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.

Makefile

@@ -61,7 +61,7 @@ sextest:
 	$(MAKE) -C ./tools/sextest sextest
 	cp ./tools/sextest/sextest .
 
-SEX_TEST_PROGRAMS = hello-world lists comments unicode serialize
+SEX_TEST_PROGRAMS = hello-world lists comments unicode serialize features
 
 # Multi-module linking is checked end to end; see tests/modules/Makefile.
 check-modules: sexc

Readme.org

@@ -31,13 +31,21 @@ Options:
  --c-compiler=ARG         Select C compiler. Defaults to value of SEX_CC
                           environment variable, or if it is empty, to cc
  -c, --compile-object     Compile object file instead of executable program
- -C, --preprocess         Emit C code
+ -f, --features=ARG       Comma-separated feature names, added to the host's own
+                          for #+ and #- feature expressions. May be given
+                          more than once
+ --no-platform-features   Leave out the host's own features. With --features,
+                          this reads a file the way another platform would
+ -C, --emit-c             Emit C code
  --public-interface       Get module's public interface
  -h, --help               Show this help
  -m, --macro-expand       Emit macro-expanded semantically processed Sex code
-                          (sort of IR). May be useful for debugging
  -o, --output=ARG         Write output to file. Default file name is a.out.
                           If -E or -m options are provided, defaults to stdout
+ --line-directives=ARG    How much #line information to emit: statement (default),
+                          toplevel, or none. `statement' is what makes a debugger
+                          land on the right source line; `none' is for reading -C
+                          output by eye
 #+end_src
 ** Compiling Hello World
 #+begin_src shell
@@ -101,6 +109,39 @@ Module's public interface consists of everything declared
 ~pub~. Structures, function, macros, types, variables can be
 public.
 
+** Read-time feature expressions
+Sex is able to use ~#+~ and ~#-~ for conditional compilation: the form that
+follows is kept only when the feature expression is true, and otherwise
+is read and thrown away.
+
+#+begin_src scheme
+#+macosx (include OpenGL/gl3.h)
+#-macosx (include GL/gl.h)
+
+#+(and unix (not macosx)) (define HAVE-EPOLL 1)
+#+end_src
+
+An expression is a feature name, or ~and~, ~or~ and ~not~ of them.
+
+This is read time, not compile time. What does not apply never reaches macro
+expansion, the type database or the generated C.
+
+The features are the host's ~(software-version)~, ~(software-type)~
+and ~(machine-type)~, e.g. ~macosx unix arm64~ or ~linux unix
+x86-64~. ~--features~ adds to them:
+
+#+begin_src shell
+sexc prog.sex -f debug,with-sdl
+sexc prog.sex --features=debug --features=with-sdl
+#+end_src
+
+A feature is never taken away. The host's features can be disabled,
+e.g. for checking output for other platform:
+
+#+begin_src shell
+sexc example/sdl3-triangle.sex -C --no-platform-features --features=linux,unix,x86-64
+#+end_src
+
 ** Syntactic macros
 Sex has support for syntactic macros. Macro definitions look like
 functions: they have a name, an argument list and a body. Macro should

reader.module.scm

@@ -1,3 +1,6 @@
 (module reader (read-from-file
-                read-raw-forms)
+                read-raw-forms
+
+                current-features
+                platform-features)
   "reader.scm")

reader.scm

@@ -7,6 +7,8 @@
 ;;;   - a leading `.' rewritten to the symbol `dot-access'
 ;;;   - `;' comments preserved as (comment "...") forms, so they can be
 ;;;     re-emitted into the generated C (keeping the source mapping)
+;;;   - #+ / #- feature expressions, which decide at read time what the
+;;;     compiler gets to see at all
 ;;; It also records the source location of every form it reads (see
 ;;; utils' form-source), so the C writer can emit #line directives.
 
@@ -15,6 +17,8 @@
   (scheme base)                         ; make-parameter
   (chicken base)
   (chicken pathname)
+  (chicken platform)                    ; software-version, machine-type
+  (only srfi-1 every any)               ; srfi-1 also has an append-reverse
   utils)
 
 ;;; Sentinels for structural tokens
@@ -160,7 +164,8 @@
    ((string->number s) => identity)
    (else (string->symbol s))))
 
-;;; #-dispatch: booleans, characters, vectors, block/datum comments
+;;; #-dispatch: booleans, characters, vectors, block/datum comments,
+;;; feature expressions
 (define (read-hash port)
   (let ((c (get-ch port)))
     (cond
@@ -171,8 +176,46 @@
      ((char=? c #\() (list->vector (read-list port close-paren)))
      ((char=? c #\|) (skip-block-comment port 1) (next-token port))
      ((char=? c #\;) (read-datum port) (next-token port)) ; datum comment
+     ((char=? c #\+) (read-conditional port #t))
+     ((char=? c #\-) (read-conditional port #f))
      (else (error "Unsupported # syntax" c)))))
 
+;;; Feature expressions
+;;;
+;;;   #+linux   (include GL/gl.h)      kept on Linux
+;;;   #-macosx  (foo)                  kept only on other than macOS
+;;;   #+(and unix (not macosx)) (bar)  and / or / not combine them
+;;;
+(define (platform-features)
+  (list (software-version) (software-type) (machine-type)))
+
+;;; The host's features are the default, so anything reading Sex sees
+;;; what the compiler would. sexc rebinds this to add --features
+(define current-features (make-parameter (platform-features)))
+
+(define (feature-true? test)
+  (cond
+   ((symbol? test) (and (memq test (current-features)) #t))
+   ((pair? test)
+    (case (car test)
+      ((and) (every feature-true? (cdr test)))
+      ((or) (any feature-true? (cdr test)))
+      ((not)
+       (if (and (pair? (cdr test)) (null? (cddr test)))
+           (not (feature-true? (cadr test)))
+           (error "Feature expression `not' takes exactly one operand" test)))
+      (else (error "Unknown operator in feature expression" (car test)))))
+   (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
+(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)))))
+
 ;;; #t / #true / #f / #false. `val' is the boolean; consume any
 ;;; trailing name characters and validate
 (define (read-bool port val)

sexc.scm

@@ -9,6 +9,7 @@
         (chicken process)
         (chicken process-context)
         (chicken port)
+        (chicken string)                ; string-split
         fmt
         fmt-c-writer
         getopt-long
@@ -31,6 +32,17 @@
                       (required #f)
                       (value #f)
                       (single-char #\c))
+      (features ,(fmt #f "Comma-separated feature names, added to the host's own" nl
+                      (pad padding) "for #+ and #- feature expressions. May be given" nl
+                      (pad padding) "more than once")
+                (required #f)
+                (value #t)
+                (single-char #\f))
+      (no-platform-features
+       ,(fmt #f "Leave out the host's own features. With --features," nl
+             (pad padding) "this reads a file the way another platform would")
+       (required #f)
+       (value #f))
       (emit-c "Emit C code"
               (required #f)
               (value #f)
@@ -98,6 +110,15 @@
           ((equal? v "none") 'none)
           (else (error "--line-directives must be statement, toplevel or none, got" v)))))
 
+
+
+;;; --features may be given more than once, and each may name several.
+;;; Collect all of them
+(define (cli-features args)
+  (append-map (lambda (entry)
+                (map string->symbol (string-split (cdr entry) ",")))
+              (filter (lambda (entry) (eq? (car entry) 'features)) args)))
+
 (define (get-input-file args)
   (let ((rest-args (get-rest-args args)))
     (if (null? rest-args)
@@ -181,6 +202,14 @@ status, which is ours to pass on."
        (when help
          (print-help)
          (return #f))
+       ;; Read time comes before everything, so the features have to be
+       ;; in place before the first form is read
+       (current-features
+        (append (if (get-arg args 'no-platform-features #f)
+                    (list)
+                    (platform-features))
+                (cli-features args)))
+
        (when (get-arg args 'public-interface #f)
          (assert (not (eq? input 'stdin)) "Error: --public-interface requires file argument")
 

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))