for instance, I would expect
(format t "~A" (f "foo" '("arg1" "arg2")))
to produce
fooarg1
arg2
which it does, correctly. Now, consider the following calls
(format t "~A" (f "a" ()))
(format t "~A" (f "b" ()))
(format t "~A" (f "c" '("d")))
Given that the only newlines I ever print out are following elements from the second argument of f, and the first two calls pass an empty list for the second f argument, I expect to see no newlines until the very end, i.e. I expect this to print
abcd
It does exactly this in SBCL. However, in GNU CLISP, I get
ab
cd
Note the newline between b and c.
Changing the code to
(format t "~S" (f "a" ()))
(format t "~S" (f "b" ()))
(format t "~S" (f "c" '("d")))
shows the following, even less illuminating, results
"a""b"
"cd
"
So the newline between b and c is not part of either of the strings. Why is CLISP deciding to print a newline in between these format statements? And how can I prevent it from doing so?
This is a feature of GNU CLISP. See the documentation of *pprint-first-newline*.
If you want a different output than that, bind either *print-pretty* or *pprint-first-newline* to nil during your format invocation.