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May 31st, 2019 - written by Kimserey with .

Last week we implemented a metacircular evaluator in Racket, evaluating a subset of Racket language. We saw that the main functions part of an evaluator are `eval`

evaluating the meaning of an expression and `apply`

, applying a procedure to arguments. We wrote the evaluator following the *order* of execution of arguments and procedures from Lisp. Lisp being an *applicative-order* language, when provided a procedure with parameters, the parameters get evaluated before the procedure is applied. As opposed to *normal-order* languages, also called *lazy* languages, which delay the evaluation of arguments until the result of the procedure application is required.
Today we will see the implication of this slight change in order of application by modifying our evaluator created last week, transforming it into a lazy evaluator.

*This post is based on our previous post where we built an evaluator for Racket. It is highly recommended to start by reading the previous post.*

Lisp being an *applicative order* language, arguments gets evaluated before the procedure is applied to them. The following example showcases the possibilities offered by a lazy language.

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(define (do-nothing a b) 'nothing)

We define `do-nothing`

, a procedure which wastefully take `a`

and `b`

and does nothing with it.

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(do-nothing (/ 1 0) 'b)

Executing this will yield a division by zero error. `(/ 1 0)`

being passed as argument is evaluated in order to apply `do-nothing`

procedure.

The reason why arguments are evaluated first lies in the implementation of the evaluator. When the compound procedure is applied, each operands are evaluated, mapped to their variables, and save in an environment frame (more on last week post).

As opposed to applicative order, *normal order* languages, also called *lazy* languages, only evaluate the parameters when needed. Instead of saving the computed value of the arguments, a *thunk* gets created and saved as value.

Thunks are delayed evaluation of arguments.

In order to support lazy evaluation, we start by changing `eval`

and `apply-local`

. The evaluation of procedure arguments used to occur when an `application`

was found, therefore instead of previously extracting the list of operands, we now then directly pass the operands to the `apply`

.

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; From eval
[(application? exp)
(apply-local
(eval (operator exp) env)
(list-of-values
(operands exp)
env))]

`List-of-values`

used to evaluate each value and `cons`

‘ing them together.

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; From updated lazy eval
[(application? exp)
(apply-local
(actual-value (operator exp) env)
(operands exp)
env)]

Instead of that, we will now simply pass the operands as expressions to `apply-local`

together with the environment. We also introduced `actual-value`

which abstracts away the `eval`

of the operator.

We then change `apply-local`

to take the environment as argument.

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(define (apply-local procedure arguments env)
(cond [(primitive-procedure? procedure)
(apply-primitive-procedure
procedure
(list-of-args-values arguments env))] ; <= For primitive procedures, we evaluate directly the arguments
[(compound-procedure? procedure)
(eval-sequence
(procedure-body procedure)
(extend-environment
(procedure-parameters procedure)
(list-of-delayed-args arguments env) ; <= For compount procedures, we delay arguments
(procedure-environment procedure)))]
[else
(error "Unknown procedure type: APPLY" procedure)]))

And we modified the primitive procedure arguments to use `list-of-args-values`

to evaluate the argument expressions directly and we use `list-of-delayed-args`

to delay the evaluation of argument expressions introducing the laziness.

We also need to modify the `eval-if`

,

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(define (eval-if exp env)
(if (true? (actual-value (if-predicate exp) env))
(eval (if-consequent exp) env)
(eval (if-alternative exp) env)))

where we have to use `actual-value`

to ensure evaluation of the predicate, whether value or thunk.
We then define `actual-value`

, `list-of-args-values`

and `list-of-delayed-args`

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(define (actual-value exp env)
(force-it (eval exp env)))
(define (list-of-args-values exps env)
(if (no-operands? exps)
'()
(mcons (actual-value
(first-operand exps)
env)
(list-of-args-values
(rest-operands exps)
env))))
(define (list-of-delayed-args exps env)
(if (no-operands? exps)
'()
(mcons (delay-it
(first-operand exps)
env)
(list-of-delayed-args
(rest-operands exps)
env))))

`actual-value`

calls `force-it`

which forces the execution of the evaluation. `List-of-args-values`

recursively forces the execution of each expression part of the list provided and `list-of-delayed-args`

transforms each expression part of the list provided to a delayed evaluation, a *thunk*.

`Delay-it`

is implemented by taking the expression and the environment and tagging them with a tag `'thunk`

.

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(define (delay-it exp env)
(mlist 'thunk exp env))

Then we can create a `thunk?`

predicate looking for the `'thunk`

tag.

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(define (thunk? obj) (tagged-list? obj 'thunk))
(define (thunk-exp thunk) (mcar (mcdr thunk)))
(define (thunk-env thunk) (mcar (mcdr (mcdr thunk))))

And we also create selectors for the expression and the environment. Lastly we need to define `force-it`

, which simply evaluates any thunk found and once evaluated, replaces the thunk expression and environment by the resulting value.

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(define (force-it obj)
(cond
[(thunk? obj)
(let ([result (actual-value (thunk-exp obj) (thunk-env obj))])
(set-mcar! obj 'evaluated-thunk)
(set-mcar! (mcdr obj) result)
(set-mcdr! (mcdr obj) '())
result)]
[(evaluated-thunk? obj)
(thunk-value obj)]
[else obj]))
(define (evaluated-thunk? obj)
(tagged-list? obj 'evaluated-thunk))
(define (thunk-value evaluated-thunk)
(mcar (mcdr evaluated-thunk)))

If the object provided was already evaluated, we directly return the result of the evaluated thunk. *This method of caching result precalculated is called memoization, the concept is explained in my previous post on dynamic programming.*

Once we have modified `eval`

and `apply`

, our evaluator will now be completely lazy. Therefore defining the same procedure `do-nothing`

as we did previously,

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(define (do-nothing a b) 'nothing)

will no longer yield a division by zero exception.

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(do-nothing (/ 1 0) 'b)

Instead it will delay the parameters until necessary, and since none of the paremeters are used, will return directly `'nothing`

.

Today we explored the difference between Applicative Order and Normal Order languages. We saw how an Applicative Order language could be turned into a Normal Order language by changing how parameters of application are evaluated and hence providing a functionality built into the language. Normal Order languages are also known as lazy languages where Lisp is a applicative order language. I hope you liked this post and I see you on the next one!

Compared to our previous implementation of the evaluator, we have to change all `list`

and `cons`

to their mutable equivalent `mlist`

and `mcons`

. The following source code contains the full code implementing the lazy evaluator:

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(require compatibility/mlist)
(define (eval exp env)
(cond [(self-evaluating? exp)
exp]
[(variable? exp)
(lookup-variable-value exp env)]
[(quoted? exp)
(text-of-quotation exp)]
[(assignment? exp)
(eval-assignment exp env)]
[(definition? exp)
(eval-definition exp env)]
[(if? exp)
(eval-if exp env)]
[(lambda? exp)
(make-procedure
(lambda-parameters exp)
(lambda-body exp)
env)]
[(begin? exp)
(eval-sequence
(begin-actions exp)
env)]
[(cond? exp)
(eval (cond->if exp) env)]
[(application? exp)
(apply-local
(actual-value (operator exp) env)
(operands exp) ; Instead of evaluating the operands, we pass them directly to apply-local
env)] ; Apply-local needs the environment as the delayed operands will need to be evaluated when needed
[else
(error "Unknown expression type: EVAL" exp)]))
(define (apply-local procedure arguments env)
(cond [(primitive-procedure? procedure)
(apply-primitive-procedure
procedure
(list-of-args-values arguments env))] ; For primitive procedures, we evaluate directly as applicative-order
[(compound-procedure? procedure)
(eval-sequence
(procedure-body procedure)
(extend-environment
(procedure-parameters procedure)
(list-of-delayed-args arguments env) ; All arguments are delayed
(procedure-environment procedure)))]
[else
(error "Unknown procedure type: APPLY" procedure)]))
(define (actual-value exp env)
(force-it (eval exp env)))
(define (list-of-args-values exps env)
(if (no-operands? exps)
'()
(mcons (actual-value
(first-operand exps)
env)
(list-of-args-values
(rest-operands exps)
env))))
(define (list-of-delayed-args exps env)
(if (no-operands? exps)
'()
(mcons (delay-it
(first-operand exps)
env)
(list-of-delayed-args
(rest-operands exps)
env))))
(define (delay-it exp env)
(mlist 'thunk exp env))
(define (thunk? obj) (tagged-list? obj 'thunk))
(define (thunk-exp thunk) (mcar (mcdr thunk)))
(define (thunk-env thunk) (mcar (mcdr (mcdr thunk))))
(define (evaluated-thunk? obj)
(tagged-list? obj 'evaluated-thunk))
(define (thunk-value evaluated-thunk)
(mcar (mcdr evaluated-thunk)))
(define (force-it obj)
(cond
[(thunk? obj)
(let ([result (actual-value (thunk-exp obj) (thunk-env obj))])
(set-mcar! obj 'evaluated-thunk)
(set-mcar! (mcdr obj) result)
(set-mcdr! (mcdr obj) '())
result)]
[(evaluated-thunk? obj)
(thunk-value obj)]
[else obj]))
(define (eval-if exp env)
(if (true? (actual-value (if-predicate exp) env))
(eval (if-consequent exp) env)
(eval (if-alternative exp) env)))
(define (eval-sequence exps env)
(cond [(last-exp? exps)
(eval (first-exp exps) env)]
[else
(eval (first-exp exps) env)
(eval-sequence (rest-exps exps) env)]))
(define (eval-assignment exp env)
(set-variable-value!
(assignment-variable exp)
(eval (assignment-value exp) env)
env)
'ok)
(define (eval-definition exp env)
(define-variable!
(definition-variable exp)
(eval (definition-value exp) env)
env)
'ok)
; Numbers and strings self evaluate
(define (self-evaluating? exp)
(cond [(number? exp) true]
[(string? exp) true]
[else false]))
(define (variable? exp)
(symbol? exp))
(define (tagged-list? exp tag)
(if (mpair? exp)
(eq? (mcar exp) tag)
false))
(define (quoted? exp)
(tagged-list? exp 'quote))
(define (text-of-quotation exp)
(mcar (mcdr exp)))
(define (assignment? exp)
(tagged-list? exp 'set!))
(define (assignment-variable exp)
(mcar (mcdr exp)))
(define (assignment-value exp)
(mcar (mcdr (mcdr exp))))
(define (definition? exp)
(tagged-list? exp 'define))
(define (definition-variable exp)
(if (symbol? (mcar (mcdr exp)))
(mcar (mcdr exp))
(mcar (mcar (mcdr exp)))))
(define (definition-value exp)
(if (symbol? (mcar (mcdr exp)))
(mcar (mcdr (mcdr exp)))
(make-lambda
(mcdr (mcar (mcdr exp)))
(mcdr (mcdr exp)))))
(define (lambda? exp)
(tagged-list? exp 'lambda))
(define (lambda-parameters exp)
(mcar (mcdr exp)))
(define (lambda-body exp)
(mcdr (mcdr exp)))
(define (make-lambda parameters body)
(mcons 'lambda (mcons parameters body)))
(define (if? exp) (tagged-list? exp 'if))
(define (if-predicate exp) (mcar (mcdr exp)))
(define (if-consequent exp) (mcar (mcdr (mcdr exp))))
(define (if-alternative exp)
(if (not (null? (mcdr (mcdr (mcdr exp)))))
(mcar (mcdr (mcdr (mcdr exp))))
'false))
(define (make-if predicate consequent alternative)
(mlist 'if predicate consequent alternative))
(define (begin? exp)
(tagged-list? exp 'begin))
(define (begin-actions exp) (mcdr exp))
(define (last-exp? seq) (null? (mcdr seq)))
(define (first-exp seq) (mcar seq))
(define (rest-exps seq) (mcdr seq))
(define (sequence->exp seq)
(cond [(null? seq) seq]
[(last-exp? seq) (first-exp seq)]
[else (make-begin seq)]))
(define (make-begin seq) (cons 'begin seq))
(define (application? exp) (mpair? exp))
(define (operator exp) (mcar exp))
(define (operands exp) (mcdr exp))
(define (no-operands? ops) (null? ops))
(define (first-operand ops) (mcar ops))
(define (rest-operands ops) (mcdr ops))
(define (cond? exp)
(tagged-list? exp 'cond))
(define (cond-clauses exp) (mcdr exp))
(define (cond-else-clause? clause)
(eq? (cond-predicate clause) 'else))
(define (cond-predicate clause)
(mcar clause))
(define (cond-actions clause)
(mcdr clause))
(define (cond->if exp)
(expand-clauses (cond-clauses exp)))
(define (expand-clauses clauses)
(if (null? clauses)
'false ;no else clause
(let ([first (mcar clauses)]
[rest (mcdr clauses)])
(if (cond-else-clause? first)
(if (null? rest)
(sequence->exp (cond-actions first))
(error "ELSE clause isn't last: COND->IF" clauses))
(make-if (cond-predicate first)
(sequence->exp (cond-actions first))
(expand-clauses rest))))))
(define (true? x)
(not (eq? x false)))
(define (make-procedure parameters body env)
(mlist 'procedure parameters body env))
(define (compound-procedure? p)
(tagged-list? p 'procedure))
(define (procedure-parameters p) (mcar (mcdr p)))
(define (procedure-body p) (mcar (mcdr (mcdr p))))
(define (procedure-environment p) (mcar (mcdr (mcdr (mcdr p)))))
(define (enclosing-environment env) (mcdr env))
(define (first-frame env) (mcar env))
(define the-empty-environment 'the-empty-environment)
(define (make-frame variables values)
(mcons variables values))
(define (frame-variables frame) (mcar frame))
(define (frame-values frame) (mcdr frame))
(define (add-binding-to-frame! var val frame)
(set-mcar! frame (mcons var (frame-variables frame)))
(set-mcdr! frame (mcons val (frame-values frame))))
(define (extend-environment vars vals base-env)
(if (= (mlength vars) (mlength vals))
(mcons (make-frame vars vals) base-env)
(if (< (mlength vars) (mlength vals))
(error "Too many arguments supplied"
vars
vals)
(error "Too few arguments supplied"
vars
vals))))
(define (lookup-variable-value var env)
(define (env-loop env)
(define (scan vars vals)
(cond [(null? vars) (env-loop (enclosing-environment env))]
[(eq? var (mcar vars))(mcar vals)]
[else (scan (mcdr vars) (mcdr vals))]))
(if (eq? env the-empty-environment)
(error "Unbound variable" var)
(let ([frame (first-frame env)])
(scan (frame-variables frame)
(frame-values frame)))))
(env-loop env))
(define (set-variable-value! var val env)
(define (env-loop env)
(define (scan vars vals)
(cond [(null? vars) (env-loop (enclosing-environment env))]
[(eq? var (mcar vars)) (set-mcar! vals val)]
[else (scan (mcdr vars) (mcdr vals))]))
(if (eq? env the-empty-environment)
(error "Unbound variable: SET!" var)
(let ([frame (first-frame env)])
(scan (frame-variables frame)
(frame-values frame)))))
(env-loop env))
(define (define-variable! var val env)
(let ([frame (first-frame env)])
(define (scan vars vals)
(cond [(null? vars) (add-binding-to-frame! var val frame)]
[(eq? var (mcar vars)) (set-mcar! vals val)]
[else (scan (mcdr vars) (mcdr vals))]))
(scan (frame-variables frame)
(frame-values frame))))
(define (setup-environment)
(let ([initial-env (extend-environment
(primitive-procedure-names)
(primitive-procedure-objects)
the-empty-environment)])
(define-variable! 'true true initial-env)
(define-variable! 'false false initial-env)
initial-env))
(define (primitive-procedure? proc)
(tagged-list? proc 'primitive))
(define (primitive-implementation proc)
(mcdr proc))
(define primitive-procedures
(mlist
(mcons 'car car)
(mcons 'cdr cdr)
(mcons 'cons cons)
(mcons 'null? null?)
(mcons '+ +)
(mcons '- -)
(mcons '/ /)
(mcons '* *)
(mcons '= =)))
(define (primitive-procedure-names)
(mmap mcar primitive-procedures))
(define (primitive-procedure-objects)
(mmap
(lambda (proc)
(mcons 'primitive (mcdr proc)))
primitive-procedures))
(define (apply-primitive-procedure proc args)
(apply (primitive-implementation proc) (if (mpair? args) (mlist->list args) args)))
(define the-global-environment (setup-environment))
(define input-prompt ";;; M-Eval input:")
(define output-prompt ";;; M-Eval value:")
(define (list->mlist/deep input)
(mmap
(lambda (value)
(if (pair? value)
(list->mlist/deep value)
value))
(list->mlist input)))
(define (driver-loop)
(prompt-for-input input-prompt)
(let ([input (read)])
(let ([output (actual-value (list->mlist/deep input) the-global-environment)])
(announce-output output-prompt)
(user-print output)))
(driver-loop))
(define (prompt-for-input string)
(newline)
(newline)
(display string)
(newline))
(define (announce-output string)
(newline)
(display string)
(newline))
(define (user-print object)
(if (compound-procedure? object)
(display
(list 'compound-procedure
(procedure-parameters object)
(procedure-body object)
'<procedure-env>))
(display object)))
(driver-loop)