Add syntax lens and exports

lenses/main.rkt

@@ -1,6 +1,10 @@
 #lang racket
 
-(require)
+(require "core.rkt"
+         "list.rkt"
+         "syntax.rkt")
 
 (provide
- (all-from-out))
+ (all-from-out "core.rkt"
+               "list.rkt"
+               "syntax.rkt"))

lenses/syntax.rkt

@@ -0,0 +1,37 @@
+#lang racket
+
+(require syntax/parse
+         (for-syntax racket/syntax
+                     syntax/stx
+                     syntax/parse))
+
+(provide syntax-lens)
+
+(define-syntax syntax-lens
+  (syntax-parser
+    [(_ target-name:id template)
+     (with-syntax* ([target ((target-stx #'target-name) #'template)]
+                    [parse-pattern (template->pattern #'template)]
+                    [rebuilder ((template-rebuilder #'target-name) #'parse-pattern)])
+       #'(syntax-parser
+           [parse-pattern
+            (values #'target rebuilder)]))]))
+
+(begin-for-syntax
+  
+  (define (target-stx target-id)
+    (syntax-parser
+      [(a ...) (ormap (target-stx target-id) (syntax->list #'(a ...)))]
+      [a (and (bound-identifier=? target-id #'a) #'a)]))
+  
+  (define template->pattern
+    (syntax-parser #:literals (_)
+      [(a ...) #`(#,@(stx-map template->pattern #'(a ...)))]
+      [_ (generate-temporary)]
+      [a #'a]))
+  
+  (define ((template-rebuilder target-id) parse-pattern)
+    (with-syntax ([pat parse-pattern])
+      #`(lambda (stx)
+          (with-syntax ([#,target-id stx])
+            #'pat)))))