lint: formatting: Reporters return #f or a warning.

* guix/lint.scm (report-tabulations, report-trailing-white-space)
(report-long-line, report-lone-parentheses): Return #f instead
of *unspecified* when there are no warnings.
(report-formatting-issues): Use 'filter-map' instead of 'map' + 'filter'.

guix/lint.scm

@@ -1031,7 +1031,7 @@ the NIST server non-fatal."
 (define (report-tabulations package line line-number)
   "Warn about tabulations found in LINE."
   (match (string-index line #\tab)
-    (#f #t)
+    (#f #f)
     (index
      (make-warning package
                    (G_ "tabulation on line ~a, column ~a")
@@ -1043,8 +1043,8 @@ the NIST server non-fatal."
 
 (define (report-trailing-white-space package line line-number)
   "Warn about trailing white space in LINE."
-  (unless (or (string=? line (string-trim-right line))
-              (string=? line (string #\page)))
+  (and (not (or (string=? line (string-trim-right line))
+                (string=? line (string #\page))))
        (make-warning package
                      (G_ "trailing white space on line ~a")
                      (list line-number)
@@ -1058,7 +1058,7 @@ the NIST server non-fatal."
   ;; Note: We don't warn at 80 characters because sometimes hashes and URLs
   ;; make it hard to fit within that limit and we want to avoid making too
   ;; much noise.
-  (when (> (string-length line) 90)
+  (and (> (string-length line) 90)
        (make-warning package
                      (G_ "line ~a is way too long (~a characters)")
                      (list line-number (string-length line))
@@ -1072,7 +1072,7 @@ the NIST server non-fatal."
 
 (define (report-lone-parentheses package line line-number)
   "Emit a warning if LINE contains hanging parentheses."
-  (when (regexp-exec %hanging-paren-rx line)
+  (and (regexp-exec %hanging-paren-rx line)
        (make-warning package
                      (G_ "parentheses feel lonely, \
 move to the previous or next line")
@@ -1130,11 +1130,9 @@ them for PACKAGE."
                          warnings
                          (if (< line-number starting-line)
                              '()
-                             (filter
-                              lint-warning?
-                              (map (lambda (report)
+                             (filter-map (lambda (report)
                                            (report package line line-number))
-                                   reporters))))))))))))
+                                         reporters)))))))))))
 
 (define (check-formatting package)
   "Check the formatting of the source code of PACKAGE."