From 6a3d4c5c15c728aa635df164ee92bacfed3a60f9 Mon Sep 17 00:00:00 2001
From: Matthew Butterick <mb@mbtype.com>
Date: Fri, 6 May 2016 22:18:41 -0700
Subject: [PATCH] syntaxing

---
 beautiful-racket-lib/br/syntax.rkt | 91 +++++++++++++++++++++++++++++-
 1 file changed, 89 insertions(+), 2 deletions(-)

diff --git a/beautiful-racket-lib/br/syntax.rkt b/beautiful-racket-lib/br/syntax.rkt
index b94d993..b4fcad4 100644
--- a/beautiful-racket-lib/br/syntax.rkt
+++ b/beautiful-racket-lib/br/syntax.rkt
@@ -1,5 +1,5 @@
 #lang racket/base
-(require (for-syntax racket/base syntax/parse) syntax/strip-context)
+(require (for-syntax racket/base syntax/parse racket/syntax) syntax/strip-context)
 (provide (all-defined-out) (all-from-out syntax/strip-context))
 
 
@@ -31,4 +31,91 @@
                        <arg>) ...)]))
 
 
-#;(define-syntax syntax-variable (make-rename-transformer #'format-id))
\ No newline at end of file
+#;(define-syntax syntax-variable (make-rename-transformer #'format-id))
+
+(define (context stx)
+  (hash-ref (syntax-debug-info stx) 'context))
+
+(define-syntax-rule (scopes stx)
+  (format "~a = ~a" 'stx
+          (cons (syntax->datum stx)
+                (for/list ([scope (in-list (context stx))])
+                          scope))))
+
+(define (->syntax x)
+  (if (syntax? x) x (datum->syntax #f x)))
+
+(define-syntax (define-scope stx)
+  (syntax-case stx ()
+    [(_ id)
+     #'(define-scope id #f)]
+    [(_ id sis)
+     (with-syntax ([id-si (format-id #'id "~a-si" #'id)]
+                   [add-id (format-id #'id "add-~a" #'id)]
+                   [flip-id (format-id #'id "flip-~a" #'id)]
+                   [remove-id (format-id #'id "remove-~a" #'id)]
+                   [id? (format-id #'id "~a?" #'id)]
+                   [id* (format-id #'id "~a*" #'id)])
+       #'(begin
+           (define id-si (or sis (make-syntax-introducer 'use-site)))
+           (define add-id (λ(x) (id-si (->syntax x) 'add)))
+           (define flip-id (λ(x) (id-si (->syntax x) 'flip)))
+           (define remove-id (λ(x) (id-si (->syntax x) 'remove)))
+           (define (id x) (add-id (datum->syntax #f (syntax-e (->syntax x)))))
+           (define (id* x) (replace-context (id-si (datum->syntax #f '_)) (->syntax x)))
+           (define (id? x)
+             (and
+              (member (car (context (id-si (datum->syntax #f '_))))
+                      (context (->syntax x)))
+              #t))))]))
+
+
+(define (scopes-equal? stxl stxr)
+  ;; "A bound-identifier=? comparison checks that two identifiers have exactly the same scope sets"
+  (bound-identifier=? (datum->syntax stxl '_) (datum->syntax stxr '_)))
+
+
+(module+ test
+  (require rackunit)
+  (define-scope red)
+  
+  (define stx (datum->syntax #f 'x))
+  (define red-stx (add-red stx))
+  (define double-red-stx (add-red (add-red stx)))
+  
+  (check-false (red? stx))
+  (check-true (red? red-stx))
+  (check-true (red? double-red-stx))
+  (check-false (scopes-equal? stx red-stx))
+  (check-true (scopes-equal? red-stx double-red-stx))
+  (check-false (scopes-equal? red-stx (remove-red double-red-stx)))
+  
+  
+  (define-scope blue) ; scope addition is commutative
+  (define blue-stx (blue stx))
+  (check-true (scopes-equal? (add-blue red-stx) (add-red blue-stx)))
+  (check-true (scopes-equal? (remove-red (add-blue red-stx)) (remove-red (add-red blue-stx))))
+  
+  
+  (define-scope green) ; replace scopes at outer layer
+  (check-true (scopes-equal? (green red-stx) (green blue-stx)))
+  
+  ;; replace scopes everywhere
+  (check-true (scopes-equal? (car (syntax->list (green* #`(#,blue-stx #,red-stx))))
+                             (car (syntax->list (green* #`(#,red-stx #,blue-stx))))))
+  
+  ;; todo: test flipping
+  
+  ;; todo: scope composition
+  #;(define-scope purple (red blue))
+  
+  #;(check-true (purple? (add-purple stx)))
+  #;(scopes-equal? (purple stx) (blue (red stx)))
+  
+  )
+
+(define-syntax (define-with-scopes stx)
+  (syntax-case stx ()
+    [(_ id (scope-id) val)
+     #'(define-syntax id (λ(stx) (scope-id (syntax val))))]))
+