Pretty-print Not Honoring ~:@_ Directive

[Symbolics]

To see this error:

? (setq *print-pretty* t)
? (format t "~:@_FAILURE DETAILS:~:@_~4:I================")
FAILURE DETAILS:================ NIL

As I read the CLHS, section 22.3, the ~:@_ directive is equivalent to a mandatory newline (i.e. ~%), but that’s not the case here.

http://www.lispworks.com/documentation/HyperSpec/Body/22_cea.htm

[PuercoPop]

The issue appears to be with with pprint-newline. CLHS 22.3.5.1 states that ~:@_ is the same as (pprint-newline :mandatory). The following snippet shows that pprint-newline does not insert a newline.

(setf *print-pretty* t)
(with-output-to-string (out)
  (princ "a" out)
  (pprint-newline :mandatory out)
  (princ "b" out))
;; => "ab"

This happens in SBCL, CLISP, ECL, ABCL as well so we should check that the standard indeed mandates a newline to be inserted. Althought it is certainly my impression that that is the case the following part on the CLHS entry on pprint-newline raises an eyebrow

:mandatory
This specifies a ``mandatory-style'' conditional newline.

[Symbolics]

From CLtL2:

"If kind is :mandatory, it specifies a `mandatory-style' conditional newline. A line break is always inserted. This implies that none of the containing sections can be printed on a single line and will therefore trigger the insertion of line breaks at linear-style conditional newlines in these sections."

emphasis is mine.

[3b]

From CLHS pprint-newline:
"If stream is a pretty printing stream and the value of print-pretty is true, a line break is inserted in the output when the appropriate condition below is satisfied; otherwise, pprint-newline has no effect. "

From CLHS glossary: pretty printing stream:
"pretty printing stream n. a stream that does pretty printing. Such streams are created by the function pprint-logical-block as a link between the output stream and the logical block. "

So since there is no pprint-logical-block, it is not a pretty printing stream, so pprint-newline has no effect.

(with-output-to-string (out)
  (pprint-logical-block (out nil)
    (format out "~:@_FAILURE DETAILS:~:@_~4:I================")
    (princ "a" out)
    (pprint-newline :mandatory out)
    (princ "b" out)))

;; =>

"
FAILURE DETAILS:
================a
    b"

;; or

;; I don't think an argument should be required with ~@< ~:>, but ccl complains
(format nil "~@<~:@_FAILURE DETAILS:~:@_~4:I================~:>" nil) 
;; =>
"
FAILURE DETAILS:
================"