X-Git-Url: https://thelambdalab.xyz/gitweb/index.cgi?a=blobdiff_plain;f=src%2Fscheme.4th;h=934775080aa276c352d852a1b9b525904bf28442;hb=ed0aaed61b10e03e5f064404f506eaa73e50c87d;hp=4d5e33bc9c1c0ff04a93bddf3d0cf6def3299a5a;hpb=346e7741d328a4521e8673f1c0418d05d59a4439;p=scheme.forth.jl.git

diff --git a/src/scheme.4th b/src/scheme.4th
index 4d5e33b..9347750 100644
--- a/src/scheme.4th
+++ b/src/scheme.4th
@@ -39,6 +39,7 @@ make-type pair-type
 make-type symbol-type
 make-type primitive-proc-type
 make-type compound-proc-type
+make-type continuation-type
 make-type port-type
 : istype? ( obj type -- obj bool )
     over = ;
@@ -62,7 +63,7 @@ variable nextexception
 make-exception recoverable-exception
 make-exception unrecoverable-exception
 
-: throw reset-term throw ;
+: throw reset-term cr throw ;
 
 \ }}}
 
@@ -90,7 +91,7 @@ variable nextfree
     nextfree !
 
     nextfree @ scheme-memsize >= if
-        collect-garbage
+      collect-garbage
     then
 
     nextfree @ scheme-memsize >= if
@@ -132,6 +133,10 @@ variable nextfree
     cdr-cells + !
 ;
 
+variable object-stack-base
+: init-object-stack-base
+  depth object-stack-base ! ;
+
 : nil 0 nil-type ;
 : nil? nil-type istype? ;
 
@@ -402,6 +407,9 @@ variable read-line-buffer-offset
 : make-frame ( vars vals -- frame )
     cons ;
 
+: add-frame-to-env ( frame env -- env )
+    cons ;
+
 : frame-vars ( frame -- vars )
     car ;
 
@@ -416,99 +424,90 @@ variable read-line-buffer-offset
 ;
 
 : extend-env ( vars vals env -- env )
-    >R >R
-    make-frame
-    R> R>
-    cons
+    -2rot make-frame
+    2swap add-frame-to-env
 ;
 
-objvar vars
-objvar vals
-
-: get-vars-vals-frame ( var frame -- bool )
-    2dup frame-vars vars obj!
-    frame-vals vals obj!
+: get-vals-frame ( var frame -- vals | nil )
+    2dup frame-vars 
+    2swap frame-vals ( var vars vals )
 
     begin
-        vars obj@ nil objeq? false =
+        nil? false =
     while
-        2dup vars obj@ car objeq? if
-            2drop true
+
+        -2rot ( vals var vars )
+        2over 2over car objeq? if
+            2drop 2drop
             exit
         then
 
-        vars obj@ cdr vars obj!
-        vals obj@ cdr vals obj!
+        cdr 2rot cdr
     repeat
 
-    2drop false
+    2drop 2drop 2drop
+    nil
 ;
 
-: get-vars-vals ( var env -- vars? vals? bool )
+: get-vals ( var env -- vals | nil )
 
     begin
         nil? false =
     while
         2over 2over first-frame
-        get-vars-vals-frame if
-            2drop 2drop
-            vars obj@ vals obj@ true
+        get-vals-frame nil? false = if
+            2swap 2drop 2swap 2drop
             exit
         then
 
+        2drop
+
         enclosing-env
     repeat
 
-    2drop 2drop
-    false
+    2swap 2drop
 ;
 
-hide vars
-hide vals
-
-objvar var
-
+objvar var \ Used only for error messages
 : lookup-var ( var env -- val )
     2over var obj!
-    get-vars-vals if
-        2swap 2drop car
-    else
-        except-message: ." tried to read unbound variable '" var obj@ print ." '." recoverable-exception  throw
+    
+    get-vals nil? if
+        except-message: ." tried to read unbound variable '" var obj@ print ." '."
+        recoverable-exception throw
     then
+
+    car
 ;
 
 : set-var ( var val env -- )
-    >R >R 2swap R> R> ( val var env )
-    2over var obj!
-    get-vars-vals if
-        2swap 2drop ( val vals )
-        set-car!
+    2rot 2dup var obj! ( val env var )
+    2swap ( val var env )
+    get-vals nil? if
+        except-message: ." tried to set unbound variable '" var obj@ print ." '."
+        recoverable-exception throw
     else
-        except-message: ." tried to set unbound variable '" var obj@ print ." '." recoverable-exception throw
+        ( val vals )
+        set-car!
     then
 ;
-
 hide var
 
-objvar env
-
 : define-var ( var val env -- )
-    env obj! 
+    first-frame ( var val frame )
+    2rot 2swap 2over 2over ( val var frame var frame )
 
-    2over env obj@ ( var val var env )
-    get-vars-vals if
-        2swap 2drop ( var val vals )
-        set-car!
+    get-vals-frame nil? if
         2drop
-    else
-        env obj@
-        first-frame ( var val frame )
+        -2rot 2swap 2rot
         add-binding
+    else
+        ( val var frame vals )
+        2swap 2drop 2swap 2drop
+        set-car!
     then
 ;
 
-hide env
-
 : make-procedure ( params body env -- proc )
     nil
     cons cons cons
@@ -521,6 +520,127 @@ global-env obj!
 
 \ }}}
 
+\ ---- Continuations ---- {{{
+
+: cons-return-stack ( -- listobj )
+  rsp@ 1- rsp0  = if
+    nil exit
+  then
+
+  nil rsp@ 1- rsp0 do
+    i 1+ @ fixnum-type 2swap cons
+  loop
+
+  rsp@ 1- rsp0 - fixnum-type 2swap cons
+;
+
+: cons-param-stack ( -- listobj )
+  nil 
+
+  depth 2- object-stack-base @ = if
+    exit
+  then
+
+  depth 2- object-stack-base @ do
+        PSP0 i + 1 + @
+        PSP0 i + 2 + @
+
+        2swap cons
+    2 +loop
+
+  depth 2- 2/ fixnum-type 2swap cons
+;
+
+: make-continuation ( -- continuation true-obj )
+    \ true-obj allows calling code to detect whether
+    \ it is being called immediately following make-continuation
+    \ or by a restore-continuation.
+
+  cons-param-stack
+  cons-return-stack
+  cons drop continuation-type
+
+  true boolean-type
+;
+
+: continuation->pstack-list
+  drop pair-type car ;
+
+: continuation->rstack-list
+  drop pair-type cdr ;
+
+: stack-list-len ( stack-list -- n )
+    car drop
+;
+
+: restore-param-stack ( continuation -- obj_stack )
+  continuation->pstack-list
+  2dup >R >R
+
+  ( Allocate stack space first using psp!,
+    then copy objects from list. )
+
+  car drop 2*
+  object-stack-base @ psp0 + + psp!
+
+  R> R> 2dup cdr
+  2swap
+  stack-list-len 1- 0 swap do
+
+      2dup car
+      PSP0 object-stack-base @ + i 2* + 2 + !
+      PSP0 object-stack-base @ + i 2* + 1 + !
+      cdr
+
+  -1 +loop
+
+  2drop
+;
+
+: restore-return-stack ( continuation -- )
+
+    continuation->rstack-list
+
+    2dup cdr 2swap stack-list-len ( list n )
+
+    dup RSP0 + RSP! \ expand return stack to accommodate entries
+
+    ( list n )
+
+    1-  \ initial offset n-1
+    0   \ final offset 0
+    swap
+    do
+        2dup cdr 2swap car drop
+        RSP0 i 1+ + !
+    -1 +loop
+
+    2drop
+;
+
+: restore-continuation-with-arg ( continuation obj -- )
+
+    >R >R \ Store obj on return stack
+
+    2dup >R >R \ Store copy of continuation on return stack
+
+    restore-param-stack
+
+    R> R> \ Pop continuation from return stack
+
+    R> R> \ Pop obj from return stack
+
+    2swap
+
+    false boolean-type \ Add flag signifying continuation restore
+
+    2swap
+
+    restore-return-stack
+;
+
+\ }}}
+
 \ ---- Primitives ---- {{{
 
 : make-primitive ( cfa -- )
@@ -1273,6 +1393,11 @@ parse-idx-stack parse-idx-sp !
         exit
     then
 
+    nextchar [char] ) = if
+        inc-parse-idx
+        except-message: ." unmatched closing parenthesis." recoverable-exception throw
+    then
+
     \ Anything else is parsed as a symbol
     readsymbol charlist>symbol
 
@@ -1286,7 +1411,7 @@ parse-idx-stack parse-idx-sp !
 
 \ }}}
 
-\ ---- Eval ---- {{{
+\ ---- Syntax ---- {{{
 
 : self-evaluating? ( obj -- obj bool )
     boolean-type istype? if true exit then
@@ -1315,95 +1440,6 @@ parse-idx-stack parse-idx-sp !
 : quote-body ( quote-obj -- quote-body-obj )
     cdr car ;
 
-: quasiquote? ( obj -- obj bool )
-    quasiquote-symbol tagged-list? ;
-
-: unquote? ( obj -- obj bool )
-    unquote-symbol tagged-list? ;
-
-: unquote-splicing? ( obj -- obj bool )
-    unquote-splicing-symbol tagged-list? ;
-
-: eval-unquote ( env obj -- res )
-    cdr ( env args )
-
-    nil? if
-        except-message: ." no arguments to unquote." recoverable-exception throw
-    then
-
-    2dup cdr
-    nil? false = if
-        except-message: ." too many arguments to unquote." recoverable-exception throw
-    then
-
-    2drop car 2swap eval
-;
-
-( Create a new list from elements of l1 consed on to l2 )
-: join-lists ( l2 l1 -- l3 )
-    nil? if 2drop exit then
-
-    2dup car
-    -2rot cdr
-    recurse cons
-;
-
-defer eval-quasiquote-item
-: eval-quasiquote-pair ( env obj -- res )
-    2over 2over ( env obj env obj )
-
-    cdr eval-quasiquote-item
-
-    -2rot car ( cdritem env objcar )
-
-    unquote-splicing? if
-        eval-unquote ( cdritems caritem )
-
-        2swap nil? if
-            2drop
-        else
-            2swap join-lists
-        then
-    else
-        eval-quasiquote-item ( cdritems caritem )
-        2swap cons
-    then
-
-;
-
-:noname ( env obj )
-    nil? if
-        2swap 2drop exit
-    then
-
-    unquote? if
-        eval-unquote exit
-    then
-
-    pair-type istype? if
-        eval-quasiquote-pair exit
-    then
-
-    2swap 2drop
-; is eval-quasiquote-item
-
-: eval-quasiquote ( obj env -- res )
-    2swap cdr ( env args )
-
-    nil? if
-        except-message: ." no arguments to quasiquote." recoverable-exception throw
-    then
-
-    2dup cdr ( env args args-cdr )
-    nil? false = if
-        except-message: ." too many arguments to quasiquote." recoverable-exception throw
-    then
-
-    2drop car ( env arg )
-
-    eval-quasiquote-item
-;
-
 : variable? ( obj -- obj bool )
     symbol-type istype? ;
 
@@ -1416,20 +1452,6 @@ defer eval-quasiquote-item
 : definition-val ( obj -- val )
     cdr cdr car ;
 
-: eval-definition ( obj env -- res )
-    2swap
-    2over 2over
-    definition-val 2swap
-    eval
-
-    2swap definition-var 2swap
-
-    2rot
-    define-var
-
-    ok-symbol
-;
-
 : assignment? ( obj -- obj bool )
     set!-symbol tagged-list? ;
 
@@ -1439,20 +1461,6 @@ defer eval-quasiquote-item
 : assignment-val ( obj -- val )
     cdr cdr car ;
 
-: eval-assignment ( obj env -- res )
-    2swap 
-    2over 2over ( env obj env obj )
-    assignment-val 2swap ( env obj valexp env )
-    eval  ( env obj val )
-    
-    2swap assignment-var 2swap ( env var val )
-
-    2rot ( var val env )
-    set-var
-
-    ok-symbol
-;
-
 : macro-definition? ( obj -- obj bool )
     define-macro-symbol tagged-list? ;
 
@@ -1465,22 +1473,6 @@ defer eval-quasiquote-item
 : macro-definition-body ( exp -- body )
     cdr cdr ;
 
-objvar env
-: eval-define-macro ( obj env -- res )
-    env obj!
-
-    2dup macro-definition-name 2swap ( name obj )
-    2dup macro-definition-params 2swap ( name params obj )
-    macro-definition-body ( name params body )
-
-    env obj@ ( name params body env )
-
-    make-macro
-
-    ok-symbol
-;
-hide env
-
 : if? ( obj -- obj bool )
     if-symbol tagged-list? ;
 
@@ -1518,32 +1510,6 @@ hide env
 : lambda-body ( obj -- body )
     cdr cdr ;
 
-: eval-sequence ( explist env -- finalexp env )
-    ( Evaluates all bar the final expressions in
-      an an expression list. The final expression
-      is returned to allow for tail optimization. )
-
-    2swap ( env explist )
-
-    \ Abort on empty list
-    nil? if
-        2drop none
-        2swap exit
-    then
-
-    begin
-        2dup cdr ( env explist nextexplist )
-        nil? false =
-    while
-        -2rot car 2over ( nextexplist env exp env )
-        eval
-        2drop \ discard result
-        2swap ( env nextexplist )
-    repeat
-
-    2drop car 2swap ( finalexp env )
-;
-
 : application? ( obj -- obj bool )
     pair-type istype? ;
 
@@ -1562,18 +1528,6 @@ hide env
 : rest-operands ( operands -- other-operands )
     cdr ;
 
-: list-of-vals ( args env -- vals )
-    2swap
-
-    2dup nooperands? if
-        2swap 2drop
-    else
-        2over 2over first-operand 2swap eval
-        -2rot rest-operands 2swap recurse
-        cons
-    then
-;
-
 : procedure-params ( proc -- params )
     drop pair-type car ;
 
@@ -1618,121 +1572,9 @@ hide env
     2swap
 ;
 
-: apply ( proc argvals -- result )
-        2swap dup case
-            primitive-proc-type of
-                drop execute     
-            endof
-
-            compound-proc-type of
-                2dup procedure-body ( argvals proc body )
-                -2rot 2dup procedure-params ( body argvals proc argnames )
-                -2rot procedure-env ( body argnames argvals procenv )
-
-                -2rot 2swap
-                flatten-proc-args
-                2swap 2rot
-
-                extend-env ( body env )
-
-                eval-sequence
-
-                R> drop ['] eval goto-deferred  \ Tail call optimization
-            endof
-
-            except-message: ." object '" drop print ." ' not applicable." recoverable-exception throw
-        endcase
-;
-
-:noname ( obj env -- result )
-    2swap
-
-    \ --- DEBUG ---
-    ( 
-      fg yellow ." Evaluating: " bold 2dup print reset-term
-      space fg green ." PS: " bold depth . reset-term
-      space fg blue  ." RS: " bold RSP@ RSP0 - . reset-term cr
-    )
-
-    self-evaluating? if
-        2swap 2drop
-        exit
-    then
-
-    quote? if
-        quote-body
-        2swap 2drop
-        exit
-    then
-
-    quasiquote? if
-        2swap eval-quasiquote
-        exit
-    then
-
-    variable? if
-        2swap lookup-var
-        exit
-    then
-
-    definition? if
-        2swap eval-definition
-        exit
-    then
-
-    assignment? if
-        2swap eval-assignment
-        exit
-    then
-
-    macro-definition? if
-        2swap eval-define-macro
-        exit
-    then
-
-    if? if
-        2over 2over
-        if-predicate
-        2swap eval 
-
-        true? if
-            if-consequent
-        else
-            if-alternative
-        then
-
-        2swap
-        ['] eval goto-deferred
-    then
-
-    lambda? if
-        2dup lambda-parameters
-        2swap lambda-body
-        2rot make-procedure
-        exit
-    then
-
-    application? if
-
-        2over 2over ( env exp env exp )
-        operator ( env exp env opname )
-
-        2swap eval ( env exp proc )
-
-        -2rot ( proc env exp )
-        operands 2swap ( proc operands env )
-        list-of-vals ( proc argvals )
-
-        apply
-        exit
-    then
-
-    except-message: ." tried to evaluate object with unknown type." recoverable-exception throw
-; is eval
-
 \ }}}
 
-\ ---- Analyze ----
+\ ---- Analyze ---- {{{
 
 : evaluate-eproc ( eproc env --- res )
 
@@ -1752,8 +1594,6 @@ hide env
     \ Final element of eproc list is primitive procedure
     drop \ dump type signifier
 
-    .s cr \ DEBUG
-
     goto \ jump straight to primitive procedure (executor)
 ;
 
@@ -1813,28 +1653,6 @@ hide env
     nil cons cons cons
 ;
 
-: if-executor ( cproc aproc pproc env -- res )
-    2swap 2over ( cproc aproc env pproc env -- res )
-    evaluate-eproc
-
-    true? if
-        2swap 2drop
-    else
-        2rot 2drop
-    then
-
-    evaluate-eproc
-;
-
-: analyze-if ( exp -- eproc )
-    2dup if-predicate analyze
-    2swap 2dup if-consequent analyze
-    2swap if-alternative analyze
-
-    ['] if-executor primitive-proc-type
-    nil cons cons cons cons
-;
-
 : sequence-executor ( eproc-list env -- res )
     2swap
 
@@ -1842,7 +1660,6 @@ hide env
         2dup cdr ( env elist elist-rest)
         nil? invert
     while
-
         -2rot car 2over ( elist-rest env elist-head env )
         evaluate-eproc  ( elist-rest env head-res )
         2drop 2swap     ( env elist-rest )
@@ -1868,6 +1685,42 @@ hide env
     nil cons cons
 ;
 
+
+: macro-definition-executor  ( name params bproc env -- ok )
+    make-macro ok-symbol
+;
+
+: analyze-macro-definition ( exp -- eproc )
+    2dup macro-definition-name
+    2swap 2dup macro-definition-params
+    2swap macro-definition-body analyze-sequence
+
+    ['] macro-definition-executor primitive-proc-type
+    nil cons cons cons cons
+;
+
+: if-executor ( cproc aproc pproc env -- res )
+    2swap 2over ( cproc aproc env pproc env -- res )
+    evaluate-eproc
+
+    true? if
+        2swap 2drop
+    else
+        2rot 2drop
+    then
+
+    ['] evaluate-eproc goto
+;
+
+: analyze-if ( exp -- eproc )
+    2dup if-consequent analyze
+    2swap 2dup if-alternative analyze
+    2swap if-predicate analyze
+
+    ['] if-executor primitive-proc-type
+    nil cons cons cons cons
+;
+
 : lambda-executor ( params bproc env -- res )
     make-procedure
     ( Although this is packaged up as a regular compound procedure,
@@ -1908,22 +1761,14 @@ hide env
     then
 ;
 
-: application-executor ( operator-proc arg-procs env -- res )
-    2rot 2over ( aprocs env fproc env )
-    evaluate-eproc ( aprocs env proc )
-
-    -2rot 2swap ( proc env aprocs )
-    evaluate-operand-eprocs ( proc vals )
-
-    2swap ( vals proc )
-    
+: apply ( vals proc )
     dup case
         primitive-proc-type of
             drop execute
         endof
 
         compound-proc-type of
-                2dup procedure-body ( argvals proc body )
+                2dup procedure-body ( argvals proc bproc )
                 -2rot 2dup procedure-params ( bproc argvals proc argnames )
                 -2rot procedure-env ( bproc argnames argvals procenv )
 
@@ -1936,10 +1781,41 @@ hide env
                ['] evaluate-eproc goto
         endof
 
+        continuation-type of
+            2swap
+            nil? if
+                except-message: ." Continuations expect exactly 1 argument."
+                recoverable-exception throw
+            then
+
+            2dup cdr
+
+            nil? invert if
+                except-message: ." Continuations expect exactly 1 argument."
+                recoverable-exception throw
+            then
+
+            2drop car
+                
+            restore-continuation-with-arg
+        endof
+
         except-message: ." object '" drop print ." ' not applicable." recoverable-exception throw
     endcase
 ;
 
+: application-executor ( operator-proc arg-procs env -- res )
+    2rot 2over ( aprocs env fproc env )
+    evaluate-eproc ( aprocs env proc )
+
+    -2rot 2swap ( proc env aprocs )
+    evaluate-operand-eprocs ( proc vals )
+
+    2swap ( vals proc )
+
+    ['] apply goto
+;
+
 : analyze-application ( exp -- eproc )
     2dup operator analyze
     2swap operands operand-eproc-list
@@ -1950,74 +1826,54 @@ hide env
 
 :noname ( exp --- eproc )
 
-    self-evaluating? if
-        analyze-self-evaluating
-        exit
-    then
+    self-evaluating? if analyze-self-evaluating exit then
 
-    quote? if
-        analyze-quoted
-        exit
-    then
+    quote? if analyze-quoted exit then
     
-    variable? if
-        analyze-variable
-        exit
-    then
+    variable? if analyze-variable exit then
 
-    definition? if
-        analyze-definition
-        exit
-    then
+    definition? if analyze-definition exit then
 
-    assignment? if
-        analyze-assignment
-        exit
-    then
+    assignment? if analyze-assignment exit then
 
-    if? if
-        analyze-if
-        exit
-    then
+    macro-definition? if analyze-macro-definition exit then
 
-    lambda? if
-        analyze-lambda
-        exit
-    then
+    if? if analyze-if exit then
 
-    application? if
-        analyze-application
-        exit
-    then
+    lambda? if analyze-lambda exit then
+
+    application? if analyze-application exit then
 
-    
     except-message: ." tried to analyze unknown expression type." recoverable-exception throw
 
 ; is analyze
 
+\ }}}
 
 \ ---- Macro Expansion ---- {{{
 
 ( Simply evaluates the given procedure with expbody as its argument. )
 : macro-eval ( proc expbody -- result )
     2swap
-    2dup procedure-body ( expbody proc procbody )
-    -2rot 2dup procedure-params ( procbody expbody proc argnames )
-    -2rot procedure-env ( procbody argnames expbody procenv )
+    2dup procedure-body ( expbody proc bproc )
+    -2rot 2dup procedure-params ( bproc expbody proc argnames )
+    -2rot procedure-env ( bproc argnames expbody procenv )
     
     -2rot 2swap
     flatten-proc-args
     2swap 2rot
 
-    extend-env eval-sequence eval
+    extend-env ( bproc env )
+
+    ['] evaluate-eproc goto
 ;
 
 : expand-macro ( exp -- result )
     pair-type istype? invert if exit then
+
     2dup car symbol-type istype? invert if 2drop exit then
-    
-    lookup-macro nil? if
-        2drop exit then
+
+    lookup-macro nil? if 2drop exit then
 
     2over cdr macro-eval
 
@@ -2030,33 +1886,6 @@ hide env
     R> drop ['] expand goto-deferred
 ;
 
-: expand-quasiquote-item ( exp -- result )
-    nil? if exit then
-
-    unquote? if
-        unquote-symbol 2swap cdr car expand nil cons cons
-        exit
-    then
-
-    unquote-splicing? if
-        unquote-splicing-symbol 2swap cdr car expand nil cons cons
-        exit
-    then
-    
-    pair-type istype? if
-        2dup car recurse
-        2swap cdr recurse
-        cons
-    then
-;
-
-: expand-quasiquote ( exp -- result )
-    quasiquote-symbol 2swap cdr
-
-    expand-quasiquote-item
-
-    cons ;
-
 : expand-definition ( exp -- result )
     define-symbol 2swap
 
@@ -2126,8 +1955,6 @@ hide env
 
     quote? if exit then
 
-    quasiquote? if expand-quasiquote exit then
-
     definition? if expand-definition exit then
 
     assignment? if expand-assignment exit then
@@ -2144,6 +1971,10 @@ hide env
 
 \ }}}
 
+:noname ( exp env -- res )
+    2swap expand analyze 2swap evaluate-eproc
+; is eval
+
 \ ---- Print ---- {{{
 
 : printfixnum ( fixnum -- ) drop 0 .R ;
@@ -2217,6 +2048,9 @@ hide env
 : printcomp ( primobj -- )
     2drop ." <compound procedure>" ;
 
+: printcont ( primobj --)
+    2drop ." <continuation>" ;
+
 : printnone ( noneobj -- )
     2drop ." Unspecified return value" ;
 
@@ -2235,6 +2069,7 @@ hide env
     pair-type istype? if ." (" printpair ." )" exit then
     primitive-proc-type istype? if printprim exit then
     compound-proc-type istype? if printcomp exit then
+    continuation-type istype? if printcont exit then
     none-type istype? if printnone exit then
     port-type istype? if printport exit then
 
@@ -2245,20 +2080,20 @@ hide env
 
 \ ---- Garbage Collection ---- {{{
 
-variable gc-enabled
-false gc-enabled !
+( Notes on garbage collection:
+  This is a mark-sweep garbage collector, invoked by cons.
+  The roots of the object tree used by the marking routine 
+  include all objects in the parameter stack, and several
+  other fixed roots such as global-env, symbol-table, macro-table,
+  and the console-i/o-port.
 
-variable gc-stack-depth
+  NO OTHER OBJECTS WILL BE MARKED!
 
-: enable-gc
-    depth gc-stack-depth !
-    true gc-enabled ! ;
+  This places implicit restrictions on when cons can be invoked.
+  Invoking cons when live objects are stored on the return stack
+  or in other variables than the above will result in possible
+  memory corruption if the cons triggers the GC. )
 
-: disable-gc
-    false gc-enabled ! ;
-
-: gc-enabled?
-    gc-enabled @ ;
 
 : pairlike? ( obj -- obj bool )
     pair-type istype? if true exit then
@@ -2266,6 +2101,7 @@ variable gc-stack-depth
     symbol-type istype? if true exit then
     compound-proc-type istype? if true exit then
     port-type istype? if true exit then
+    continuation-type istype? if true exit then
 
     false
 ;
@@ -2335,7 +2171,7 @@ variable gc-stack-depth
     console-i/o-port obj@ gc-mark-obj
     global-env obj@ gc-mark-obj
 
-    depth gc-stack-depth @ do
+    depth object-stack-base @ do
         PSP0 i + 1 + @
         PSP0 i + 2 + @
 
@@ -2359,8 +2195,14 @@ variable gc-stack-depth
     ok-symbol ( port res )
 
     begin
+        \ DEBUG
+        \ bold fg blue ." READ from " 2over drop . ." ==> " reset-term
+
         2over read-port ( port res obj )
 
+        \ DEBUG
+        \ 2dup print cr
+
         2dup EOF character-type objeq? if
             2drop 2swap close-port
             exit
@@ -2368,8 +2210,6 @@ variable gc-stack-depth
 
         2swap 2drop ( port obj )
 
-        expand
-
         global-env obj@ eval ( port res )
     again
 ;
@@ -2380,7 +2220,8 @@ variable gc-stack-depth
 
     include scheme-primitives.4th
 
-    \ s" scheme-library.scm" load 2drop
+    init-object-stack-base
+    s" scheme-library.scm" load 2drop
     
 \ }}}
 
@@ -2398,8 +2239,6 @@ variable gc-stack-depth
         true exit
     then
 
-    expand
-
     global-env obj@ eval
 
     fg cyan ." ; " print reset-term
@@ -2410,10 +2249,10 @@ variable gc-stack-depth
 : repl
     empty-parse-str
 
-    enable-gc
+    init-object-stack-base
 
     \ Display welcome message
-    \ welcome-symbol nil cons global-env obj@ eval 2drop
+    welcome-symbol nil cons global-env obj@ eval 2drop
 
     begin
         ['] repl-body catch