fmt-c-writer.scm

@@ -135,9 +135,6 @@ forms, and what remains."
       (car type)
       type))
 
-(define (comment-form? f)
-  (and (pair? f) (eq? (car f) 'comment)))
-
 ;;; Strip ;s so ;;; Foo becomes /* Foo */ and not /* ;; Foo */
 (define (strip-comment-marker text)
   (string-trim-both (string-trim text #\;)))

reader.scm

@@ -115,6 +115,21 @@
      ((eq? tok dot-token) (error "Unexpected ."))
      (else tok))))
 
+;;; A token that stands for a datum
+(define (datum-token? tok)
+  (not (or (eof-object? tok)
+           (eq? tok close-paren)
+           (eq? tok close-bracket)
+           (eq? tok dot-token))))
+
+;;; next-token, sans the comments
+(define (next-code-token port)
+  (let loop ()
+    (let ((tok (next-token port)))
+      (if (comment-form? tok)
+          (loop)
+          tok))))
+
 ;;; Read list elements up to close-paren or close-bracket,
 ;;; honoring dotted-pair notation (a b . c)
 (define (read-list port closer)
@@ -208,13 +223,24 @@
    (else (error "Malformed feature expression" test))))
 
 ;;; The #-/#+ preceded datum is always read -- there is no other way
-;;; to know where it ends -- and then either returned or dropped
+;;; to know where it ends -- and then either returned or dropped. What
+;;; follows a dropped datum is read in its place: `#+x #+y (a) (b)'
+;;; with only x is (b).
+;;;
+;;; That next thing may be nothing: the end of the file, or the
+;;; paren closing the list we are in
 (define (read-conditional port keep-when)
-  (let ((keep (eq? keep-when (feature-true? (read-datum port)))))
-    (if keep
-        (read-datum port)
-        (begin (read-datum port)
-               (next-token port)))))
+  (let* ((test (next-code-token port))
+         (keep (begin
+                 (unless (datum-token? test)
+                   (error "Unexpected end of input in feature expression"))
+                 (eq? keep-when (feature-true? test))))
+         (guarded (next-code-token port)))
+    (cond
+     (keep guarded)
+     ;; A datum was dropped, so the next one stands in for it
+     ((datum-token? guarded) (next-token port))
+     (else guarded))))
 
 ;;; #t / #true / #f / #false. `val' is the boolean; consume any
 ;;; trailing name characters and validate

semen.scm

@@ -145,9 +145,6 @@
 ;;; C code it will be placed as a C commentary just before the function
 ;;; definition (actually that works for all blocky things: enum, struct, union as well).
 
-(define (comment-form? f)
-  (and (pair? f) (eq? (car f) 'comment)))
-
 (define (fn-header-length fn-form)
   (if (memq (first fn-form) '(pub extern)) 5 4))
 

tests/reader.scm

@@ -80,6 +80,22 @@
   ;; guards nest
   (feature-test '((a)) (x y) "#+x #+y (a)")
   (feature-test '((b)) (x) "#+x #+y (a) (b)")
+  ;; ...and the inner one may leave nothing behind: the end of the
+  ;; file, or the paren closing the list, is what the outer one then
+  ;; produces, and neither is an error
+  (feature-test '()      (x) "#+x #+y (a)")
+  (feature-test '((f))   (x) "(f #+x #+y 1)")
+  (feature-test '((f 2)) (x) "(f #+x #+y 1 2)")
+  (feature-test '((a))   (x) "(a) #+x #+y (b)")
+
+  ;; a comment between a guard and the form it guards describes the
+  ;; guard. Taking it for the guarded datum would leave the form itself
+  ;; unconditional
+  (feature-test '((a)) (x) "#+x ;; why\n (a)")
+  (feature-test '()    (y) "#+x ;; why\n (a)")
+  (feature-test '((b)) (y) "#+x ;; why\n (a) (b)")
+  ;; a comment after the guarded form is an ordinary form, and stays
+  (feature-test '((comment "; tail")) (y) "#+x (a) ;; tail")
 
   ;; a feature the program was not given is simply absent
   (feature-test '() () "#+anything (a)")

tools/sextest/utils.module.scm

@@ -2,6 +2,7 @@
     (get-env-var
      set-working-directory
      to-absolute-pathname
+     comment-form?
      list-split
      list-join
      recons

utils.module.scm

@@ -2,6 +2,7 @@
     (get-env-var
      set-working-directory
      to-absolute-pathname
+     comment-form?
      list-split
      list-join
      recons

utils.scm

@@ -42,6 +42,9 @@
        (current-directory)
        pathname)))
 
+(define (comment-form? form)
+  (and (pair? form) (eq? (car form) 'comment)))
+
 (define (list-split src-list split-elt)
   ;; split '(1 2 / 3 4 / 5 6) by '/ -> '((1 2) (3 4) (5 6))
   (fold (lambda (elt acc)