#| -*-Scheme-*-

$Id: rep.scm,v 14.68 2007/01/05 21:19:28 cph Exp $

Copyright (C) 1986, 1987, 1988, 1989, 1990, 1991, 1992, 1993, 1994,
    1995, 1996, 1997, 1998, 1999, 2000, 2001, 2002, 2003, 2004, 2005,
    2006, 2007 Massachusetts Institute of Technology

This file is part of MIT/GNU Scheme.

MIT/GNU Scheme is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 2 of the License, or (at
your option) any later version.

MIT/GNU Scheme is distributed in the hope that it will be useful, but
WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

You should have received a copy of the GNU General Public License
along with MIT/GNU Scheme; if not, write to the Free Software
Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301,
USA.

|#

;;;; Read-Eval-Print Loop
;;; package: (runtime rep)

(declare (usual-integrations))

(define repl:allow-restart-notifications? #t)
(define repl:write-result-hash-numbers? #t)

(define (initialize-package!)
  (set! *nearest-cmdl* #f)
  (set! hook/repl-read default/repl-read)
  (set! hook/repl-eval default/repl-eval)
  (set! hook/repl-write default/repl-write)
  (set! hook/set-default-environment default/set-default-environment)
  (set! hook/error-decision #f)
  (initialize-breakpoint-condition!)
  unspecific)

(define (initial-top-level-repl)
  (call-with-current-continuation
   (lambda (continuation)
     (set! root-continuation continuation)
     (repl/start (make-repl #f
			    console-i/o-port
			    user-initial-environment
			    #f
			    `((SET-DEFAULT-DIRECTORY
			       ,top-level-repl/set-default-directory))
			    user-initial-prompt)
		 (cmdl-message/strings "Cold load finished")))))

(define root-continuation)

(define (top-level-repl/set-default-directory cmdl pathname)
  cmdl
  ((ucode-primitive set-working-directory-pathname! 1)
   (->namestring pathname)))

;;;; Command Loops

(define-record-type <cmdl>
    (%make-cmdl level parent port driver state operations properties)
    cmdl?
  (level cmdl/level)
  (parent cmdl/parent)
  (port cmdl/port set-cmdl/port!)
  (driver cmdl/driver)
  (state cmdl/state set-cmdl/state!)
  (operations cmdl/operations)
  (properties cmdl/properties))

(define-guarantee cmdl "command loop")

(define (make-cmdl parent port driver state operations)
  (if (not (or (not parent) (cmdl? parent)))
      (error:wrong-type-argument parent "cmdl" 'MAKE-CMDL))
  (if (not (or parent port))
      (error:bad-range-argument port 'MAKE-CMDL))
  (%make-cmdl (if parent (+ (cmdl/level parent) 1) 1)
	      parent
	      (or port (and parent (cmdl/child-port parent)))
	      driver
	      state
	      (parse-operations-list operations 'MAKE-CMDL)
	      (make-1d-table)))

(define (cmdl/child-port cmdl)
  (or (let ((operation (cmdl/local-operation cmdl 'CHILD-PORT)))
	(and operation
	     (operation cmdl)))
      (cmdl/port cmdl)))

(define (push-cmdl driver state operations)
  (make-cmdl (nearest-cmdl) #f driver state operations))

(define (cmdl/base cmdl)
  (let ((parent (cmdl/parent cmdl)))
    (if parent
	(cmdl/base parent)
	cmdl)))

(define (cmdl/set-default-directory cmdl pathname)
  (let ((operation (cmdl/local-operation cmdl 'SET-DEFAULT-DIRECTORY)))
    (if operation
	(operation cmdl pathname)))
  (port/set-default-directory (cmdl/port cmdl) pathname))

(define (cmdl/start cmdl message)
  (let ((port (cmdl/port cmdl)))
    (let ((thunk
	   (lambda ()
	     (fluid-let ((*nearest-cmdl* cmdl)
			 (dynamic-handler-frames '())
			 (*bound-restarts*
			  (if (cmdl/parent cmdl) *bound-restarts* '()))
			 (standard-error-hook #f)
			 (standard-warning-hook #f)
			 (standard-breakpoint-hook #f)
			 (*working-directory-pathname*
			  *working-directory-pathname*)
			 (*default-pathname-defaults*
			  *default-pathname-defaults*)
			 (*current-input-port* #f)
			 (*current-output-port* #f)
			 (*notification-output-port* #f)
			 (*trace-output-port* #f)
			 (*interaction-i/o-port* #f))
	       (let loop ((message message))
		 (loop
		  (bind-abort-restart cmdl
		    (lambda ()
		      (deregister-all-events)
		      (with-interrupt-mask interrupt-mask/all
			(lambda (interrupt-mask)
			  interrupt-mask
			  (unblock-thread-events)
			  (ignore-errors
			   (lambda ()
			     ((->cmdl-message message) cmdl)))
			  (call-with-current-continuation
			   (lambda (continuation)
			     (with-create-thread-continuation continuation
			       (lambda ()
				 ((cmdl/driver cmdl) cmdl))))))))))))))
	  (mutex (port/thread-mutex port)))
      (let ((thread (current-thread))
	    (owner (thread-mutex-owner mutex)))
	(cond ((and owner (not (eq? thread owner)))
	       (signal-thread-event owner
		 (let ((signaller
			(or (cmdl/local-operation cmdl 'START-NON-OWNED)
			    (lambda (cmdl thread)
			      cmdl
			      (error "Non-owner thread can't start CMDL:"
				     thread)))))
		   (lambda ()
		     (unblock-thread-events)
		     (signaller cmdl thread))))
	       (stop-current-thread))
	      ((let ((parent (cmdl/parent cmdl)))
		 (and parent
		      (cmdl/local-operation parent 'START-CHILD)))
	       => (lambda (operation) (operation cmdl thunk)))
	      (else
	       (with-thread-mutex-locked mutex thunk)))))))

(define (bind-abort-restart cmdl thunk)
  (call-with-current-continuation
   (lambda (continuation)
     (with-restart 'ABORT
	 (string-append "Return to "
			(if (repl? cmdl)
			    "read-eval-print"
			    "command")
			" level "
			(number->string (cmdl/level cmdl))
			".")
	 (lambda (#!optional message)
	   (continuation
	    (cmdl-message/append
	     (cmdl-message/active
	      (lambda (port)
		;; Inform the port that the default directory has changed.
		(port/set-default-directory port
					    (working-directory-pathname))))
	     (if (default-object? message) "Abort!" message))))
	 values
       (lambda ()
	 (restart/put! (first-bound-restart) cmdl-abort-restart-tag cmdl)
	 (thunk))))))

(define (cmdl-abort-restart? restart)
  (restart/get restart cmdl-abort-restart-tag))

(define *nearest-cmdl*)

(define (nearest-cmdl)
  (if (not *nearest-cmdl*) (error "NEAREST-CMDL: no cmdl"))
  *nearest-cmdl*)

(define (nearest-cmdl/port)
  (let ((cmdl *nearest-cmdl*))
    (if cmdl
	(cmdl/port cmdl)
	console-i/o-port)))

(define (nearest-cmdl/level)
  (let ((cmdl *nearest-cmdl*))
    (if cmdl
	(cmdl/level cmdl)
	0)))

(define (nearest-cmdl/batch-mode?)
  (let ((cmdl *nearest-cmdl*))
    (if cmdl
	(cmdl/batch-mode? cmdl)
	#f)))

(define (cmdl/batch-mode? cmdl)
  (and (= (cmdl/level cmdl) 1)
       (implemented-primitive-procedure? (ucode-primitive batch-mode? 0))
       ((ucode-primitive batch-mode? 0))))

;;;; Operations

(define (parse-operations-list operations procedure)
  (if (not (list? operations))
      (error:wrong-type-argument operations "list" procedure))
  (map (lambda (operation)
	 (if (not (and (pair? operation)
		       (symbol? (car operation))
		       (pair? (cdr operation))
		       (procedure? (cadr operation))
		       (null? (cddr operation))))
	     (error:wrong-type-argument operation
					"operation binding"
					procedure))
	 (cons (car operation) (cadr operation)))
       operations))

(define (cmdl/local-operation cmdl name)
  (let ((binding (assq name (cmdl/operations cmdl))))
    (and binding
	 (cdr binding))))

(define (cmdl/operation cmdl name)
  (let loop ((cmdl cmdl))
    (or (cmdl/local-operation cmdl name)
	(let ((parent (cmdl/parent cmdl)))
	  (and parent
	       (loop parent))))))

(define (cmdl/operation-names cmdl)
  (let cmdl-loop ((cmdl cmdl) (names '()))
    (let loop ((bindings (cmdl/operations cmdl)) (names names))
      (if (pair? bindings)
	  (loop (cdr bindings)
		(if (memq (caar bindings) names)
		    names
		    (cons (caar bindings) names)))
	  (let ((parent (cmdl/parent cmdl)))
	    (if parent
		(cmdl-loop parent names)
		names))))))

;;;; Messages

(define (->cmdl-message object)
  (cond ((not object) (cmdl-message/null))
	((string? object) (cmdl-message/strings object))
	(else object)))

(define ((cmdl-message/strings . strings) cmdl)
  (let ((port (cmdl/port cmdl)))
    (port/with-output-terminal-mode port 'COOKED
      (lambda ()
	(for-each (lambda (string)
		    (fresh-line port)
		    (write-string ";" port)
		    (write-string string port))
		  strings)))))

(define ((cmdl-message/active actor) cmdl)
  (let ((port (cmdl/port cmdl)))
    (port/with-output-terminal-mode port 'COOKED
      (lambda ()
	(actor port)))))

(define (cmdl-message/append . messages)
  (do ((messages messages (cdr messages)))
      ((not (pair? messages)))
    (set-car! messages (->cmdl-message (car messages))))
  (let ((messages (delq! %cmdl-message/null messages)))
    (if (pair? messages)
	(if (pair? (cdr messages))
	    (lambda (cmdl)
	      (for-each (lambda (message) (message cmdl)) messages))
	    (car messages))
	(cmdl-message/null))))

(define-integrable (cmdl-message/null)
  %cmdl-message/null)

(define (%cmdl-message/null cmdl)
  cmdl
  #f)

;;;; Interrupts

(define (cmdl-interrupt/breakpoint)
  ((or (cmdl/operation (nearest-cmdl) 'INTERRUPT/BREAKPOINT)
       breakpoint)))

(define (cmdl-interrupt/abort-nearest)
  ((or (cmdl/operation (nearest-cmdl) 'INTERRUPT/ABORT-NEAREST)
       abort->nearest)))

(define (cmdl-interrupt/abort-previous)
  ((or (cmdl/operation (nearest-cmdl) 'INTERRUPT/ABORT-PREVIOUS)
       abort->previous)))

(define (cmdl-interrupt/abort-top-level)
  ((or (cmdl/operation (nearest-cmdl) 'INTERRUPT/ABORT-TOP-LEVEL)
       abort->top-level)))

(define (abort->nearest #!optional message)
  (invoke-abort (let ((restart (find-restart 'ABORT)))
		  (if (not restart)
		      (error:no-such-restart 'ABORT))
		  restart)
		(if (default-object? message) "Abort!" message)))

(define (abort->previous #!optional message)
  (invoke-abort (let ((restarts (find-restarts 'ABORT (bound-restarts))))
		  (let ((next (find-restarts 'ABORT (cdr restarts))))
		    (cond ((pair? next) (car next))
			  ((pair? restarts) (car restarts))
			  (else (error:no-such-restart 'ABORT)))))
		(if (default-object? message) "Up!" message)))

(define (abort->top-level #!optional message)
  (invoke-abort (let loop ((restarts (find-restarts 'ABORT (bound-restarts))))
		  (let ((next (find-restarts 'ABORT (cdr restarts))))
		    (cond ((pair? next) (loop next))
			  ((pair? restarts) (car restarts))
			  (else (error:no-such-restart 'ABORT)))))
		(if (default-object? message) "Quit!" message)))

(define (find-restarts name restarts)
  (let loop ((restarts restarts))
    (if (or (not (pair? restarts))
	    (eq? name (restart/name (car restarts))))
	restarts
	(loop (cdr restarts)))))

(define (invoke-abort restart message)
  (let ((effector (restart/effector restart)))
    (if (cmdl-abort-restart? restart)
	(effector message)
	(effector))))

(define cmdl-abort-restart-tag
  (list 'CMDL-ABORT-RESTART-TAG))

;;;; REP Loops

(define (make-repl parent port environment
		   #!optional condition operations prompt)
  (make-cmdl parent
	     port
	     repl-driver
	     (let ((inherit
		    (let ((repl (and parent (skip-non-repls parent))))
		      (lambda (argument default name check-arg)
			(if (eq? 'INHERIT argument)
			    (begin
			      (if (not repl)
				  (error "Can't inherit -- no REPL ancestor:"
					 name))
			      (default repl))
			    (check-arg argument 'MAKE-REPL))))))
	       (make-repl-state
		(inherit (if (default-object? prompt) 'INHERIT prompt)
			 repl/prompt
			 'PROMPT
			 (lambda (object procedure)
			   (if (not (string? object))
			       (error:wrong-type-argument object
							  "string"
							  procedure))
			   object))
		(inherit environment
			 repl/environment
			 'ENVIRONMENT
			 ->environment)
		(if (default-object? condition) #f condition)))
	     (append (if (default-object? operations) '() operations)
		     default-repl-operations)))

(define default-repl-operations
  `((START-CHILD ,(lambda (cmdl thunk) cmdl (with-history-disabled thunk)))
    (START-NON-OWNED
     ,(lambda (repl thread)
	(let ((condition (repl/condition repl)))
	  (if condition
	      (error:derived-thread thread condition)
	      (error "Non-owner thread can't start REPL:" thread)))))))

(define (push-repl environment
		   #!optional condition operations prompt)
  (let ((parent (nearest-cmdl)))
    (make-repl parent
	       #f
	       environment
	       (if (default-object? condition) #f condition)
	       (if (default-object? operations) '() operations)
	       (if (default-object? prompt) 'INHERIT prompt))))

(define (repl-driver repl)
  (let ((condition (repl/condition repl)))
    (if (and condition (condition/error? condition))
	(cond ((cmdl/operation repl 'ERROR-DECISION)
	       => (lambda (operation)
		    (operation repl condition)))
	      (hook/error-decision
	       (hook/error-decision repl condition)))))
  (port/set-default-environment (cmdl/port repl) (repl/environment repl))
  (let ((queue (repl/input-queue repl)))
    (do () (#f)
      (if (queue-empty? queue)
	  (let ((environment (repl/environment repl)))
	    (%repl-eval/write (hook/repl-read environment repl)
			      environment
			      repl))
	  ((dequeue! queue) repl)))))

(define (run-in-nearest-repl procedure)
  (guarantee-procedure-of-arity procedure 1 'run-in-nearest-repl)
  (enqueue! (repl/input-queue (nearest-repl)) procedure))

(define (repl-read #!optional environment repl)
  (receive (environment repl) (optional-er environment repl 'REPL-READ)
    (hook/repl-read environment repl)))

(define hook/repl-read)
(define (default/repl-read environment repl)
  (prompt-for-command-expression (cons 'STANDARD (repl/prompt repl))
				 (cmdl/port repl)
				 environment))

(define (repl-eval s-expression #!optional environment repl)
  (receive (environment repl) (optional-er environment repl 'REPL-EVAL)
    (%repl-eval s-expression environment repl)))

(define (%repl-eval s-expression environment repl)
  (repl-history/record! (repl/reader-history repl) s-expression)
  (let ((value (hook/repl-eval s-expression environment repl)))
    (repl-history/record! (repl/printer-history repl) value)
    value))

(define hook/repl-eval)
(define (default/repl-eval s-expression environment repl)
  (%repl-scode-eval (syntax s-expression environment) environment repl))

(define (repl-scode-eval scode #!optional environment repl)
  (receive (environment repl) (optional-er environment repl 'REPL-SCODE-EVAL)
    (%repl-scode-eval scode environment repl)))

(define (%repl-scode-eval scode environment repl)
  (with-repl-eval-boundary repl
    (lambda ()
      (extended-scode-eval scode environment))))

(define (with-repl-eval-boundary repl thunk)
  ((ucode-primitive with-stack-marker 3)
   (lambda () (with-new-history thunk))
   with-repl-eval-boundary
   repl))

(define (repl-write value s-expression #!optional environment repl)
  (receive (environment repl) (optional-er environment repl 'REPL-WRITE)
    (hook/repl-write value s-expression environment repl)))

(define hook/repl-write)
(define (default/repl-write object s-expression environment repl)
  (port/write-result (cmdl/port repl)
		     s-expression
		     object
		     (and repl:write-result-hash-numbers?
			  (object-pointer? object)
			  (not (interned-symbol? object))
			  (not (number? object))
			  (object-hash object))
		     environment))

(define (repl-eval/write s-expression #!optional environment repl)
  (receive (environment repl) (optional-er environment repl 'REPL-EVAL/WRITE)
    (%repl-eval/write s-expression environment repl)))

(define (%repl-eval/write s-expression environment repl)
  (hook/repl-write (%repl-eval s-expression environment repl)
		   s-expression
		   environment
		   repl))

(define (optional-er environment repl caller)
  (let ((repl
	 (if (default-object? repl)
	     (nearest-repl)
	     (begin
	       (guarantee-repl repl caller)
	       repl))))
    (values (if (default-object? environment)
		(repl/environment repl)
		(begin
		  (guarantee-environment environment caller)
		  environment))
	    repl)))

(define (repl/start repl #!optional message)
  (cmdl/start repl
	      (make-repl-message repl
				 (if (default-object? message)
				     #f
				     message))))

(define (make-repl-message repl message)
  (let ((condition (repl/condition repl)))
    (cmdl-message/append
     (or message
	 (and condition
	      (cmdl-message/strings
	       (fluid-let ((*unparser-list-depth-limit* 25)
			   (*unparser-list-breadth-limit* 100)
			   (*unparser-string-length-limit* 500))
		 (condition/report-string condition)))))
     (and condition
	  repl:allow-restart-notifications?
	  (condition-restarts-message condition))
     repl/set-default-environment)))

(define hook/error-decision)

(define (repl/set-default-environment repl)
  (let ((parent (cmdl/parent repl))
	(environment (repl/environment repl)))
    (if (not (and parent
		  (repl? parent)
		  (eq? (repl/environment parent) environment)))
	(let ((operation (cmdl/operation repl 'SET-DEFAULT-ENVIRONMENT)))
	  (if operation
	      (operation repl environment)
	      (hook/set-default-environment repl environment))))))

(define hook/set-default-environment)
(define (default/set-default-environment port environment)
  (let ((port (cmdl/port port)))
    (port/with-output-terminal-mode port 'COOKED
      (lambda ()
	(if (not (interpreter-environment? environment))
	    (begin
	      (fresh-line port)
	      (write-string
	       ";Warning! this environment is a compiled-code environment:
; Assignments to most compiled-code bindings are prohibited,
; as are certain other environment operations."
	       port)))
	(let ((package (environment->package environment)))
	  (if package
	      (begin
		(fresh-line port)
		(write-string ";Package: " port)
		(write (package/name package) port))))))))

(define (restart #!optional n)
  (let ((condition (nearest-repl/condition)))
    (let ((restarts
	   (filter-restarts
	    (if condition
		(condition/restarts condition)
		(bound-restarts)))))
      (let ((n-restarts (length restarts)))
	(if (zero? n-restarts)
	    (error "Can't RESTART: no options available."))
	(invoke-restart-interactively
	 (list-ref
	  restarts
	  (- n-restarts
	     (if (default-object? n)
		 (let ((port (interaction-i/o-port)))
		   (fresh-line port)
		   (write-string ";Choose an option by number:" port)
		   (write-restarts restarts port
		     (lambda (index port)
		       (write-string ";" port)
		       (write-string (string-pad-left (number->string index) 3)
				     port)
		       (write-string ":" port)))
		   (let loop ()
		     (let ((n
			    (prompt-for-evaluated-expression
			     "Option number"
			     (nearest-repl/environment)
			     port)))
		       (if (and (exact-integer? n) (<= 1 n n-restarts))
			   n
			   (begin
			     (beep port)
			     (fresh-line port)
			     (write-string
			      ";Option must be an integer between 1 and "
			      port)
			     (write n-restarts port)
			     (write-string ", inclusive.")
			     (loop))))))
		 (begin
		   (if (not (exact-integer? n))
		       (error:wrong-type-argument n "exact integer" 'RESTART))
		   (if (not (<= 1 n n-restarts))
		       (error:bad-range-argument n 'RESTART))
		   n))))
	 condition)))))

(define (write-restarts restarts port write-index)
  (newline port)
  (do ((restarts restarts (cdr restarts))
       (index (length restarts) (- index 1)))
      ((not (pair? restarts)))
    (write-index index port)
    (write-string " " port)
    (write-restart-report (car restarts) port)
    (newline port)))

(define (filter-restarts restarts)
  (let loop ((restarts restarts))
    (if (pair? restarts)
	(let ((rest
	       (if (cmdl-abort-restart? (car restarts))
		   (list-transform-positive (cdr restarts) cmdl-abort-restart?)
		   (loop (cdr restarts)))))
	  (if (restart/interactor (car restarts))
	      (cons (car restarts) rest)
	      rest))
	'())))

(define (condition-restarts-message condition)
  (cmdl-message/active
   (lambda (port)
     (fresh-line port)
     (write-string ";To continue, call RESTART with an option number:" port)
     (write-restarts (filter-restarts (condition/restarts condition)) port
       (lambda (index port)
	 (write-string "; (RESTART " port)
	 (write index port)
	 (write-string ") =>" port))))))

(define-structure (repl-state
		   (conc-name repl-state/)
		   (constructor make-repl-state
				(prompt environment condition)))
  prompt
  environment
  (condition #f read-only #t)
  (reader-history (make-repl-history repl-reader-history-size))
  (printer-history (make-repl-history repl-printer-history-size))
  (input-queue (make-queue) read-only #t))

(define (repl? object)
  (and (cmdl? object)
       (repl-state? (cmdl/state object))))

(define-guarantee repl "read-eval-print loop")

(define-integrable (repl/prompt repl)
  (repl-state/prompt (cmdl/state repl)))

(define-integrable (set-repl/prompt! repl prompt)
  (set-repl-state/prompt! (cmdl/state repl) prompt))

(define-integrable (repl/environment repl)
  (repl-state/environment (cmdl/state repl)))

(define (set-repl/environment! repl environment)
  (set-repl-state/environment! (cmdl/state repl) environment)
  (repl/set-default-environment repl)
  (port/set-default-environment (cmdl/port repl) environment))

(define-integrable (repl/condition repl)
  (repl-state/condition (cmdl/state repl)))

(define-integrable (repl/reader-history repl)
  (repl-state/reader-history (cmdl/state repl)))

(define-integrable (set-repl/reader-history! repl reader-history)
  (set-repl-state/reader-history! (cmdl/state repl) reader-history))

(define-integrable (repl/printer-history repl)
  (repl-state/printer-history (cmdl/state repl)))

(define-integrable (set-repl/printer-history! repl printer-history)
  (set-repl-state/printer-history! (cmdl/state repl) printer-history))

(define-integrable (repl/input-queue repl)
  (repl-state/input-queue (cmdl/state repl)))

(define (repl/parent repl)
  (skip-non-repls (cmdl/parent repl)))

(define (nearest-repl)
  (or (skip-non-repls (nearest-cmdl))
      (error "NEAREST-REPL: no REPLs")))

(define (skip-non-repls cmdl)
  (and cmdl
       (if (repl-state? (cmdl/state cmdl))
	   cmdl
	   (skip-non-repls (cmdl/parent cmdl)))))

(define (repl/base repl)
  (let ((parent (repl/parent repl)))
    (if parent
	(repl/base parent)
	repl)))

(define (nearest-repl/environment)
  (repl/environment (nearest-repl)))

(define (nearest-repl/condition)
  (repl/condition (nearest-repl)))

;;;; History

(define repl-reader-history-size 5)
(define repl-printer-history-size 10)

(define-structure (repl-history (constructor %make-repl-history)
				(conc-name repl-history/))
  (size #f read-only #t)
  elements)

(define (make-repl-history size)
  (%make-repl-history size (make-circular-list size '())))

(define (repl-history/record! history object)
  (let ((elements (repl-history/elements history)))
    (if (pair? elements)
	(begin
	  (set-car! elements object)
	  (set-repl-history/elements! history (cdr elements))))))

(define (repl-history/replace-current! history object)
  (let ((elements (repl-history/elements history)))
    (if (pair? elements)
	(set-car! (list-tail elements (- (repl-history/size history) 1))
		  object))))

(define (repl-history/read history n)
  (if (not (and (exact-nonnegative-integer? n)
		(< n (repl-history/size history))))
      (error:wrong-type-argument n "history index" 'REPL-HISTORY/READ))
  (list-ref (repl-history/elements history)
	    (- (- (repl-history/size history) 1) n)))

;;; User Interface Stuff

(define (pe)
  (let ((environment (nearest-repl/environment)))
    (let ((package (environment->package environment)))
      (if package
	  (package/name package)
	  environment))))

(define (ge environment)
  (let ((environment (->environment environment 'GE)))
    (set-repl/environment! (nearest-repl) environment)
    environment))

(define (->environment object #!optional caller)
  (let ((caller (if (default-object? caller) '->ENVIRONMENT caller)))
    (cond ((environment? object) object)
	  ((package? object) (package/environment object))
	  ((procedure? object) (procedure-environment object))
	  ((promise? object) (promise-environment object))
	  (else
	   (let ((package
		  (let ((package-name
			 (cond ((symbol? object) (list object))
			       ((list? object) object)
			       (else #f))))
		    (and package-name
			 (name->package package-name)))))
	     (if (not package)
		 (error:wrong-type-argument object "environment" caller))
	     (package/environment package))))))

(define (re #!optional index)
  (repl-eval (repl-history/read (repl/reader-history (nearest-repl))
				(if (default-object? index) 1 index))))

(define (in #!optional index)
  (repl-history/read (repl/reader-history (nearest-repl))
		     (if (default-object? index) 1 index)))

(define (out #!optional index)
  (repl-history/read (repl/printer-history (nearest-repl))
		     (- (if (default-object? index) 1 index) 1)))

(define (read-eval-print environment message prompt)
  (repl/start (push-repl environment #f '() prompt) message))

(define (ve environment)
  (read-eval-print (->environment environment 'VE) #f 'INHERIT))

(define (proceed #!optional value)
  (if (default-object? value)
      (continue)
      (use-value value))
  (let ((port (notification-output-port)))
    (fresh-line port)
    (write-string ";Unable to PROCEED" port)))

;;;; Breakpoints

(define (bkpt datum . arguments)
  (apply breakpoint-procedure 'CONTINUATION-ENVIRONMENT datum arguments))

(define (breakpoint-procedure environment datum . arguments)
  (signal-breakpoint-1 #f
		       environment
		       (cmdl-message/active
			(lambda (port)
			  (fresh-line port)
			  (format-error-message datum arguments port)))
		       "bkpt>"
		       "Return from BKPT."))

(define (breakpoint #!optional message environment continuation)
  (signal-breakpoint (if (default-object? continuation)
			 #f
			 continuation)
		     (if (default-object? environment)
			 (nearest-repl/environment)
			 environment)
		     (if (default-object? message)
			 "Break!"
			 message)))

(define (signal-breakpoint continuation environment message #!optional prompt)
  (signal-breakpoint-1 continuation
		       environment
		       message
		       (if (default-object? prompt) "break>" prompt)
		       "Continue from breakpoint."))

(define (signal-breakpoint-1 continuation environment message prompt reporter)
  (call-with-current-continuation
   (lambda (restart-continuation)
     (let ((continuation (or continuation restart-continuation)))
       (with-restart 'CONTINUE reporter
	   (lambda () (restart-continuation unspecific))
	   values
	 (lambda ()
	   (call-with-values
	       (lambda ()
		 (get-breakpoint-environment continuation environment message))
	     (lambda (environment message)
	       (%signal-breakpoint continuation
				   environment
				   message
				   prompt)))))))))

(define (get-breakpoint-environment continuation environment message)
  (let ((environment
	 (if (eq? 'CONTINUATION-ENVIRONMENT environment)
	     (continuation/first-subproblem-environment continuation)
	     environment)))
    (if (eq? 'NO-ENVIRONMENT environment)
	(values (nearest-repl/environment)
		(cmdl-message/append
		 message
		 (cmdl-message/strings
		  "Breakpoint environment unavailable;"
		  "using REPL environment instead.")))
	(values environment message))))

(define (continuation/first-subproblem-environment continuation)
  (let ((frame (continuation/first-subproblem continuation)))
    (if frame
	(call-with-values (lambda () (stack-frame/debugging-info frame))
	  (lambda (expression environment subexpression)
	    expression subexpression
	    (if (debugging-info/undefined-environment? environment)
		'NO-ENVIRONMENT
		environment)))
	'NO-ENVIRONMENT)))

(define condition-type:breakpoint)
(define condition/breakpoint?)
(define breakpoint/environment)
(define breakpoint/message)
(define breakpoint/prompt)
(define %signal-breakpoint)

(define (initialize-breakpoint-condition!)
  (set! condition-type:breakpoint
	(make-condition-type 'BREAKPOINT #f '(ENVIRONMENT MESSAGE PROMPT)
	  (lambda (condition port)
	    condition
	    (write-string "Breakpoint." port))))
  (set! condition/breakpoint?
	(condition-predicate condition-type:breakpoint))
  (set! breakpoint/environment
	(condition-accessor condition-type:breakpoint 'ENVIRONMENT))
  (set! breakpoint/message
	(condition-accessor condition-type:breakpoint 'MESSAGE))
  (set! breakpoint/prompt
	(condition-accessor condition-type:breakpoint 'PROMPT))
  (set! %signal-breakpoint
	(let ((make-condition
	       (condition-constructor condition-type:breakpoint
				      '(ENVIRONMENT MESSAGE PROMPT))))
	  (lambda (continuation environment message prompt)
	    (let ((condition
		   (make-condition continuation
				   'BOUND-RESTARTS
				   environment
				   message
				   prompt)))
	      (signal-condition condition)
	      (standard-breakpoint-handler condition)))))
  unspecific)

(define (standard-breakpoint-handler condition)
  (let ((hook standard-breakpoint-hook))
    (if hook
	(fluid-let ((standard-breakpoint-hook #f))
	  (hook condition))))
  (repl/start (push-repl (breakpoint/environment condition)
			 condition
			 '()
			 (breakpoint/prompt condition))
	      (breakpoint/message condition)))

(define standard-breakpoint-hook #f)