CHECK-ARG-TYPE

[David A. Moon <Moon at SCRC-TENEX at MIT-MC>]

See p.275 of the 29 July Common Lisp manual and p.275 of the revision
handed out at the Lisp conference.

I suggest that we include CHECK-ARG-TYPE in the language.  Although
CHECK-ARG, CHECK-ARG-TYPE, and ASSERT have partially-overlapping
functionality, each has its own valuable uses and I think all three
ought to be in the language.

Note that CHECK-ARG and CHECK-ARG-TYPE are used when you want explicit
run-time checking, including but not limited to writing the interpreter
(which of course is written in Lisp, not machine language!).

The details:
CHECK-ARG-TYPE arg-name type &OPTIONAL type-string	[macro]

If (TYPEP arg-name 'type) is false, signal an error.  The error message
includes arg-name and a "pretty" English-language form of type, which
can be overridden by specifying type-string (this override is rarely
used).  Proceeding from the error sets arg-name to a new value and
makes the test again.

Currently arg-name must be a variable, but it should be generalized to
any SETF'able place.

type and type-string are not evaluated.

This isn't always used for checking arguments, since the value of any
variable can be checked, but it is usually used for arguments and there
isn't an alternate name that more clearly describes what it does.