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\begin{center}
MASSACHVSETTS INSTITVTE OF TECHNOLOGY \\
Department of Electrical Engineering and Computer Science \\
6.001---Structure and Interpretation of Computer Programs \\
#1 \\
\medskip
{\bf #2} \\
{\bf #3}
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\begin{document}
%\twocolumn
\begin{landscape}
\mypsetheader{Spring 2004}{Recitation 26}{Explicit Control Eval}

\begin{multicols}{2}

{\small
\begin{verbatim}
read-eval-print-loop
  (perform (op initialize-stack))
  (perform
   (op prompt-for-input) (const ";;; EC-Eval input:"))
  (assign exp (op read))
  (assign env (op get-global-environment))
  (assign continue (label print-result))
  (goto (label eval-dispatch))
print-result
  (perform (op print-stack-statistics))
  (perform
   (op announce-output) (const ";;; EC-Eval value:"))
  (perform (op user-print) (reg val))
  (goto (label read-eval-print-loop))

unknown-expression-type
  (assign val (const unknown-expression-type-error))
  (goto (label signal-error))
unknown-procedure-type
  (restore continue)
  (assign val (const unknown-procedure-type-error))
  (goto (label signal-error))

signal-error
  (perform (op user-print) (reg val))
  (goto (label read-eval-print-loop))

eval-dispatch
  (test (op self-evaluating?) (reg exp))
  (branch (label ev-self-eval))
  (test (op variable?) (reg exp))
  (branch (label ev-variable))
  (test (op quoted?) (reg exp))
  (branch (label ev-quoted))
  (test (op assignment?) (reg exp))
  (branch (label ev-assignment))
  (test (op definition?) (reg exp))
  (branch (label ev-definition))
  (test (op if?) (reg exp))
  (branch (label ev-if))
  (test (op lambda?) (reg exp))
  (branch (label ev-lambda))
  (test (op begin?) (reg exp))
  (branch (label ev-begin))
  (test (op application?) (reg exp))
  (branch (label ev-application))
  (goto (label unknown-expression-type))

ev-self-eval
  (assign val (reg exp))
  (goto (reg continue))
ev-variable
  (assign val (op lookup-variable-value) (reg exp) (reg env))
  (goto (reg continue))
ev-quoted
  (assign val (op text-of-quotation) (reg exp))
  (goto (reg continue))
ev-lambda
  (assign unev (op lambda-parameters) (reg exp))
  (assign exp (op lambda-body) (reg exp))
  (assign val (op make-procedure)
              (reg unev) (reg exp) (reg env))
  (goto (reg continue))

ev-application
  (save continue)
  (save env)
  (assign unev (op operands) (reg exp))
  (save unev)
  (assign exp (op operator) (reg exp))
  (assign continue (label ev-appl-did-operator))
  (goto (label eval-dispatch))
ev-appl-did-operator
  (restore unev)
  (restore env)
  (assign argl (op empty-arglist))
  (assign proc (reg val))
  (test (op no-operands?) (reg unev))
  (branch (label apply-dispatch))
  (save proc)
ev-appl-operand-loop
  (save argl)
  (assign exp (op first-operand) (reg unev))
  (test (op last-operand?) (reg unev))
  (branch (label ev-appl-last-arg))
  (save env)
  (save unev)
  (assign continue (label ev-appl-accumulate-arg))
  (goto (label eval-dispatch))
ev-appl-accumulate-arg
  (restore unev)
  (restore env)
  (restore argl)
  (assign argl (op adjoin-arg) (reg val) (reg argl))
  (assign unev (op rest-operands) (reg unev))
  (goto (label ev-appl-operand-loop))

ev-appl-last-arg
  (assign continue (label ev-appl-accum-last-arg))
  (goto (label eval-dispatch))
ev-appl-accum-last-arg
  (restore argl)
  (assign argl (op adjoin-arg) (reg val) (reg argl))
  (restore proc)
  (goto (label apply-dispatch))
apply-dispatch
  (test (op primitive-procedure?) (reg proc))
  (branch (label primitive-apply))
  (test (op compound-procedure?) (reg proc))  
  (branch (label compound-apply))
  (goto (label unknown-procedure-type))

primitive-apply
  (assign val (op apply-primitive-procedure)
              (reg proc)
              (reg argl))
  (restore continue)
  (goto (reg continue))

compound-apply
  (assign unev (op procedure-parameters) (reg proc))
  (assign env (op procedure-environment) (reg proc))
  (assign env (op extend-environment)
              (reg unev) (reg argl) (reg env))
  (assign unev (op procedure-body) (reg proc))
  (goto (label ev-sequence))

ev-begin
  (assign unev (op begin-actions) (reg exp))
  (save continue)
  (goto (label ev-sequence))

ev-sequence
  (assign exp (op first-exp) (reg unev))
  (test (op last-exp?) (reg unev))
  (branch (label ev-sequence-last-exp))
  (save unev)
  (save env)
  (assign continue (label ev-sequence-continue))
  (goto (label eval-dispatch))
ev-sequence-continue
  (restore env)
  (restore unev)
  (assign unev (op rest-exps) (reg unev))
  (goto (label ev-sequence))
ev-sequence-last-exp
  (restore continue)
  (goto (label eval-dispatch))

ev-assignment
  (assign unev (op assignment-variable) (reg exp))
  (save unev)
  (assign exp (op assignment-value) (reg exp))
  (save env)
  (save continue)
  (assign continue (label ev-assignment-1))
  (goto (label eval-dispatch))
ev-assignment-1
  (restore continue)
  (restore env)
  (restore unev)
  (perform
   (op set-variable-value!) (reg unev) (reg val) (reg env))
  (assign val (const ok))
  (goto (reg continue))

ev-definition
  (assign unev (op definition-variable) (reg exp))
  (save unev)
  (assign exp (op definition-value) (reg exp))
  (save env)
  (save continue)
  (assign continue (label ev-definition-1))
  (goto (label eval-dispatch))
ev-definition-1
  (restore continue)
  (restore env)
  (restore unev)
  (perform
   (op define-variable!) (reg unev) (reg val) (reg env))
  (assign val (const ok))
  (goto (reg continue))

ev-if
\end{verbatim}}
%%   (save exp)
%%   (save env)
%%   (save continue)
%%   (assign continue (label ev-if-decide))
%%   (assign exp (op if-predicate) (reg exp))
%%   (goto (label eval-dispatch))
%% ev-if-decide
%%   (restore continue)
%%   (restore env)
%%   (restore exp)
%%   (test (op true?) (reg val))
%%   (branch (label ev-if-consequent))
%%  ev-if-alternative
%%   (assign exp (op if-alternative) (reg exp))
%%   (goto (label eval-dispatch))
%% ev-if-consequent
%%   (assign exp (op if-consequent) (reg exp))
%%   (goto (label eval-dispatch))
%% \end{verbatim}}

%% \newpage
%% \section*{Adding let}

%% {\small
%% \begin{verbatim}
%% ev-let
%%       (save continue)
%%       ___________
%%       (assign argl (op empty-arglist))
%%       (assign proc (op binding-names) (reg exp))
%%       __________
%%       (assign unev (op binding-values) (reg exp))
%% ev-let-operand-loop
%%       (save argl)
%%       (assign exp (op first-binding) (reg unev))
%%       (test (op last-operand?) (reg unev))
%%       (branch (label ev-let-last-binding))
%%       (save env)
%%       (save unev)
%%       (assign continue (label ev-let-accumulate-binding))
%%       (goto (label eval-dispatch))

%% ev-let-accumulate-binding
%%       (restore unev)
%%       (restore env)
%%       (restore argl)
%%       (assign argl (op adjoin-arg) (reg val) (reg argl))
%%       (assign unev (op rest-operands) (reg unev))
%%       (goto (label ev-let-operand-loop))

%% ev-let-last-binding
%%       (assign continue (label ev-let-accum-last-binding))
%%       (goto (label eval-dispatch))
%%    ev-let-accum-last-binding
%%       (restore argl)
%%       (assign argl (op adjoin-arg) (reg val) (reg argl))
%%       (restore proc)
%%       (goto (label ev-final-let))

%% ev-final-let
%%       (restore exp)
%%       (assign env (op extend-environment)
%%                   ___________________________
%%       (assign unev (op let-body) ___________)
%%       (goto (label ev-sequence))
%% \end{verbatim}}

%% \begin{enumerate}
%% \item By reference to the rest of EC-eval, and knowledge of the
%% desugaring of let, where was most of this code appropriated from? (It
%% was copied with label modification from eceval)
%% \item Fill in the blanks.
%% \item There's a bug in this code, can you spot it?  (Hint: it has to
%% do with contracts).
%% \end{enumerate}

\end{multicols}
\end{landscape}
\end{document}
