Preliminary support for engine classes (not working).

Patches applied:

 * skribilo--devel--1.2  (patch 75)

   - Adding missing patch logs from `lcourtes@laas.fr--2005-libre'.

 * skribilo--engine-classes--1.2  (base, patch 1)

   - tag of lcourtes@laas.fr--2005-libre/skribilo--engine-classes--1.2--base-0
   - Preliminary support for engine classes (not working).

git-archimport-id: lcourtes@laas.fr--2005-libre/skribilo--engine-classes--1.2--patch-1

15 files changed, 454 insertions, 231 deletions

diff --git a/src/guile/skribilo/biblio/bibtex.scm b/src/guile/skribilo/biblio/bibtex.scm
index 319df1d..d3d1cca 100644
--- a/src/guile/skribilo/biblio/bibtex.scm
+++ b/src/guile/skribilo/biblio/bibtex.scm
@@ -22,7 +22,8 @@
 (define-module (skribilo biblio bibtex)
   :autoload   (skribilo utils strings) (make-string-replace)
   :autoload   (skribilo ast)           (markup-option ast->string)
-  :autoload   (skribilo engine)        (engine-filter find-engine)
+  :autoload   (skribilo engine)        (engine-class-filter
+					lookup-engine-class)
   :use-module (skribilo biblio author)
   :use-module (srfi srfi-39)
   :export     (print-as-bibtex-entry))
@@ -60,8 +61,8 @@
 	    (markup-ident entry))
     (for-each (lambda (opt)
 		(let* ((o (show-option opt))
-		       (tex-filter (engine-filter
-				    (find-engine 'latex)))
+		       (tex-filter (engine-class-filter
+				    (lookup-engine-class 'latex)))
 		       (filter (lambda (n)
 				 (tex-filter (ast->string n))))
 		       (id (lambda (a) a)))
diff --git a/src/guile/skribilo/engine.scm b/src/guile/skribilo/engine.scm
index 06667ad..0622e11 100644
--- a/src/guile/skribilo/engine.scm
+++ b/src/guile/skribilo/engine.scm
@@ -1,7 +1,7 @@
 ;;; engine.scm	-- Skribilo engines.
 ;;;
-;;; Copyright 2003-2004  Erick Gallesio - I3S-CNRS/ESSI <eg@essi.fr>
-;;; Copyright 2005  Ludovic Courtès  <ludovic.courtes@laas.fr>
+;;; Copyright 2003, 2004  Erick Gallesio - I3S-CNRS/ESSI <eg@essi.fr>
+;;; Copyright 2005, 2006  Ludovic Courtès  <ludovic.courtes@laas.fr>
 ;;;
 ;;;
 ;;; This program is free software; you can redistribute it and/or modify
@@ -32,25 +32,38 @@
   :use-module (ice-9 optargs)
   :autoload   (srfi srfi-39)  (make-parameter)
 
-  :export (<engine> engine? engine-ident engine-format
-		    engine-customs engine-filter engine-symbol-table
+  :export (<engine-class> engine-class? engine-class-ident
+			  engine-class-format
+			  engine-class-customs engine-class-filter
+			  engine-class-symbol-table
+			  copy-engine-class
+
+	   <engine> engine? engine-custom
+	            engine-custom-set! engine-custom-add!
+		    engine-format?
+
+		    engine-ident engine-format
+		    engine-filter engine-symbol-table
 
 	   *current-engine*
-	   default-engine default-engine-set!
-	   make-engine copy-engine find-engine lookup-engine
-	   engine-custom engine-custom-set! engine-custom-add!
-	   engine-format? engine-add-writer!
+	   default-engine-class default-engine-class-set!
+	   push-default-engine-class pop-default-engine-class
+
+	   make-engine-class lookup-engine-class
+	   make-engine copy-engine engine-class
+	   engine-class-add-writer!
+
 	   processor-get-engine
-	   push-default-engine pop-default-engine
 
-	   engine-loaded? when-engine-is-loaded))
+	   engine-class-loaded? when-engine-class-is-loaded
+	   when-engine-is-instantiated))
 
 
 (fluid-set! current-reader %skribilo-module-reader)
 
 
 ;;;
-;;; Class definition.
+;;; Class definitions.
 ;;;
 
 ;; Note on writers
@@ -81,7 +94,7 @@
 ;; For more details, see `markup-writer-get' and `lookup-markup-writer' in
 ;; `(skribilo writer)'.
 
-(define-class <engine> ()
+(define-class <engine-class> (<class>)
   (ident		:init-keyword :ident		:init-value '???)
   (format		:init-keyword :format		:init-value "raw")
   (info		        :init-keyword :info		:init-value '())
@@ -92,109 +105,182 @@
   (free-writers         :init-value '())
   (filter		:init-keyword :filter		:init-value #f)
   (customs		:init-keyword :custom		:init-value '())
-  (symbol-table	:init-keyword :symbol-table	:init-value '()))
+  (symbol-table		:init-keyword :symbol-table	:init-value '()))
 
+(define-class <engine> ()
+  (customs	:init-keyword :customs	:init-value '())
+  :metaclass <engine-class>)
+
+(define-method (compute-cpl (class <engine-class>))
+  ;; Automatically set the class precedence list of <engine> subclasses.
+  (format (current-error-port) "computing CPL for ~a~%" class)
+  (list class <engine> <top>))
+
+(define-method (initialize (engine-class <engine-class>) . args)
+  ;; Set the name of <engine> subclasses.
+  (let ((result (next-method))
+	(ident (slot-ref engine-class 'ident)))
+    (slot-set! engine-class 'name
+	       (symbol-append '<engine: ident '>))
+    result))
+
+
+(define %format format)
+(define* (make-engine-class ident :key (version 'unspecified)
+				       (format "raw")
+				       (filter #f)
+				       (delegate #f)
+				       (symbol-table '())
+				       (custom '())
+				       (info '()))
+  (let ((e (make <engine-class>
+	     :ident ident :version version :format format
+	     :filter filter :delegate delegate
+	     :symbol-table symbol-table
+	     :custom custom :info info)))
+    (%format (current-error-port) "make-engine-class returns ~a~%" e)
+    e))
 
-(define (engine? obj)
-  (is-a? obj <engine>))
+(define (engine-class? obj)
+  (is-a? obj <engine-class>))
 
-(define (engine-ident obj)
+(define (engine-class-ident obj)
   (slot-ref obj 'ident))
 
-(define (engine-format obj)
+(define (engine-class-format obj)
   (slot-ref obj 'format))
 
-(define (engine-customs obj)
+(define (engine-class-customs obj)
   (slot-ref obj 'customs))
 
-(define (engine-filter obj)
+(define (engine-class-filter obj)
   (slot-ref obj 'filter))
 
-(define (engine-symbol-table obj)
+(define (engine-class-symbol-table obj)
   (slot-ref obj 'symbol-table))
 
 
 
 ;;;
-;;; Default engines.
+;;; Engine instances.
 ;;;
 
-(define *default-engine*	#f)
-(define *default-engines*	'())
 
+(define (engine? obj)
+  (is-a? obj <engine>))
 
-(define (default-engine)
-   *default-engine*)
+;; A mapping of engine classes to hooks.
+(define %engine-instantiate-hook (make-hash-table))
+
+(define-method (initialize (engine <engine>) . args)
+  (format (current-error-port) "initializing engine ~a~%" engine)
+  engine)
+
+(define-method (make-instance (class <engine-class>) . args)
+  (format #t "making engine of class ~a~%" class)
+  (let ((engine (next-method)))
+    (if (engine? engine)
+	(let ((hook (hashq-ref %engine-instantiate-hook engine-class)))
+	  (format (current-error-port) "engine made: ~a~%" engine)
+	  (if (hook? hook)
+	      (run-hook hook engine class))
+	  engine)
+	engine)))
+
+(define (make-engine engine-class)
+  (make engine-class
+    :customs (list-copy (slot-ref engine-class 'customs))))
+
+;; Convenience functions.
+
+(define (engine-ident obj) (engine-class-ident (engine-class obj)))
+(define (engine-format obj) (engine-class-format (engine-class obj)))
+;;(define (engine-customs obj) (engine-class-customs (engine-class obj)))
+(define (engine-filter obj) (engine-class-filter (engine-class obj)))
+(define (engine-symbol-table obj)
+  (engine-class-symbol-table (engine-class obj)))
 
+(define (engine-format? fmt . e)
+  (let ((e (cond
+	     ((pair? e) (car e))
+	     (else (*current-engine*)))))
+    (if (not (engine? e))
+	(skribe-error 'engine-format? "no engine" e)
+	(string=? fmt (engine-format e)))))
 
-(define (default-engine-set! e)
-  (with-debug 5 'default-engine-set!
-     (debug-item "engine=" e)
 
-     (if (not (engine? e))
-	 (skribe-error 'default-engine-set! "bad engine ~S" e))
-     (set! *default-engine* e)
-     (set! *default-engines* (cons e *default-engines*))
-     e))
+(define (engine-class-add-writer! e ident pred upred opt before action
+				  after class valid)
+  ;; Add a writer to engine class E.  If IDENT is a symbol, then it should
+  ;; denote a markup name and the writer being added is specific to that
+  ;; markup.  If IDENT is `#t' (for instance), then it is assumed to be a
+  ;; ``free writer'' that may apply to any kind of markup for which PRED
+  ;; returns true.
 
+  (define (check-procedure name proc arity)
+    (cond
+      ((not (procedure? proc))
+	 (skribe-error ident "Illegal procedure" proc))
+      ((not (equal? (%procedure-arity proc) arity))
+	 (skribe-error ident
+		       (format #f "Illegal ~S procedure" name)
+		       proc))))
 
-(define (push-default-engine e)
-   (set! *default-engines* (cons e *default-engines*))
-   (default-engine-set! e))
+  (define (check-output name proc)
+    (and proc (or (string? proc) (check-procedure name proc 2))))
 
-(define (pop-default-engine)
-   (if (null? *default-engines*)
-       (skribe-error 'pop-default-engine "Empty engine stack" '())
-       (begin
-	  (set! *default-engines* (cdr *default-engines*))
-	  (if (pair? *default-engines*)
-	      (default-engine-set! (car *default-engines*))
-	      (set! *default-engine* #f)))))
+  ;;
+  ;; Engine-add-writer! starts here
+  ;;
+  (if (not (is-a? e <engine-class>))
+      (skribe-error ident "Illegal engine" e))
 
+  ;; check the options
+  (if (not (or (eq? opt 'all) (list? opt)))
+      (skribe-error ident "Illegal options" opt))
 
-(define (processor-get-engine combinator newe olde)
-  (cond
-    ((procedure? combinator)
-     (combinator newe olde))
-    ((engine? newe)
-     newe)
-    (else
-     olde)))
+  ;; check the correctness of the predicate
+  (if pred
+      (check-procedure "predicate" pred 2))
 
+  ;; check the correctness of the validation proc
+  (if valid
+      (check-procedure "validate" valid 2))
 
-(define (engine-format? fmt . e)
-  (let ((e (cond
-	     ((pair? e) (car e))
-	     (else (*current-engine*)))))
-    (if (not (engine? e))
-	(skribe-error 'engine-format? "no engine" e)
-	(string=? fmt (engine-format e)))))
+  ;; check the correctness of the three actions
+  (check-output "before" before)
+  (check-output "action" action)
+  (check-output "after" after)
 
-;;;
-;;; MAKE-ENGINE
-;;;
-(define* (make-engine ident :key (version 'unspecified)
-				(format "raw")
-				(filter #f)
-				(delegate #f)
-				(symbol-table '())
-				(custom '())
-				(info '()))
-  (let ((e (make <engine> :ident ident :version version :format format
-			  :filter filter :delegate delegate
-			  :symbol-table symbol-table
-			  :custom custom :info info)))
-    e))
+  ;; create a new writer and bind it
+  (let ((n (make <writer>
+	     :ident (if (symbol? ident) ident 'all)
+	     :class class :pred pred :upred upred :options opt
+	     :before before :action action :after after
+	     :validate valid)))
+    (if (symbol? ident)
+	(let ((writers (slot-ref e 'writers)))
+	  (hashq-set! writers ident
+		      (cons n (hashq-ref writers ident '()))))
+	(slot-set! e 'free-writers
+		   (cons n (slot-ref e 'free-writers))))
+    n))
 
 
 ;;;
 ;;; COPY-ENGINE
 ;;;
-(define* (copy-engine ident e :key (version 'unspecified)
-				  (filter #f)
-				  (delegate #f)
-				  (symbol-table #f)
-				  (custom #f))
+(define (copy-engine e)
+  (let ((new (shallow-clone e)))
+    (slot-set! new 'class   (engine-class e))
+    (slot-set! new 'customs (list-copy (slot-ref e 'customs)))
+    new))
+
+(define* (copy-engine-class ident e :key (version 'unspecified)
+					 (filter #f)
+					 (delegate #f)
+					 (symbol-table #f)
+					 (custom #f))
   (let ((new (shallow-clone e)))
     (slot-set! new 'ident	 ident)
     (slot-set! new 'version	 version)
@@ -212,14 +298,11 @@
       (hash-for-each (lambda (m w*)
 		       (hashq-set! new-writers m w*))
 		     (slot-ref e 'writers))
-      (slot-set! new 'writers new-writers))
-
-    new))
-
+      (slot-set! new 'writers new-writers))))
 
 
 ;;;
-;;; Engine loading.
+;;; Engine class loading.
 ;;;
 
 ;; Each engine is to be stored in its own module with the `(skribilo engine)'
@@ -229,39 +312,47 @@
 (define (engine-id->module-name id)
   `(skribilo engine ,id))
 
-(define (engine-loaded? id)
+(define (engine-class-loaded? id)
   "Check whether engine @var{id} is already loaded."
   ;; Trick taken from `resolve-module' in `boot-9.scm'.
   (nested-ref the-root-module
 	      `(%app modules ,@(engine-id->module-name id))))
 
 ;; A mapping of engine names to hooks.
-(define %engine-load-hook (make-hash-table))
+(define %engine-class-load-hook (make-hash-table))
 
 (define (consume-load-hook! id)
   (with-debug 5 'consume-load-hook!
-    (let ((hook (hashq-ref %engine-load-hook id)))
+    (let ((hook (hashq-ref %engine-class-load-hook id)))
       (if hook
 	  (begin
 	    (debug-item "running hook " hook " for engine " id)
-	    (hashq-remove! %engine-load-hook id)
+	    (hashq-remove! %engine-class-load-hook id)
 	    (run-hook hook))))))
 
-(define (when-engine-is-loaded id thunk)
+(define (when-engine-class-is-loaded id thunk)
   "Run @var{thunk} only when engine with identifier @var{id} is loaded."
-  (if (engine-loaded? id)
+  (if (engine-class-loaded? id)
       (begin
 	;; Maybe the engine had already been loaded via `use-modules'.
 	(consume-load-hook! id)
 	(thunk))
-      (let ((hook (or (hashq-ref %engine-load-hook id)
+      (let ((hook (or (hashq-ref %engine-class-load-hook id)
 		      (let ((hook (make-hook)))
-			(hashq-set! %engine-load-hook id hook)
+			(hashq-set! %engine-class-load-hook id hook)
 			hook))))
 	(add-hook! hook thunk))))
 
+(define (when-engine-is-instantiated engine-class proc)
+  (let loop ((hook (hashq-ref %engine-instantiate-hook engine-class)))
+    (if (not hook)
+	(let ((hook (make-hook 2)))
+	  (hashq-set! %engine-instantiate-hook engine-class hook)
+	  (loop hook))
+	(add-hook! hook proc))))
 
-(define* (lookup-engine id :key (version 'unspecified))
+
+(define* (lookup-engine-class id :key (version 'unspecified))
   "Look for an engine named @var{name} (a symbol) in the @code{(skribilo
 engine)} module hierarchy.  If no such engine was found, an error is raised,
 otherwise the requested engine is returned."
@@ -276,11 +367,6 @@ otherwise the requested engine is returned."
 	     e)
 	   (error "no such engine" id)))))
 
-(define* (find-engine id :key (version 'unspecified))
-  (false-if-exception (apply lookup-engine (list id version))))
-
-
-
 
 
 ;;;
@@ -294,7 +380,6 @@ otherwise the requested engine is returned."
 	(cadr c)
 	'unspecified)))
 
-
 (define (engine-custom-set! e id val)
   (let* ((customs (slot-ref e 'customs))
 	 (c       (assq id customs)))
@@ -308,61 +393,47 @@ otherwise the requested engine is returned."
 	  (engine-custom-set! e id (list val))
 	  (engine-custom-set! e id (cons val old)))))
 
-(define (engine-add-writer! e ident pred upred opt before action
-			    after class valid)
-  ;; Add a writer to engine E.  If IDENT is a symbol, then it should denote
-  ;; a markup name and the writer being added is specific to that markup.  If
-  ;; IDENT is `#t' (for instance), then it is assumed to be a ``free writer''
-  ;; that may apply to any kind of markup for which PRED returns true.
+(define (processor-get-engine combinator newe olde)
+  (cond
+    ((procedure? combinator)
+     (combinator newe olde))
+    ((engine? newe)
+     newe)
+    (else
+     olde)))
 
-  (define (check-procedure name proc arity)
-    (cond
-      ((not (procedure? proc))
-	 (skribe-error ident "Illegal procedure" proc))
-      ((not (equal? (%procedure-arity proc) arity))
-	 (skribe-error ident
-		       (format #f "Illegal ~S procedure" name)
-		       proc))))
 
-  (define (check-output name proc)
-    (and proc (or (string? proc) (check-procedure name proc 2))))
+
+;;;
+;;; Default engines.
+;;;
 
-  ;;
-  ;; Engine-add-writer! starts here
-  ;;
-  (if (not (is-a? e <engine>))
-      (skribe-error ident "Illegal engine" e))
+(define *default-engine-classes*	'(html))
 
-  ;; check the options
-  (if (not (or (eq? opt 'all) (list? opt)))
-      (skribe-error ident "Illegal options" opt))
+(define (default-engine-class)
+  (let ((class (car *default-engine-classes*)))
+    (cond ((symbol? class) (lookup-engine-class class))
+	  (else class))))
 
-  ;; check the correctness of the predicate
-  (if pred
-      (check-procedure "predicate" pred 2))
+(define (default-engine-class-set! e)
+  (with-debug 5 'default-engine-set!
+     (debug-item "engine=" e)
 
-  ;; check the correctness of the validation proc
-  (if valid
-      (check-procedure "validate" valid 2))
+     (if (not (or (symbol? e) (engine-class? e)))
+	 (skribe-error 'default-engine-class-set! "bad engine class ~S" e))
+     (set-car! *default-engine-classes* e)
+     e))
 
-  ;; check the correctness of the three actions
-  (check-output "before" before)
-  (check-output "action" action)
-  (check-output "after" after)
 
-  ;; create a new writer and bind it
-  (let ((n (make <writer>
-	     :ident (if (symbol? ident) ident 'all)
-	     :class class :pred pred :upred upred :options opt
-	     :before before :action action :after after
-	     :validate valid)))
-    (if (symbol? ident)
-	(let ((writers (slot-ref e 'writers)))
-	  (hashq-set! writers ident
-		      (cons n (hashq-ref writers ident '()))))
-	(slot-set! e 'free-writers
-		   (cons n (slot-ref e 'free-writers))))
-    n))
+(define (push-default-engine-class e)
+  (set! *default-engine-classes*
+	(cons e *default-engine-classes*))
+  e)
+
+(define (pop-default-engine-class)
+  (if (null? *default-engine-classes*)
+      (skribe-error 'pop-default-engine-class "empty engine class stack" '())
+      (set! *default-engine-classes* (cdr *default-engine-classes*))))
 
 
 
@@ -374,14 +445,20 @@ otherwise the requested engine is returned."
 (use-modules (skribilo module))
 
 ;; At this point, we're almost done with the bootstrap process.
-;(format #t "base engine: ~a~%" (lookup-engine 'base))
+(format (current-error-port) "HERE~%")
+(format #t "base engine: ~a~%" (lookup-engine-class 'base))
+(format (current-error-port) "THERE~%")
 
 (define *current-engine*
   ;; By default, use the HTML engine.
-  (make-parameter (lookup-engine 'html)
+  (make-parameter #f ;'html ;(make-engine (lookup-engine-class 'html))
 		  (lambda (val)
-		    (cond ((symbol? val) (lookup-engine val))
+		    (cond ((symbol? val)
+			   (make-engine (lookup-engine-class val)))
+			  ((engine-class? val)
+			   (make-engine val))
 			  ((engine? val) val)
+			  ((not val) val)
 			  (else
 			   (error "invalid value for `*current-engine*'"
 				  val))))))
diff --git a/src/guile/skribilo/engine/base.scm b/src/guile/skribilo/engine/base.scm
index 8418e8b..c7f7dd2 100644
--- a/src/guile/skribilo/engine/base.scm
+++ b/src/guile/skribilo/engine/base.scm
@@ -1,6 +1,7 @@
 ;;; base.scm  --  BASE Skribe engine
 ;;;
 ;;; Copyright 2003, 2004  Manuel Serrano
+;;; Copyright 2006  Ludovic Courtès <ludovic.courtes@laas.fr>
 ;;;
 ;;;
 ;;; This program is free software; you can redistribute it and/or modify
@@ -18,14 +19,26 @@
 ;;; Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301,
 ;;; USA.
 
-(define-skribe-module (skribilo engine base))
+(define-module (skribilo engine base)
+  :use-module (skribilo ast)
+  :use-module (skribilo engine)
+  :use-module (skribilo writer)
+  :autoload   (skribilo output) (output)
+  :use-module (skribilo evaluator)
 
+  ;; syntactic sugar
+  :use-module (skribilo reader)
+  :use-module (skribilo utils syntax))
+
+(fluid-set! current-reader (make-reader 'skribe))
+
+
 ;*---------------------------------------------------------------------*/
 ;*    base-engine ...                                                  */
 ;*---------------------------------------------------------------------*/
 (define base-engine
-   (default-engine-set!
-      (make-engine 'base
+   (default-engine-class-set!
+      (make-engine-class 'base
 		   :version 'plain
 		   :symbol-table '(("iexcl" "!")
 				   ("cent" "c")
@@ -166,7 +179,7 @@
 			    (format #f "?~a " k))))
 		     (msg (list f (markup-body n)))
 		     (n (list "[" (color :fg "red" (bold msg)) "]")))
-		 (skribe-eval n e))))
+		 (evaluate-document n e))))
 
 ;*---------------------------------------------------------------------*/
 ;*    &the-bibliography ...                                            */
@@ -314,14 +327,14 @@
 ;*---------------------------------------------------------------------*/
 (markup-writer '&bib-entry-title
    :action (lambda (n e)
-	      (skribe-eval (bold (markup-body n)) e)))
+	      (evaluate-document (bold (markup-body n)) e)))
 
 ;*---------------------------------------------------------------------*/
 ;*    &bib-entry-publisher ...                                         */
 ;*---------------------------------------------------------------------*/
 (markup-writer '&bib-entry-publisher
    :action (lambda (n e)
-	      (skribe-eval (it (markup-body n)) e)))
+	      (evaluate-document (it (markup-body n)) e)))
 
 ;*---------------------------------------------------------------------*/
 ;*    &the-index ...  @label the-index@                                */
@@ -441,7 +454,7 @@
 			       ;;:&skribe-eval-location loc
 			       :class "index-table"
 			       (make-sub-tables ie nc pref))))))
-		 (output (skribe-eval t e) e))))
+		 (output (evaluate-document t e) e))))
 
 ;*---------------------------------------------------------------------*/
 ;*    &the-index-header ...                                            */
@@ -458,7 +471,7 @@
 (markup-writer '&prog-line
    :before (lambda (n e)
 	      (let ((n (markup-ident n)))
-		 (if n (skribe-eval (it (list n) ": ") e))))
+		 (if n (evaluate-document (it (list n) ": ") e))))
    :after "\n")
 
 ;*---------------------------------------------------------------------*/
@@ -469,7 +482,7 @@
    :action (lambda (n e)
 	      (let ((o (markup-option n :offset))
 		    (n (markup-ident (handle-body (markup-body n)))))
-		 (skribe-eval (it (if (integer? o) (+ o n) n)) e))))
+		 (evaluate-document (it (if (integer? o) (+ o n) n)) e))))
 
 
 
diff --git a/src/guile/skribilo/evaluator.scm b/src/guile/skribilo/evaluator.scm
index 8502d51..4450298 100644
--- a/src/guile/skribilo/evaluator.scm
+++ b/src/guile/skribilo/evaluator.scm
@@ -27,7 +27,8 @@
   :autoload (skribilo location)   (<location>)
   :autoload (skribilo ast)        (ast? markup?)
   :autoload (skribilo engine)     (*current-engine*
-				   engine? find-engine engine-ident)
+				   engine? lookup-engine-class
+				   engine-ident)
   :autoload (skribilo reader)     (*document-reader*)
 
   :autoload (skribilo verify)     (verify)
@@ -99,7 +100,9 @@
      (debug-item "engine=" engine)
      (debug-item "reader=" reader)
 
-     (let ((e (if (symbol? engine) (find-engine engine) engine)))
+     (let ((e (if (symbol? engine)
+		  (make-engine (lookup-engine-class engine))
+		  engine)))
        (debug-item "e=" e)
        (if (not (engine? e))
 	   (skribe-error 'evaluate-document-from-port "cannot find engine" engine)
diff --git a/src/guile/skribilo/module.scm b/src/guile/skribilo/module.scm
index ac8eee0..b709079 100644
--- a/src/guile/skribilo/module.scm
+++ b/src/guile/skribilo/module.scm
@@ -58,8 +58,8 @@
     (skribilo biblio)
     (skribilo lib)        ;; `define-markup', `unwind-protect', etc.
     (skribilo resolve)
-    (skribilo engine)
-    (skribilo writer)
+    ;;(skribilo engine)
+    ;;(skribilo writer)
     (skribilo output)
     (skribilo evaluator)
     (skribilo debug)
diff --git a/src/guile/skribilo/output.scm b/src/guile/skribilo/output.scm
index a33c040..27906ec 100644
--- a/src/guile/skribilo/output.scm
+++ b/src/guile/skribilo/output.scm
@@ -222,7 +222,7 @@
 
 
 (define-method (out (node <markup>) e)
-  (let ((w (lookup-markup-writer node e)))
+  (let ((w (lookup-markup-writer node (engine-class e))))
     (if (writer? w)
 	(%out/writer node e w)
 	(output (slot-ref node 'body) e))))
diff --git a/src/guile/skribilo/package/eq.scm b/src/guile/skribilo/package/eq.scm
index 4f5020e..eec84d6 100644
--- a/src/guile/skribilo/package/eq.scm
+++ b/src/guile/skribilo/package/eq.scm
@@ -258,7 +258,7 @@ a symbol representing the mathematical operator denoted by @var{m} (e.g.,
 ;;;
 
 
-(markup-writer 'eq (find-engine 'base)
+(markup-writer 'eq (lookup-engine-class 'base)
    :action (lambda (node engine)
 	     ;; The `:renderer' option should be a symbol (naming an engine
 	     ;; class) or an engine or engine class.  This allows the use of
@@ -276,7 +276,7 @@ a symbol representing the mathematical operator denoted by @var{m} (e.g.,
 			 (let ((lout-code
 				(with-output-to-string
 				  (lambda ()
-				    (output node (find-engine 'lout))))))
+				    (output node (lookup-engine-class 'lout))))))
 			   (output (lout-illustration
 				    :ident (markup-ident node)
 				    lout-code)
@@ -292,7 +292,7 @@ a symbol representing the mathematical operator denoted by @var{m} (e.g.,
   ;; Note: The text-only rendering is less ambiguous if we parenthesize
   ;; without taking operator precedence into account.
   (let ((precedence (operator-precedence op)))
-    `(markup-writer ',(symbol-append 'eq: op) (find-engine 'base)
+    `(markup-writer ',(symbol-append 'eq: op) (lookup-engine-class 'base)
        :action (lambda (node engine)
 		  (let loop ((operands (markup-body node)))
 		   (if (null? operands)
@@ -334,14 +334,14 @@ a symbol representing the mathematical operator denoted by @var{m} (e.g.,
 (simple-markup-writer >= (symbol "ge"))
 (simple-markup-writer <= (symbol "le"))
 
-(markup-writer 'eq:sqrt (find-engine 'base)
+(markup-writer 'eq:sqrt (lookup-engine-class 'base)
    :action (lambda (node engine)
 	     (display "sqrt(")
 	     (output (markup-body node) engine)
 	     (display ")")))
 
 (define-macro (simple-binary-markup-writer op obj)
-  `(markup-writer ',(symbol-append 'eq: op) (find-engine 'base)
+  `(markup-writer ',(symbol-append 'eq: op) (lookup-engine-class 'base)
      :action (lambda (node engine)
 	       (let ((body (markup-body node)))
 		 (if (= (length body) 2)
@@ -358,7 +358,7 @@ a symbol representing the mathematical operator denoted by @var{m} (e.g.,
 				   "wrong argument type"
 				   body))))))
 
-(markup-writer 'eq:expt (find-engine 'base)
+(markup-writer 'eq:expt (lookup-engine-class 'base)
    :action (lambda (node engine)
 	     (let ((body (markup-body node)))
 		 (if (= (length body) 2)
@@ -372,7 +372,7 @@ a symbol representing the mathematical operator denoted by @var{m} (e.g.,
 (simple-binary-markup-writer in    (symbol "in"))
 (simple-binary-markup-writer notin (symbol "notin"))
 
-(markup-writer 'eq:apply (find-engine 'base)
+(markup-writer 'eq:apply (lookup-engine-class 'base)
    :action (lambda (node engine)
 	     (let ((func (car (markup-body node))))
 	       (output func engine)
@@ -387,7 +387,7 @@ a symbol representing the mathematical operator denoted by @var{m} (e.g.,
 		       (loop (cdr operands)))))
 	       (display ")"))))
 
-(markup-writer 'eq:sum (find-engine 'base)
+(markup-writer 'eq:sum (lookup-engine-class 'base)
    :action (lambda (node engine)
 	     (let ((from (markup-option node :from))
 		   (to (markup-option node :to)))
@@ -400,7 +400,7 @@ a symbol representing the mathematical operator denoted by @var{m} (e.g.,
 	       (output (markup-body node) engine)
 	       (display ")"))))
 
-(markup-writer 'eq:prod (find-engine 'base)
+(markup-writer 'eq:prod (lookup-engine-class 'base)
    :action (lambda (node engine)
 	     (let ((from (markup-option node :from))
 		   (to (markup-option node :to)))
@@ -413,7 +413,7 @@ a symbol representing the mathematical operator denoted by @var{m} (e.g.,
 	       (output (markup-body node) engine)
 	       (display ")"))))
 
-(markup-writer 'eq:script (find-engine 'base)
+(markup-writer 'eq:script (lookup-engine-class 'base)
    :action (lambda (node engine)
 	     (let ((body (markup-body node))
 		   (sup* (markup-option node :sup))
@@ -429,7 +429,7 @@ a symbol representing the mathematical operator denoted by @var{m} (e.g.,
 ;;; Initialization.
 ;;;
 
-(when-engine-is-loaded 'lout
+(when-engine-class-is-loaded 'lout
   (lambda ()
     (resolve-module '(skribilo package eq lout))))
 
diff --git a/src/guile/skribilo/package/eq/lout.scm b/src/guile/skribilo/package/eq/lout.scm
index c487b85..c38e74c 100644
--- a/src/guile/skribilo/package/eq/lout.scm
+++ b/src/guile/skribilo/package/eq/lout.scm
@@ -37,14 +37,13 @@
 ;;; Initialization.
 ;;;
 
-(let ((lout (find-engine 'lout)))
-  (if (not lout)
-      (skribe-error 'eq "Lout engine not found" lout)
-      (let ((includes (engine-custom lout 'includes)))
-	;; Append the `eq' include file
-	(engine-custom-set! lout 'includes
-			    (string-append includes "\n"
-					   "@SysInclude { eq }\n")))))
+(when-engine-is-instantiated (lookup-engine-class 'lout)
+  (lambda (lout class)
+    (let ((includes (engine-custom lout 'includes)))
+      ;; Append the `eq' include file
+      (engine-custom-set! lout 'includes
+			  (string-append includes "\n"
+					 "@SysInclude { eq }\n")))))
 
 
 ;;;
@@ -52,7 +51,7 @@
 ;;;
 
 
-(markup-writer 'eq (find-engine 'lout)
+(markup-writer 'eq (lookup-engine-class 'lout)
    :options '(:inline?)
    :before "{ "
    :action (lambda (node engine)
@@ -81,7 +80,7 @@
 	 (close-par `(if need-paren? "{ @VScale ) }" "")))
 
     `(markup-writer ',(symbol-append 'eq: sym)
-		    (find-engine 'lout)
+		    (lookup-engine-class 'lout)
 		    :action (lambda (node engine)
 			      (let loop ((operands (markup-body node)))
 				(if (null? operands)
@@ -132,7 +131,7 @@
 (simple-lout-markup-writer >=)
 
 (define-macro (binary-lout-markup-writer sym lout-name)
-  `(markup-writer ',(symbol-append 'eq: sym) (find-engine 'lout)
+  `(markup-writer ',(symbol-append 'eq: sym) (lookup-engine-class 'lout)
      :action (lambda (node engine)
 	       (let ((body (markup-body node)))
 		 (if (= (length body) 2)
@@ -154,7 +153,7 @@
 (binary-lout-markup-writer in "element")
 (binary-lout-markup-writer notin "notelement")
 
-(markup-writer 'eq:apply (find-engine 'lout)
+(markup-writer 'eq:apply (lookup-engine-class 'lout)
    :action (lambda (node engine)
 	     (let ((func (car (markup-body node))))
 	       (output func engine)
@@ -176,7 +175,7 @@
 ;;;
 
 (define-macro (range-lout-markup-writer sym lout-name)
-  `(markup-writer ',(symbol-append 'eq: sym) (find-engine 'lout)
+  `(markup-writer ',(symbol-append 'eq: sym) (lookup-engine-class 'lout)
       :action (lambda (node engine)
 		(let ((from (markup-option node :from))
 		      (to (markup-option node :to))
@@ -193,7 +192,7 @@
 (range-lout-markup-writer sum "sum")
 (range-lout-markup-writer product "prod")
 
-(markup-writer 'eq:script (find-engine 'lout)
+(markup-writer 'eq:script (lookup-engine-class 'lout)
    :action (lambda (node engine)
 	     (let ((body (markup-body node))
 		   (sup (markup-option node :sup))
diff --git a/src/guile/skribilo/package/pie.scm b/src/guile/skribilo/package/pie.scm
index 8ccf858..c5ae84a 100644
--- a/src/guile/skribilo/package/pie.scm
+++ b/src/guile/skribilo/package/pie.scm
@@ -251,7 +251,7 @@ the string \"hello\".  Implement `sliceweight' markups too."
 		     "\n")
 		   `("colors: " ,@colors "\n")))))
 
-(markup-writer 'pie (find-engine 'base)
+(markup-writer 'pie (lookup-engine-class 'base)
   :action (lambda (node engine)
 	    (let* ((fmt (select-output-format engine))
 		   (pie-file (string-append (markup-ident node) "."
@@ -291,12 +291,12 @@ the string \"hello\".  Implement `sliceweight' markups too."
 				   "A Pie Chart"))
 			engine))))
 
-(markup-writer 'slice (find-engine 'base)
+(markup-writer 'slice (lookup-engine-class 'base)
   :action (lambda (node engine)
 	    ;; Nothing to do here
 	    (error "slice: this writer should never be invoked")))
 
-(markup-writer 'sliceweight (find-engine 'base)
+(markup-writer 'sliceweight (lookup-engine-class 'base)
   :action (lambda (node engine)
 	    ;; Nothing to do here.
 	    (error "sliceweight: this writer should never be invoked")))
@@ -306,7 +306,7 @@ the string \"hello\".  Implement `sliceweight' markups too."
 ;;; Initialization.
 ;;;
 
-(when-engine-is-loaded 'lout
+(when-engine-class-is-loaded 'lout
   (lambda ()
     (resolve-module '(skribilo package pie lout))))
 
diff --git a/src/guile/skribilo/package/pie/lout.scm b/src/guile/skribilo/package/pie/lout.scm
index 61dbcb7..1841960 100644
--- a/src/guile/skribilo/package/pie/lout.scm
+++ b/src/guile/skribilo/package/pie/lout.scm
@@ -38,11 +38,11 @@
 ;;; Helper functions.
 ;;;
 
-(let ((lout (find-engine 'lout)))
-   (if lout
-       (engine-custom-set! lout 'includes
-	  (string-append (engine-custom lout 'includes)
-			 "\n@SysInclude { pie } # Pie Charts\n"))))
+(when-engine-is-instantiated (lookup-engine-class 'lout)
+  (lambda (lout class)
+    (engine-custom-set! lout 'includes
+			(string-append (engine-custom lout 'includes)
+				       "\n@SysInclude { pie }\n"))))
 
 
 
@@ -50,7 +50,7 @@
 ;;; Writers.
 ;;;
 
-(markup-writer 'pie (find-engine 'lout)
+(markup-writer 'pie (lookup-engine-class 'lout)
    :before (lambda (node engine)
 	      (let* ((weights (map (lambda (slice)
 				     (markup-option slice :weight))
@@ -102,7 +102,7 @@
 		 (display "{\n")))
    :after "\n} # @Pie\n")
 
-(markup-writer 'slice (find-engine 'lout)
+(markup-writer 'slice (lookup-engine-class 'lout)
    :options '(:weight :detach? :color)
    :action (lambda (node engine)
 	     (display "  @Slice\n")
@@ -120,7 +120,7 @@
 	     (output (markup-body node) engine)
 	     (display " }\n")))
 
-(markup-writer 'sliceweight (find-engine 'base)
+(markup-writer 'sliceweight (lookup-engine-class 'base)
    ;; This writer should work for every engine, provided the `pie' markup has
    ;; a proper `&total-weight' option.
    :action (lambda (node engine)
diff --git a/src/guile/skribilo/package/slide.scm b/src/guile/skribilo/package/slide.scm
index 380fdc5..12955ce 100644
--- a/src/guile/skribilo/package/slide.scm
+++ b/src/guile/skribilo/package/slide.scm
@@ -257,16 +257,16 @@
 (format (current-error-port) "Slides initializing...~%")
 
 ;; Register specific implementations for lazy loading.
-(when-engine-is-loaded 'base
+(when-engine-class-is-loaded 'base
   (lambda ()
     (resolve-module '(skribilo package slide base))))
-(when-engine-is-loaded 'latex
+(when-engine-class-is-loaded 'latex
   (lambda ()
     (resolve-module '(skribilo package slide latex))))
-(when-engine-is-loaded 'html
+(when-engine-class-is-loaded 'html
   (lambda ()
     (resolve-module '(skribilo package slide html))))
-(when-engine-is-loaded 'lout
+(when-engine-class-is-loaded 'lout
   (lambda ()
     (resolve-module '(skribilo package slide lout))))
 
diff --git a/src/guile/skribilo/package/slide/base.scm b/src/guile/skribilo/package/slide/base.scm
index c8e652c..1d8d84c 100644
--- a/src/guile/skribilo/package/slide/base.scm
+++ b/src/guile/skribilo/package/slide/base.scm
@@ -40,7 +40,7 @@
 ;;;
 ;;; Simple markups.
 ;;;
-(let ((be (find-engine 'base)))
+(let ((be (lookup-engine-class 'base)))
 
    ;; slide-pause
    (markup-writer 'slide-pause be
@@ -164,7 +164,7 @@
             engine)))
 
 
-(markup-writer 'slide-topic (find-engine 'base)
+(markup-writer 'slide-topic (lookup-engine-class 'base)
    :options '(:title :outline? :class :ident)
    :action (lambda (n e)
 	      (if (markup-option n :outline?)
@@ -172,7 +172,7 @@
 
 	      (output (markup-body n) e)))
 
-(markup-writer 'slide-subtopic (find-engine 'base)
+(markup-writer 'slide-subtopic (lookup-engine-class 'base)
    ;; FIXME: Largely untested.
    :options '(:title :outline? :class :ident)
    :action (lambda (n e)
diff --git a/src/guile/skribilo/utils/compat.scm b/src/guile/skribilo/utils/compat.scm
index 118f294..787d9b9 100644
--- a/src/guile/skribilo/utils/compat.scm
+++ b/src/guile/skribilo/utils/compat.scm
@@ -35,6 +35,8 @@
   :autoload   (skribilo lib)       (type-name)
   :autoload   (skribilo resolve)   (*document-being-resolved*)
   :autoload   (skribilo output)    (*document-being-output*)
+  :use-module ((skribilo engine)   :renamer (symbol-prefix-proc 'orig:))
+  :use-module ((skribilo writer)   :renamer (symbol-prefix-proc 'orig:))
   :use-module (skribilo debug)
 
   :re-export (file-size)  ;; re-exported from `(skribilo utils files)'
@@ -142,6 +144,7 @@
     ("acmproc.skr"      . (skribilo package acmproc))))
 
 (define*-public (skribe-load file :rest args)
+  ;; FIXME: Convert the `:engine' arg to an engine class.
   (guard (c ((file-search-error? c)
 	     ;; Regular file loading failed.  Try built-ins.
 	     (let ((mod-name (assoc-ref %skribe-known-files file)))
@@ -178,6 +181,131 @@
       (set! %skribe-reader (make-reader 'skribe)))
   (%skribe-reader port))
 
+
+;;;
+;;; Engines and engine classes.
+;;;
+
+(define %per-class-current-engines
+  (make-hash-table))
+
+(define (engine-class-singleton class)
+  (let ((e (hashq-ref %per-class-current-engines class)))
+    (if e
+	e
+	(let ((e (orig:make-engine class)))
+	  (format (current-error-port) "e: ~a~%" e)
+	  (hashq-set! %per-class-current-engines class e)
+	  e))))
+
+(define-public (find-engine class-name)
+  ;; The old `find-engine' gave the illusion of a single instance per engine
+  ;; class.
+  (let ((class (false-if-exception (orig:lookup-engine-class class-name))))
+    (if class
+	(engine-class-singleton class)
+	#f)))
+
+(define (make-engine-compat-proc wrapped-engine-class-proc)
+  (lambda (e)
+    (if (orig:engine? e)
+	(wrapped-engine-class-proc (orig:engine-class e))
+	(wrapped-engine-class-proc e))))
+
+(define-public engine?
+  (make-engine-compat-proc orig:engine-class?))
+(define engine-format
+  (make-engine-compat-proc orig:engine-class-format))
+(define-public engine-customs
+  (make-engine-compat-proc orig:engine-class-customs))
+(define-public engine-filter
+  (make-engine-compat-proc orig:engine-class-filter))
+(define-public engine-symbol-table
+  (make-engine-compat-proc orig:engine-class-symbol-table))
+
+(define-public (make-engine first-arg . args)
+  ;; The old-style `make-engine' should translate to a `make-engine-class'.
+  ;; Its `:delegate' argument should be an engine class rather than an
+  ;; engine.
+
+  (define (rewrite-delegate-arg args)
+    (let loop ((args args)
+	       (result '()))
+      (if (null? args)
+	  (reverse! result)
+	  (let ((arg (car args)))
+	    (if (eq? arg :delegate)
+		(let ((delegate (cadr args)))
+		  (loop (cddr args)
+			(cons* :delegate
+			       (if (orig:engine? delegate)
+				   (orig:engine-class delegate)
+				   delegate)
+			       result)))
+		(loop (cdr args) (cons arg result)))))))
+
+  (if (symbol? first-arg)
+      (apply orig:make-engine-class first-arg (rewrite-delegate-arg args))
+      (apply orig:make-engine-class first-arg args)))
+
+(define-public (markup-writer markup . args)
+  ;; In old-style `markup-writer', the first arg could (optionally) be an
+  ;; engine.  Now, this must be (optionally) an engine class instead of an
+  ;; engine.
+  (if (null? args)
+      (apply orig:markup-writer markup args)
+      (let loop ((first-arg (car args)))
+	(if (orig:engine? first-arg)
+	    (loop (orig:engine-class first-arg))
+	    (apply orig:markup-writer markup first-arg (cdr args))))))
+
+
+(define (ensure-engine engine-or-class)
+  (cond ((orig:engine-class? engine-or-class)
+	 (engine-class-singleton engine-or-class))
+	(else engine-or-class)))
+
+(define %default-engines '())
+
+(define-public (default-engine)
+  (ensure-engine
+   (if (null? %default-engines)
+       (let* ((class  (orig:default-engine-class))
+	      (engine (find-engine (orig:engine-class-ident class))))
+	 (set! %default-engine (list engine))
+	 engine)
+       (car %default-engines))))
+
+(define-public (default-engine-set! e)
+  (let ((e (ensure-engine e)))
+    (format (current-error-port) "DEFAULT-ENGINE-SET! ~a~%" e)
+    (if (null? %default-engines)
+	(set! %default-engines (list e))
+	(set-car! %default-engines e))
+    e))
+
+(define-public (push-default-engine e)
+  (let ((e (ensure-engine e)))
+    (set! %default-engines (cons e %default-engines))
+    e))
+
+(define-public (pop-default-engine)
+  (ensure-engine
+   (if (null? %default-engines)
+       (skribe-error 'pop-default-engine "empty stack" '())
+       (let ((e (car %default-engines)))
+	 (set! %default-engines (cdr %default-engines))
+	 e))))
+
+(define-public (copy-engine ident e . args)
+  (cond ((engine? e)
+	 (orig:copy-engine e))
+	(else
+	 (apply orig:copy-engine-class ident e args))))
+
+(define-public engine-custom      orig:engine-custom)
+(define-public engine-custom-set! orig:engine-custom-set!)
+(define-public engine-format?     orig:engine-format?)
 
 
 ;;;
diff --git a/src/guile/skribilo/verify.scm b/src/guile/skribilo/verify.scm
index 052b5cc..6b67156 100644
--- a/src/guile/skribilo/verify.scm
+++ b/src/guile/skribilo/verify.scm
@@ -127,7 +127,7 @@
 
      (next-method)
 
-     (let ((w (lookup-markup-writer node e)))
+     (let ((w (lookup-markup-writer node (engine-class e))))
        (when (writer? w)
 	 (check-required-options node w e)
 	 (when (pair? (writer-options w))
diff --git a/src/guile/skribilo/writer.scm b/src/guile/skribilo/writer.scm
index b16819d..df12eaa 100644
--- a/src/guile/skribilo/writer.scm
+++ b/src/guile/skribilo/writer.scm
@@ -28,7 +28,8 @@
 
   :use-module (skribilo utils syntax)
   :autoload (srfi srfi-1)     (find filter)
-  :autoload (skribilo engine) (engine? engine-ident? default-engine))
+  :autoload (skribilo engine) (engine-class? engine-ident
+			       default-engine-class))
 
 
 (use-modules (skribilo debug)
@@ -133,13 +134,13 @@
   (let ((e (or (if (and (list? engine) (not (keyword? (car engine))))
 		   (car engine)
 		   #f)
-	       (default-engine))))
+	       (default-engine-class))))
 
     (cond
       ((and (not (symbol? markup)) (not (eq? markup #t)))
        (skribe-error 'markup-writer "illegal markup" markup))
-      ((not (engine? e))
-	  (skribe-error 'markup-writer "illegal engine" e))
+      ((not (engine-class? e))
+       (skribe-error 'markup-writer "illegal engine class" e))
       ((and (not predicate)
 	    (not class)
 	    (null? options)
@@ -152,8 +153,9 @@
 	     (ac (if (eq? action 'unspecified)
 		     (lambda (n e) (output (markup-body n) e))
 		     action)))
-	 (engine-add-writer! e markup m predicate
-			     options before ac after class validate))))))
+	 (engine-class-add-writer! e markup m predicate
+				   options before ac after
+				   class validate))))))
 
 
 
@@ -162,8 +164,8 @@
 ;;;
 
 (define (lookup-markup-writer node e)
-  ;; Find the writer that applies best to NODE.  See also `markup-writer-get'
-  ;; and `markup-writer-get*'.
+  ;; Find the writer that applies best to NODE.  E should be an engine class.
+  ;; See also `markup-writer-get' and `markup-writer-get*'.
 
   (define (matching-writer writers)
     (find (lambda (w)
@@ -179,14 +181,14 @@
 
     (or (matching-writer node-writers)
 	(matching-writer (slot-ref e 'free-writers))
-	(and (engine? delegate)
+	(and (engine-class? delegate)
 	     (lookup-markup-writer node delegate)))))
 
 
 (define* (markup-writer-get markup :optional engine :key (class #f) (pred #f))
-  ;; Get a markup writer for MARKUP (a symbol) in ENGINE, with class CLASS
-  ;; and user predicate PRED.  [FIXME: Useless since PRED is a procedure and
-  ;; therefore not comparable?]
+  ;; Get a markup writer for MARKUP (a symbol) in engine class ENGINE, with
+  ;; class CLASS and user predicate PRED.  [FIXME: Useless since PRED is a
+  ;; procedure and therefore not comparable?]
 
   (define (matching-writer writers)
     (find (lambda (w)
@@ -195,19 +197,19 @@
 		     (eq? (slot-ref w 'upred) pred))))
 	  writers))
 
-  (let ((e (or engine (default-engine))))
+  (let ((e (or engine (default-engine-class))))
     (cond
       ((not (symbol? markup))
        (skribe-error 'markup-writer-get "illegal symbol" markup))
-      ((not (engine? e))
-       (skribe-error 'markup-writer-get "illegal engine" e))
+      ((not (engine-class? e))
+       (skribe-error 'markup-writer-get "illegal engine class" e))
       (else
        (let* ((writers (slot-ref e 'writers))
 	      (markup-writers (hashq-ref writers markup '()))
 	      (delegate (slot-ref e 'delegate)))
 
 	 (or (matching-writer markup-writers)
-	     (and (engine? delegate)
+	     (and (engine-class? delegate)
 		  (markup-writer-get markup delegate
 				     :class class :pred pred))))))))
 
@@ -222,19 +224,19 @@
 		  (equal? (writer-class w) class)))
 	    writers))
 
-  (let ((e (or engine (default-engine))))
+  (let ((e (or engine (default-engine-class))))
     (cond
       ((not (symbol? markup))
        (skribe-error 'markup-writer "illegal symbol" markup))
-      ((not (engine? e))
-       (skribe-error 'markup-writer "illegal engine" e))
+      ((not (engine-class? e))
+       (skribe-error 'markup-writer "illegal engine class" e))
       (else
        (let* ((writers (slot-ref e 'writers))
 	      (markup-writers (hashq-ref writers markup '()))
 	      (delegate (slot-ref e 'delegate)))
 
 	 (append (matching-writers writers)
-		 (if (engine? delegate)
+		 (if (engine-class? delegate)
 		     (markup-writer-get* markup delegate :class class)
 		     '())))))))