Re: Nil is a sequence

[Taiichi Yuasa <yuasa%tutics.tut.junet%utokyo-relay.csnet@RELAY.CS.NET>]

	From: Quiroz Gonzalez
	Subject: Nil is a sequence

	This is a nit discovered due to a typo:

		>#1a()

		Error: NIL is not a sequence.
		Error signalled by SYSTEM:SHARP-A-READER.

	I am reasonable sure (upon careful exegesis of p.357) that my typo
	should have produced a vector of size zero.  [If not persuaded,
	consider (make-array 0 :initial-contents ())]

	Even if this interpretation is not right, the error message lies.

	= Cesar

There were several bugs related to this error.

1. There was a bug in the definition of the Lisp function SHARP-A-READER
in file lsp/iolib.lsp.  To fix the bug, replace the ELT expression

	(elt ic 0)
with
	(if (zerop (length ic)) ic (elt ic 0))

2. There was a bug in the definition of the C function elt
in file c/sequence.d.  To fix the bug, add the following line (marked =>)
in the definition.

	default:
=>		if (seq == Cnil) goto E;
		FEerror("~S is not a sequence.", 1, seq);

Incidentally, I also found there was a similar bug in the definition of
the C function elt_set in file c/sequence.d.  To fix the bug, add the
following line (marked =>) in the definition.

	default:
=>		if (seq == Cnil) goto E;
		FEerror("~S is not a sequence.", 1, seq);

3. There was a bug in the definition of the Lisp function MAKE-ARRAY
in file lsp/arraylib.lsp.  To fix the bug, surround the LET expression

	(defun make-array (...)
	  (when ...)
	  (cond ( ... )
	        (t
		 (let (...) (when ...) (when ...) x)
		 )))

with the following UNLESS expression.

	(defun make-array (...)
	  (when ...)
	  (cond ( ... )
	        (t
=>		 (unless (member 0 dimensions)
		   (let (...) (when ...) (when ...) x)
=>	           )
		 )))

-- Taiichi