Improve interface for `find-mutated-vars', now supports mutable *and* immutable versions.

collects/unstable/mutated-vars.rkt

@@ -1,17 +1,17 @@
 #lang racket/base
 
 (require (for-template racket/base) racket/dict
-         racket/trace
          syntax/id-table syntax/kerncase)
 
-;; samth : this should use sets, not dicts
-;; but sets do not have extensible comparisons
-;; shouldn't be promoted until this is fixed
-
 ;; find and add to mapping all the set!'ed variables in form
-;; syntax -> table
-(define (find-mutated-vars form)
-  (let loop ([stx form] [tbl (make-immutable-free-id-table)])
+;; if the supplied mapping is mutable, mutates it
+;; default is immutability
+;; syntax [table] -> table
+(define (find-mutated-vars form [tbl (make-immutable-free-id-table)])
+  (define add (if (dict-mutable? tbl)
+                  (lambda (t i) (dict-set! t i #t) t)
+                  (lambda (t i) (dict-set t i #t))))
+  (let loop ([stx form] [tbl tbl])
     ;; syntax-list -> table
     (define (fmv/list lstx)
       (for/fold ([tbl tbl])
@@ -20,7 +20,7 @@
     (kernel-syntax-case* stx #f (#%top-interaction)
       ;; what we care about: set!
       [(set! v e)
-       (dict-set (loop #'e tbl) #'v #t)]
+       (add (loop #'e tbl) #'v)]
       ;; forms with expression subforms
       [(define-values (var ...) expr)
        (loop #'expr tbl)]

collects/unstable/scribblings/mutated-vars.scrbl

@@ -4,28 +4,35 @@
           (for-label unstable/mutated-vars
                      racket/contract
                      racket/dict
+                     syntax/id-table
                      racket/base))
 
 @title[#:tag "mutated-vars"]{Finding Mutated Variables}
 
 @(define the-eval (make-base-eval))
-@(the-eval '(require unstable/mutated-vars racket/dict))
+@(the-eval '(require unstable/mutated-vars syntax/id-table racket/dict))
 
 @defmodule[unstable/mutated-vars]
 
 @unstable[@author+email["Sam Tobin-Hochstadt" "samth@ccs.neu.edu"]]
 
 
-@defproc[(find-mutated-vars [stx syntax?]) dict?]{Traverses
+@defproc[(find-mutated-vars [stx syntax?] [dict dict? (make-immutable-free-id-table)]) dict?]{Traverses
 @racket[stx], which should be @racket[module-level-form] in the sense
 of the grammar for 
 @tech[#:doc '(lib "scribblings/reference/reference.scrbl")]{fully-expanded} forms,
-and records all of the variables that are mutated.  The result is a
-dictionary that maps each mutated identifier to @racket[#t].} 
+and records all of the variables that are mutated.
+Each mutated variable is added to @racket[dict], mapped to
+@racket[#t].  If @racket[dict] is mutable, as determined by
+@racket[dict-mutable?], then the table is updated destructively.
+Otherwise, the table is updated functionally.} 
 
 @examples[#:eval the-eval
 (define t (find-mutated-vars #'(begin (set! var 'foo) 'bar)))
 (dict-ref t #'var #f)
 (dict-ref t #'other-var #f)
+(define tbl (make-free-id-table))
+(find-mutated-vars #'(begin (set! var 'foo) 'bar) tbl)
+(dict-ref tbl #'var #f)
 ]
 }