read-time feature expressions

'#+' and '#-' introduce conditional compilation: the form that follows
is kept only when the feature expression is true, and otherwise is read
and thrown away.  An expression is a feature name, or and / or / not
of them.

They are read time, not compile time.

Default features are the host's software-version, software-type and
machine-type as CHICKEN reports them, plus what --features flag adds.

tests/reader.scm

@@ -1,6 +1,14 @@
 (import (chicken port)
         reader)
 
+(define-syntax feature-test
+  (syntax-rules ()
+    ((feature-test result features string)
+     (test result
+           (parameterize ((current-features 'features))
+             (with-input-from-string string
+               (lambda () (read-raw-forms 'stdin))))))))
+
 (define-syntax reader-test
   (syntax-rules ()
     ((reader-test result string)
@@ -38,4 +46,41 @@
   (reader-test '((a b) (comment " t")) "(a b) ; t")
   ;; a `;' inside a string is not a comment
   (reader-test '("a;b") "\"a;b\"")
+
+  ;; #+ / #- feature expressions. What does not apply is read and
+  ;; dropped, so it never reaches the compiler at all
+  (feature-test '((a)) (linux) "#+linux (a)")
+  (feature-test '()    (macosx) "#+linux (a)")
+  (feature-test '()    (linux) "#-linux (a)")
+  (feature-test '((a)) (macosx) "#-linux (a)")
+  ;; the guarded datum can be anything, not only a list
+  (feature-test '(42) (x) "#+x 42")
+  (feature-test '("s") (x) "#+x \"s\"")
+
+  ;; and / or / not
+  (feature-test '((a)) (unix linux) "#+(and unix linux) (a)")
+  (feature-test '()    (unix) "#+(and unix linux) (a)")
+  (feature-test '((a)) (unix) "#+(or linux unix) (a)")
+  (feature-test '()    (bsd)  "#+(or linux unix) (a)")
+  (feature-test '((a)) (unix) "#+(and unix (not macosx)) (a)")
+  (feature-test '()    (unix macosx) "#+(and unix (not macosx)) (a)")
+  ;; (and) is true and (or) is false, as they are in CL
+  (feature-test '((a)) () "#+(and) (a)")
+  (feature-test '()    () "#+(or) (a)")
+
+  ;; a guard inside a form, including as the last element -- dropping
+  ;; continues with the next token, so the closing paren still arrives
+  (feature-test '((f 1 3)) (a) "(f #+a 1 #-a 2 3)")
+  (feature-test '((f 2 3)) (b) "(f #+a 1 #-a 2 3)")
+  (feature-test '((f 1))   (a) "(f #+a 1 #+b 2)")
+  (feature-test '((f))     (b) "(f #+a 1)")
+  ;; ...and as the last form in the file
+  (feature-test '((a)) (x) "(a) #+y (b)")
+
+  ;; guards nest
+  (feature-test '((a)) (x y) "#+x #+y (a)")
+  (feature-test '((b)) (x) "#+x #+y (a) (b)")
+
+  ;; a feature the program was not given is simply absent
+  (feature-test '() () "#+anything (a)")
 )

tests/sex-programs/features.sex

@@ -0,0 +1,23 @@
+(compilation "--features=test-on")
+(input)
+(output "selected" "on" "and-not")
+(return 0)
+
+;;; #+ and #- pick what the compiler gets to see. `test-on' is handed
+;;; to sexc by the (compilation ...) form above, so this program reads
+;;; the same way on every platform.
+
+(include stdio.h)
+
+#+test-on (define GREETING "on")
+#-test-on (define GREETING "off")
+
+#-test-on (pub fn main () int (puts "the whole function is dropped") (return 1))
+
+(pub fn main () int
+  ;; ...and inside a form, not only at toplevel
+  (puts #+test-on "selected" #-test-on "rejected")
+  (puts GREETING)
+  #+(and test-on (not test-off)) (puts "and-not")
+  #-test-on (puts "never printed")
+  (return 0))