Support for pprint of backquote forms, adapted from SBCL

[alanruttenberg]

(pprint '`(b ,a)) -> `(b ,a)

Instead of

(system::backq-list 'b a)

[easye]

This commit fails five more ANSI-TESTS--namely PRINT.BACKQUOTE.RANDOM.6
PRINT.BACKQUOTE.RANDOM.7, PRINT.BACKQUOTE.RANDOM.8, PRINT.BACKQUOTE.RANDOM.9, and PRINT.BACKQUOTE.RANDOM.12--which should be investigated.

[alanruttenberg]

If you look at .6 it's testing '`(,@A ,@b). But if you look at the output it is actually testing (system::backq-append) i.e it's lost the splices. The output is a bunch of printer bindings which the printing is tested under. If you bind those variables and pprint '`(,@A ,@b) it prints fine.

I think the problem is in the tests. The code for those tests reads:

(randomly-check-readability (list (car x)) :test #'is-similar)

but should be

(randomly-check-readability x :test #'is-similar)

By doing that the test is effectively for nil. But `nil is read a nil because we don't have a quasiquote operator, and the reader is even dropping the quote - it does that for any self-evaluating form.

I submitted an issue at the ansi-test repo

[alanruttenberg]

Oh, and those tests fail even without the pull