Look at the following dialogue:
guile> (define x '(1 2 . 3))
guile> (append x x)
ERROR: In procedure append in expression (append x x):
ERROR: Wrong type argument (expecting NULLP): 3
ABORT: (wrong-type-arg)
guile>
This error message is confusing.
1. Guile says that I have given the argument 3 to append which isn't true.
2. It is not obvious to the user that "expecting NULLP" means
"expecting the empty list".
3. Guile is *not* expecting the empty list as argument.
Problem 1 and 3 are due to misuse of an SCM_VALIDATE_XXX macro. Such
macros can only be used when the type test is applied directly to the
procedure argument.
Problem 2 is a problem with the current validate macros that we have
to solve.
I propose that we let SCM_MAKE_VALIDATE take a fourth argument which
is the expected type in plain English.
Note that sometimes might want to make several validate macros for the
same test:
#define SCM_VALIDATE_CONS(pos, scm) \
SCM_MAKE_VALIDATE (pos, scm, CONSP, "pair")
#define SCM_SLOPPY_VALIDATE_LIST(pos, scm) \
SCM_MAKE_VALIDATE (pos, scm, CONSP, "list")
