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Continuation-Passing Interpreters
Gul Agha CS 421 Spring 2006
4/19/2006 CS 421 Spring 2006 2
Representing and Manipulating
Control Contexts
- Let interpreter explicitly pass continuations
- Add language facilities to manipulate continuations: - Exceptions - First-class continuations - Threads
- Add a new parameter to eval-expression
- expression, environment, continuation
4/19/2006 CS 421 Spring 2006 3
Review
- Environment passing interpreters
- Simple expressions
- Recursive definitions
- Conditional expressions
- Primitive applications
- Procedure applications
- Let expressions
- Assignments 4/19/2006 CS 421 Spring 2006 4
Environment Passing Interpreter
(define eval-expression (lambda (exp env) (cases expression exp (lit-exp (datum) datum ) (var-exp (id) ..) (proc-exp (id body) … ) (letrec-exp (proc-names idss bodies letrec-body) ..) (if-exp (test-exp true-exp false-exp) … ) (primapp-exp (prim rands) … ) (app-exp (rator rands) …) (let-exp (ids rands body) …) (varassign-exp (id rhs-exp) …))))
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Evaluating simple expressions
(define eval-expression (lambda (exp env) (cases expression exp (lit-exp (datum) datum ) (var-exp (id) (apply-env env id)) (proc-exp (ids body) (closure ids body env)))))
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Evaluating Recursive Definitions
(define eval-expression (lambda (exp env) (cases expression exp (letrec-exp (proc-names idss bodies letrec-body) (eval-expression letrec-body (extend-env-recursively proc-names idss bodies env)))
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Evaluating Conditional Expressions
(if-exp (test-exp true-exp false-exp) (if (true-value? (eval-expression test-exp env)) (eval-expression true-exp env) (eval-expression false-exp env)))
4/19/2006 CS 421 Spring 2006 8
Evaluating Primitive Applications
(define eval-expression (lambda (exp env) (cases expression exp (primapp-exp (prim rands) (let ((args (eval-rands rands env))) (apply-primitive prim args)) ))))
4/19/2006 CS 421 Spring 2006 9
Evaluating Procedure
Applications
(app-exp (rator rands) (let ((proc (eval-expression rator env)) (args (eval-rands rands env))) (if (procval? proc) (apply-procval proc args) (eopl:error 'eval-expression "Attempt to apply non-procedure ~s" proc)))) 4/19/2006 CS 421 Spring 2006 10
Evaluating Let Expressions
- Retain call-by-value behavior for let expressions (let-exp (ids rands body) (let ((args (eval-let-exp-rands rands env))) (eval-expression body (extend-env ids args env))))
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Evaluating Assignments
(varassign-exp (id rhs-exp) (begin (setref! (apply-env-ref env id) (eval-expression rhs-exp env)) 1))
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Continuation Passing Interpreter
- Add continuations
- Pass continuation to each expression
- What about top level continuation?
- Read-eval-print loop
- Introduce a continuation to which the result of evaluating the initial (main body) expression goes
- This is the “final” continuation…
- What should this continuation do?
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Recursive Procedure Definitions
- Creates a new environment without calling eval-expression
- Evaluates the body in the new environment
- Returns the value of the body
- Control context of the body is the same is that of the entire expression
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Recursive procedures
(letrec-exp (proc-names idss bodies letrec-body) (eval-expression letrec-body (extend-env-recursively proc-names idss bodies env) cont)) ;not halt continuation
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Evaluation of Conditionals
(if-exp (test-exp true-exp false-exp) (eval-expression test-exp env (test-cont true-exp false-exp env cont)))
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Continuation for Tests
(define test-cont (lambda (true-exp false-exp env cont) (lambda (val) (if (true-value? val) (eval-expression true-exp env cont) (eval-expression false-exp env cont)
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Applying Continuations
(define apply-cont
(lambda (cont v)
(cont v)))
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Data Structure Representation of
Continuations
(define-datatype continuation continuation? (halt-cont) (test-cont (true-exp expression?) (false-exp expression?) (env environment?) (cont continuation?)))
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Applying Continuations in
Records
(define apply-cont (lambda (cont val) (cases continuation cont (halt-cont ( ) (begin (write val) (newline)) … )
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Applying Continuation (contd.)
(test-cont (true-exp false-exp env cont) (if (true-value? val) (eval-expression true-exp env cont) (eval-expression false-exp env cont)))
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Evaluating Local Bindings
(let-exp (ids rands body) (eval-rands rands env (let-exp-cont ids env body cont)))
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Continuations for Let
Expressions
(define-datatype continuation continuation? …… (let-exp-cont (ids (list-of symbol?)) (env environment?) (body expression?) (cont continuation?)) … )
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Applying Let Continuations
(define apply-cont (lambda (cont val) (cases continuation cont … (let-exp-cont (ids env body cont) (let ((new-env (extend-env ids val env))) (eval-expression body new-env cont))) … )))
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Primitive Operations
(primapp-exp (prim rands) (eval-rands rands env (prim-args-cont prim cont)))
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Procedures
(define-datatype procval procval? (closure (ids (list-of symbol?)) (body expression?) (env environment?)))
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Applying Procedures
(define apply-procval (lambda (proc args cont) (cases procval proc (closure (ids body env) (eval-expression body (extend-env ids args env) cont)))))
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Primitive Arguments
Continuation
(define-datatype continuation continuation? … (prim-args-cont (prim primitive?) (cont continuation?)) … ) (define apply-cont (lambda (cont val) … (prim-args-cont (prim cont) (let ((args val)) (apply-cont cont (apply-primitive prim args))))..) 4/19/2006 CS 421 Spring 2006 40
An Imperative Interpreter
- Shared variables in place of bindings (eval-expression exp env cont) (eval-rands rands env cont) (apply-procval proc args cont) (apply-cont cont val)
- Add seven global registers
- Use assignments to the register
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Exception Handling
Æ handle Æ raise
- Evaluate the second expression and install it as an exception handler_._
- If an exception is raised invoke the most recent exception handler. 4/19/2006 CS 421 Spring 2006 42
Record Representation for
Exceptions AST
try-exp (body-exp handler-exp) raise-exp (exp)
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Example
- list-index numbers returns the first occurrence of a given number in a list of letrec index (n,1) = if null?(1) then sub1 (0) else if equal? (n, car(l)) then 0 else let p = (index n cdr(l)) in if equal?(p, sub1(0)) then sub1(0) else add1(p) in … 4/19/2006 CS 421 Spring 2006 44
List Index using Exception Handling
let index = proc (n, 1) letrec loop ( l ) = if null? (1) then raise sub1 (0) else if equal? (n, car(l)) then 0 else let add1 ((loop cdr(l))) in try (loop l) handle proc (x) x in …
4/19/2006 CS 421 Spring 2006 45
Implementing Exception Handling
- Add two new continuation handlers (handler-cont (body expression?) (env environment?) (cont continuation?)) (try-cont (handler expval?) (cont continuation?)) 4/19/2006 CS 421 Spring 2006 46
Applying Continuations
(try-exp (body-exp handler-exp) (eval-expression handler-exp env (handler-cont body-exp env cont))) (apply-cont (handler-cont body-exp env cont) handler-val) = (if (procval? handler-val) (eval-expression body-exp env (try-cont handler-val cont)) (eopl:error ‘eval-exp “Error handler not procedure …))
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Normal Execution
- If the body returns normally: (apply-cont (try-cont handler cont) val) = (apply-cont cont val)
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Exception Raised
- If an exception is raised, search through the continuation for the nearest handler. In eval-expression: (raise-exp (exp) (eval-expression exp env (raise-cont cont))) In continuations: (apply-cont (raise-cont cont) val) = (find-handler val cont)
