improve features semantics

fontland/fontland/font.rkt

@@ -74,18 +74,21 @@ https://github.com/mbutterick/fontkit/blob/master/src/TTFFont.js
 ;; ram cache in pitfall suffices
 
 (define (layout font str
-                [features null]
-                [script 'HB_SCRIPT_LATIN]
-                [lang #"en"]
-                [direction 'HB_DIRECTION_LTR])
+                #:features [features null]
+                #:script [script #f]
+                #:language [lang #f]
+                #:direction [direction #f])
   (define buf (hb-buf font))
   (hb_buffer_reset buf)
-  (hb_buffer_set_script buf script)
-  (hb_buffer_set_language buf (hb_language_from_string lang))
-  (hb_buffer_set_direction buf direction)
+  (when script
+    (hb_buffer_set_script buf script))
+  (when lang
+    (hb_buffer_set_language buf (hb_language_from_string lang)))
+  (when direction
+    (hb_buffer_set_direction buf direction))
   (define codepoints (for/list ([c (in-string str)]) (char->integer c)))
   (hb_buffer_add_codepoints buf codepoints)
-  (hb_shape (hb-font font) buf (map tag->hb-feature features))
+  (hb_shape (hb-font font) buf (map (λ (fpr) (tag->hb-feature (car fpr) (cdr fpr))) features))
   (define gis (hb_buffer_get_glyph_infos buf))
   (define hb-gids (map hb_glyph_info_t-codepoint gis))
   (define hb-clusters (break-at codepoints (map hb_glyph_info_t-cluster gis)))

fontland/fontland/unsafe/harfbuzz.rkt

@@ -182,17 +182,17 @@
 
 (define HB_FEATURE_GLOBAL_START 0)
 (define HB_FEATURE_GLOBAL_END 4294967295)
-(define (tag->hb-feature bs)
+(define (tag->hb-feature bs [val 1])
   (unless (and (bytes? bs) (= (bytes-length bs) 4))
     (raise-argument-error 'tag->hb-feature "4 bytes" bs))
-  (hb_feature_from_string bs))
+  (make-hb_feature_t (->tag bs) val HB_FEATURE_GLOBAL_START HB_FEATURE_GLOBAL_END))
   
 (define liga_on (tag->hb-feature #"liga"))
-(define liga_off (make-hb_feature_t (->tag #"liga") 0 HB_FEATURE_GLOBAL_START HB_FEATURE_GLOBAL_END))
+(define liga_off (tag->hb-feature #"liga" 0))
 (define kern_on (tag->hb-feature #"kern"))
-(define kern_off (make-hb_feature_t (->tag #"kern") 0 HB_FEATURE_GLOBAL_START HB_FEATURE_GLOBAL_END))
+(define kern_off (tag->hb-feature #"kern" 0))
 (define onum_on (tag->hb-feature #"onum"))
-(define onum_off (make-hb_feature_t (->tag #"onum") 0 HB_FEATURE_GLOBAL_START HB_FEATURE_GLOBAL_END))
+(define onum_off (tag->hb-feature #"onum" 0))
 
 (define (shape font text [feats null])
   (define buf (hb_buffer_create))