add support for firing up a REPL to poke at DrRacket

collects/drracket/drracket.rkt

@@ -6,6 +6,8 @@
 
 (define first-parallel? (getenv "PLTDRPAR"))
 
+(define repl (getenv "PLTDRREPL"))
+
 (define install-cm? (and (not debugging?)
                          (getenv "PLTDRCM")))
 
@@ -149,3 +151,14 @@
     ((dynamic-require 'drracket/private/profile-drs 'start-profile) orig-cust)))
 
 (dynamic-require 'drracket/private/drracket-normal #f)
+
+(when repl
+  (printf "Welcome to DrRacket, v~a\n" (version))
+  (namespace-require 'racket)
+  (namespace-require 'drracket/tool-lib)
+  (unless (equal? repl "-q")
+    (define init-file (find-system-path 'init-file))
+    (when (file-exists? init-file)
+      (load init-file)))
+  (void (thread read-eval-print-loop)))
+

collects/scribblings/drracket/extending.scrbl

@@ -120,6 +120,19 @@ Several environment variables can affect DrRacket's behavior:
        those collections are loaded (The contents of the environment
        variable are @racket[read] and expected to be a single symbol
        or a list of symbols).}
+ 
+ @item{@indexed-envvar{PLTDRREPL} : When this environment variable is
+        set, DrRacket starts a read-eval-print loop with all of
+        the forms and functions from the @racketmodname[racket]
+        module and those described in @other-doc['(lib "scribblings/tools/tools.scrbl")].
+        
+        If it is not set to @litchar{-q}, then @racket[(find-system-path 'init-file)]
+        is loaded as well.
+        
+        A new thread is created to evaluate REPL expressions, so be 
+        sure to use @racket[queue-callback] to evaluate expressions
+        that mutate the GUI (to avoid race-conditions).
+        }
 
  @item{@indexed-envvar{PLTDRCM} : When this environment variable is
        set, DrRacket installs the compilation manager before starting