1

The metacircular evaluator (Cont.)

2

6. Defining new procedures 

(define (lambda? e) (tag-check e 'lambda)) 

(define (eval exp env) (cond ((number? exp) exp) ((symbol? exp) (lookup exp env)) ((define? exp) (eval-define exp env)) ((if? exp) (eval-if exp env)) ((lambda? exp) (eval-lambda exp env)) ((application? exp) (apply (eval (car exp) env) (map (lambda (e) (eval e env)) (cdr exp)))) (else (error "unknown expression " exp)))) (define (eval-lambda exp env) (make-procedure (lambda-parameters exp) (lambda-body exp) env))

(define (lambda-parameters exp) (cadr exp))(define (lambda-body exp) (cddr exp))

(define (make-procedure parameters body env) (list 'procedure parameters body env))

3

  

(eval '(define twice (lambda (x) (+ x x))) GE)

4

Implementation of lambda

(eval '(lambda (x) (+ x x)) GE)

(eval-lambda '(lambda (x) (+ x x)) GE)

(make-procedure '(x) ’((+ x x)) GE)

(list ’procedure '(x) ’((+ x x)) GE)

symbolprocedure

symbol+

symbolx

GE

This datastructure isa procedure!

5

Naming the procedure

(eval '(define twice (lambda (x) (+ x x))) GE)

names values z 9true #t+twice

symbolprimitive

schemeprocedure +

symbolprocedure

symbol+

symbolx

6

6. Defining new procedures 

(define (apply procedure arguments) (cond ((primitive-procedure? procedure) (apply-primitive-procedure procedure arguments)) ((compound-procedure? procedure) (eval-sequence (procedure-body procedure) (extend-environment (procedure-parameters procedure) arguments (procedure-env procedure)))) (else (error "Unknown procedure type -- APPLY" procedure)))) 

(define (compound-procedure? exp) (tag-check exp ‘procedure)) (define (procedure-parameters compound) (cadr compound))(define (procedure-body compound) (caddr compound))(define (procedure-env compound) (cadddr compound))

7

(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 (last-exp? seq) (null? (cdr seq)))(define (first-exp seq) (car seq))(define (rest-exps seq) (cdr seq))

8

How the Environment Works

• Abstractly – in our environment diagrams:

• Concretely – our implementation (as in SICP)

E2 x: 10plus: (procedure ...)

E1

environmentmanipulation

3.

list ofvalues

enclosing-environment

list ofvariables

frame

x plus 10

E2

procedure

9

Extending the Environment

•(extend-environment '(x y) (list 4 5) E2)

E1

E2 x: 10plus: (procedure ...)

E3 x: 4y: 5

Abstractly

ConcretelyE2

list ofvalues

list ofvariables

frame

x y 4

E3

5

E1

10

(define (extend-environment vars vals base-env) (if (= (length vars) (length vals)) (cons (make-frame vars vals) base-env) (if (< (length vars) (length vals)) (error "Too many arguments supplied" vars vals) (error "Too few arguments supplied" vars vals))))

11

(eval '(twice 4) GE)

12

Implementation of apply (1)

(eval '(twice 4) GE)

(apply (eval 'twice GE) (map (lambda (e) (eval e GE)) '(4)))

(apply (list 'procedure '(x) ’((+ x x)) GE) '(4))

(eval-seq ’((+ x x)) (extend-environment '(x) '(4) GE))

(eval '(+ x x) E1)

name valuex 4

GE

E1

A

13

Implementation of apply (2)(eval '(+ x x) E1)

(apply (eval + E1) (map (lambda (e) (eval e E1)) '(x x)))

(apply '(primitive #[add]) (list (eval 'x E1) (eval 'x E1)))

(apply '(primitive #[add]) '(4 4))

(scheme-apply #[add] '(4 4))

8

name valuex 4

GE

E1

A

14

"Scanning" the environment

• Look for a variable in the environment...

• Look for a variable in a frame...– loop through the list of vars and list of vals in parallel– detect if the variable is found in the frame

• If not found in frame (out of variables in the frame),look in enclosing environment

15

(define (lookup-variable-value var env) (define (env-loop env) (define (scan vars vals) (cond ((null? vars) (env-loop (enclosing-environment env))) ((eq? var (car vars)) (car vals)) (else (scan (cdr vars) (cdr 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 (frame-variables frame) (car frame))(define (frame-values frame) (cdr frame))

(define (enclosing-environment env) (cdr env))

16

(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 (car vars)) (set-car! vals val)) (else (scan (cdr vars) (cdr vals))))) (scan (frame-variables frame) (frame-values frame))))

(define (eval-define exp env) (let ((name (cadr exp)) (defined-to-be (eval (caddr exp) env))) (define-variable! name defined-to-be env) ‘undefined))

17

The Initial (Global) Environment primitives andinitial env.

4.

(define the-empty-environment '())

(define the-global-environment (setup-environment))

(define (setup-environment) (let ((initial-env (extend-environment (primitive-procedure-names) (primitive-procedure-objects) the-empty-environment))) (define-variable! 'true #t initial-env) (define-variable! 'false #f initial-env) initial-env))

21

Summary

• Cycle between eval and apply is the core of the evaluator• eval calls apply with operator and argument values• apply calls eval with expression and environment

• What is still missing from scheme ?• Some special forms• data types other than numbers and booleans

22

SICP’s version of eval

(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 (eval (operator exp) env) (list-of-values (operands exp) env))) (else (error "Unknown expression type -- EVAL" exp))))

23

Note some minor differences like

(define (self-evaluating? exp) (cond ((number? exp) true) ((string? exp) true) (else false)))

(define (variable? exp) (symbol? exp))

(define (list-of-values exps env) (if (no-operands? exps) '() (cons (eval (first-operand exps) env) (list-of-values (rest-operands exps) env))))

24

Note: Syntactic Abstraction

• Semantics• What the language means• Model of computation

• Syntax• Particulars of writing expressions• E.g. how to signal different expressions

• Separation of syntax and semantics: allows one to easily alter syntax

eval/applysyntax

procedures

25

Basic Syntax

• Routines to detect expressions(define (if? exp) (tagged-list? exp 'if))(define (lambda? exp) (tagged-list? exp 'lambda))(define (application? exp) (pair? exp))

• Routines to get information out of expressions(define (operator app) (car app))(define (operands app) (cdr app))(define (first-operand args) (car args))(define (rest-operands args) (cdr args))

• Routines to build expressions(define (make-if predicate consequent alternative)

(list 'if predicate consequent alternative))

26

Example – Changing Syntax

• Suppose you wanted a "verbose" application syntax:

(CALL <proc> ARGS <arg1> <arg2> ...)

• Changes – only in the syntax routines!

(define (application? exp) (tagged-list? 'CALL))

(define (operator app) (cadr app))

(define (operands app) (cdddr app))

27

Implementing "Syntactic Sugar"

• Idea:• Implement a simple fundamental "core" in the evaluator• Easy way to add alternative/convenient syntax?

• "let" as sugared procedure application:

(let ((<name1> <val1>) (<name2> <val2>)) <body>)

((lambda (<name1> <name2>) <body>) <val1> <val2>)

28

Detect and Transform the Alternative Syntax

(define (eval exp env) (cond ((self-evaluating? exp) exp) ((variable? exp) (lookup-variable-value exp env)) ((quoted? exp) (text-of-quotation exp)) . . . ((cond? exp) (eval (cond->if exp) env)) ((let? exp) (eval (let->combination exp) env)) ((application? exp) (apply (eval (operator exp) env)

(list-of-values (operands exp) env))) (else (error "Unknown expression" exp))))

29

Implementing cond: Syntax procedures

(define (cond-clauses exp) (cdr exp))

(define (cond-else-clause? clause) (eq? (cond-predicate clause) 'else))

(define (cond-predicate clause) (car clause))

(define (cond-actions clause) (cdr clause))

30

Cond syntax

(cond ((= x 23) (+ x 1))

(else (- x 1)))

cond

else

23x=

1x+

1x-

31

Transforming sequence of expression toan expression(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))

2x*

car y

begin

32

Implementing cond (Cont.)

(cond ((= x 23) (+ x 1))

(else (- x 1)))

(if (= x 23) (+ x 1) (- x 1))

33

Implementing cond

(define (cond->if exp) (expand-clauses (cond-clauses exp)))

(define (expand-clauses clauses) (if (null? clauses) 'false ; no else clause (let ((first (car clauses)) (rest (cdr 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))))))

34

Details of cond syntax transformation

(cond ((= x 23) (+ x 1))

(else (- x 1)))

cond

else

23x=

1x+

1x-

35

Details of cond syntax transformation

(expand-clauses

else

23x=

1x+

1x-

)

36

Details of cond syntax transformation

first

else

23x=

1x+

1x-

rest

37

Details of cond syntax transformation

23x=

1x+

else

1x-

(make-if

(expand-clauses ))

38

Details of cond syntax transformation

23x=

1x+

1x-

(make-if

)

39

Details of cond syntax transformation

23x=

1x+

1x-

if

40

Named Procedures – Syntax vs. Semantics (define (foo <parm>) <body>)

• Semantic implementation – just another define:(define (eval-definition exp env)

(define-variable! (definition-variable exp)

(eval (definition-value exp) env)

env))

(define (definition-variable exp)

(if (symbol? (cadr exp))

(cadr exp)

(caadr exp)))

(define (definition-value exp)

(if (symbol? (cadr exp))

(caddr exp)

(make-lambda (cdadr exp) ;formal params

(cddr exp)))) ;body

41

Read-Eval-Print Loop(define (driver-loop)

(prompt-for-input input-prompt)

(let ((input (read)))

(let ((output (eval input the-global-env)))

(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)))

(define input-prompt ";;; M-Eval input:")(define output-prompt ";;; M-Eval value:")