read-print: Read and render vertical space.

* guix/read-print.scm (<vertical-space>, vertical-space?)
(vertical-space, vertical-space-height): New variables.
(combine-vertical-space, canonicalize-vertical-space)
(read-vertical-space): New procedures.
(read-with-comments): Use it in the #\newline case.
(pretty-print-with-comments): Add #:format-vertical-space and honor it.
Add case for 'vertical-space?'.
* guix/scripts/style.scm (format-package-definition): Pass
 #:format-vertical-space to 'object->string*'.
* tests/read-print.scm ("read-with-comments: list with blank line")
("read-with-comments: list with multiple blank lines")
("read-with-comments: top-level blank lines")
("pretty-print-with-comments, canonicalize-vertical-space"): New tests.
Add a couple of additional round-trip tests.

guix/read-print.scm

@@ -30,6 +30,11 @@
 
             blank?
 
+            vertical-space
+            vertical-space?
+            vertical-space-height
+            canonicalize-vertical-space
+
             comment
             comment?
             comment->string
@@ -58,6 +63,26 @@
 
 (define blank? (record-predicate <blank>))
 
+(define <vertical-space>
+  (make-record-type '<vertical-space> '(height)
+                    #:parent <blank>
+                    #:extensible? #f))
+
+(define vertical-space?       (record-predicate <vertical-space>))
+(define vertical-space        (record-type-constructor <vertical-space>))
+(define vertical-space-height (record-accessor <vertical-space> 'height))
+
+(define (combine-vertical-space x y)
+  "Return vertical space as high as the combination of X and Y."
+  (vertical-space (+ (vertical-space-height x)
+                     (vertical-space-height y))))
+
+(define canonicalize-vertical-space
+  (let ((unit (vertical-space 1)))
+    (lambda (space)
+      "Return a vertical space corresponding to a single blank line."
+      unit)))
+
 (define <comment>
   ;; Comments.
   (make-record-type '<comment> '(str margin?)
@@ -80,6 +105,19 @@ end with newline, otherwise an error is raised."
             (&message (message "invalid comment string")))))
   (string->comment str margin?))
 
+(define (read-vertical-space port)
+  "Read from PORT until a non-vertical-space character is met, and return a
+single <vertical-space> record."
+  (define (space? chr)
+    (char-set-contains? char-set:whitespace chr))
+
+  (let loop ((height 1))
+    (match (read-char port)
+      (#\newline (loop (+ 1 height)))
+      ((? eof-object?) (vertical-space height))
+      ((? space?) (loop height))
+      (chr (unread-char chr port) (vertical-space height)))))
+
 (define (read-with-comments port)
   "Like 'read', but include <blank> objects when they're encountered."
   ;; Note: Instead of implementing this functionality in 'read' proper, which
@@ -107,7 +145,9 @@ end with newline, otherwise an error is raised."
        eof)                                       ;oops!
       (chr
        (cond ((eqv? chr #\newline)
-              (loop #t return))
+              (if blank-line?
+                  (read-vertical-space port)
+                  (loop #t return)))
              ((char-set-contains? char-set:whitespace chr)
               (loop blank-line? return))
              ((memv chr '(#\( #\[))
@@ -297,6 +337,7 @@ semicolons."
 (define* (pretty-print-with-comments port obj
                                      #:key
                                      (format-comment identity)
+                                     (format-vertical-space identity)
                                      (indent 0)
                                      (max-width 78)
                                      (long-list 5))
@@ -306,7 +347,8 @@ included in the output.
 
 Lists longer than LONG-LIST are written as one element per line.  Comments are
 passed through FORMAT-COMMENT before being emitted; a useful value for
-FORMAT-COMMENT is 'canonicalize-comment'."
+FORMAT-COMMENT is 'canonicalize-comment'.  Vertical space is passed through
+FORMAT-VERTICAL-SPACE; a useful value of 'canonicalize-vertical-space'."
   (define (list-of-lists? head tail)
     ;; Return true if HEAD and TAIL denote a list of lists--e.g., a list of
     ;; 'let' bindings.
@@ -394,6 +436,14 @@ FORMAT-COMMENT is 'canonicalize-comment'."
                       port)))
        (display (make-string indent #\space) port)
        indent)
+      ((? vertical-space? space)
+       (unless delimited? (newline port))
+       (let loop ((i (vertical-space-height (format-vertical-space space))))
+         (unless (zero? i)
+           (newline port)
+           (loop (- i 1))))
+       (display (make-string indent #\space) port)
+       indent)
       (('quote lst)
        (unless delimited? (display " " port))
        (display "'" port)

guix/scripts/style.scm

@@ -316,7 +316,8 @@ PACKAGE."
        (object->string* exp
                         (location-column
                          (package-definition-location package))
-                        #:format-comment canonicalize-comment)))))
+                        #:format-comment canonicalize-comment
+                        #:format-vertical-space canonicalize-vertical-space)))))
 
 (define (package-location<? p1 p2)
   "Return true if P1's location is \"before\" P2's."

tests/read-print.scm

@@ -19,7 +19,8 @@
 (define-module (tests-style)
   #:use-module (guix read-print)
   #:use-module (guix gexp)                        ;for the reader extensions
-  #:use-module (srfi srfi-64))
+  #:use-module (srfi srfi-64)
+  #:use-module (ice-9 match))
 
 (define-syntax-rule (test-pretty-print str args ...)
   "Test equality after a round-trip where STR is passed to
@@ -40,6 +41,35 @@
   (call-with-input-string "(a . b)"
     read-with-comments))
 
+(test-equal "read-with-comments: list with blank line"
+  `(list with ,(vertical-space 1) blank line)
+  (call-with-input-string "\
+(list with
+
+      blank line)\n"
+    read-with-comments))
+
+(test-equal "read-with-comments: list with multiple blank lines"
+  `(list with ,(comment ";multiple\n" #t)
+         ,(vertical-space 3) blank lines)
+  (call-with-input-string "\
+(list with ;multiple
+
+
+
+      blank lines)\n"
+    read-with-comments))
+
+(test-equal "read-with-comments: top-level blank lines"
+  (list (vertical-space 2) '(a b c) (vertical-space 2))
+  (call-with-input-string "
+
+(a b c)\n\n"
+    (lambda (port)
+      (list (read-with-comments port)
+            (read-with-comments port)
+            (read-with-comments port)))))
+
 (test-pretty-print "(list 1 2 3 4)")
 (test-pretty-print "((a . 1) (b . 2))")
 (test-pretty-print "(a b c . boom)")
@@ -181,6 +211,24 @@ mnopqrstuvwxyz.\")"
    `(cons \"--without-any-problem\"
           ,flags)))")
 
+(test-pretty-print "\
+(vertical-space one:
+
+                two:
+
+
+                three:
+
+
+
+                end)")
+
+(test-pretty-print "\
+(vertical-space one
+
+                ;; Comment after blank line.
+                two)")
+
 (test-equal "pretty-print-with-comments, canonicalize-comment"
   "\
 (list abc
@@ -206,4 +254,30 @@ mnopqrstuvwxyz.\")"
                                     #:format-comment
                                     canonicalize-comment)))))
 
+(test-equal "pretty-print-with-comments, canonicalize-vertical-space"
+  "\
+(list abc
+
+      def
+
+      ;; last one
+      ghi)"
+  (let ((sexp (call-with-input-string
+                  "\
+(list abc
+
+
+
+  def
+
+
+;; last one
+  ghi)"
+                read-with-comments)))
+    (call-with-output-string
+      (lambda (port)
+        (pretty-print-with-comments port sexp
+                                    #:format-vertical-space
+                                    canonicalize-vertical-space)))))
+
 (test-end)