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;;; -*- Mode:Scheme -*-
(declare (usual-integrations))
;;; Self-evaluating entities
(define (self-evaluating? exp)
(or (number? exp)
(eq? exp #t)
(eq? exp #f)
(string? exp))) ; Our prompt (viz., "EVAL==> ") is a string.
;;; Variables
(define (variable? exp) (symbol? exp))
(define (same-variable? var1 var2) (eq? var1 var2)) ;; Nice abstraction
;;; Special forms (in general)
(define (tagged-list? exp tag)
(and (pair? exp)
(eq? (car exp) tag)))
;;; Quotations
(define (quoted? exp) (tagged-list? exp 'quote))
(define (text-of-quotation quot) (cadr quot))
;;; Assignment--- SET!
(define (assignment? exp) (tagged-list? exp 'set!))
(define (permanent-assignment? exp) (tagged-list? exp 'set!!))
(define (assignment-variable assn) (cadr assn))
(define (assignment-value assn) (caddr assn))
;;; Definitions
(define (definition? exp) (tagged-list? exp 'define))
(define (definition-variable defn)
(if (variable? (cadr defn)) ;; (DEFINE foo ...)
(cadr defn)
(caadr defn))) ;; (DEFINE (foo ...) ...)
(define (definition-value defn)
(if (variable? (cadr defn)) ;; (DEFINE foo ...)
(caddr defn)
(cons 'lambda ;; (DEFINE (foo p...) b...)
(cons (cdadr defn) ;; = (DEFINE foo
(cddr defn))))) ;; (LAMBDA (p...) b...))
;;; LAMBDA expressions
(define (lambda? exp) (tagged-list? exp 'lambda))
(define (lambda-parameters lambda-exp) (cadr lambda-exp))
(define (lambda-body lambda-exp)
(let ((full-body (cddr lambda-exp)))
(sequence->begin full-body)))
(define declaration? pair?)
(define (parameter-name var-decl)
(if (pair? var-decl)
(car var-decl)
var-decl))
(define (lazy? var-decl)
(and (pair? var-decl)
(memq 'lazy (cdr var-decl))
(not (memq 'memo (cdr var-decl)))))
(define (lazy-memo? var-decl)
(and (pair? var-decl)
(memq 'lazy (cdr var-decl))
(memq 'memo (cdr var-decl))))
(define (sequence->begin seq)
(cond ((null? seq) seq)
((null? (cdr seq)) (car seq))
((begin? (car seq)) seq)
(else (make-begin seq))))
(define (make-begin exp) (cons 'begin exp))
;;; If conditionals
(define (if? exp) (tagged-list? exp 'if))
(define (if-predicate exp) (cadr exp))
(define (if-consequent exp) (caddr exp))
(define (if-alternative exp)
(if (not (null? (cdddr exp)))
(cadddr exp)
'the-unspecified-value))
(define (make-if pred conseq alternative)
(list 'IF pred conseq alternative))
;;; COND Conditionals
(define (cond? exp) (tagged-list? exp 'cond))
(define (clauses cndl) (cdr cndl))
(define (no-clauses? clauses) (null? clauses))
(define (first-clause clauses) (car clauses))
(define (rest-clauses clauses) (cdr clauses))
(define (else-clause? clause) (eq? (predicate clause) 'else))
(define (predicate clause) (car clause))
(define (actions clause)
(sequence->begin (cdr clause)))
(define (cond->if cond-exp)
(define (expand clauses)
(cond ((no-clauses? clauses)
(list 'error "COND: no values matched"))
((else-clause? (car clauses))
(if (no-clauses? (cdr clauses))
(actions (car clauses))
(error "else clause isn't last -- INTERP" exp)))
(else
(make-if (predicate (car clauses))
(actions (car clauses))
(expand (cdr clauses))))))
(expand (clauses cond-exp)))
;;; BEGIN expressions (a.k.a. sequences)
(define (begin? exp) (tagged-list? exp 'begin))
(define (begin-actions begin-exp) (cdr begin-exp))
(define (last-exp? seq) (null? (cdr seq)))
(define (first-exp seq) (car seq))
(define (rest-exps seq) (cdr seq))
(define no-more-exps? null?) ; for non-tail-recursive vers.
;;; LET expressions
(define (let? exp) (tagged-list? exp 'let))
(define (let-bound-variables let-exp)
(map car (cadr let-exp)))
(define (let-values let-exp) (map cadr (cadr let-exp)))
(define (let-body let-exp) (sequence->begin (cddr let-exp)))
(define (let->combination let-exp)
(let ((names (let-bound-variables let-exp))
(values (let-values let-exp))
(body (let-body let-exp)))
(cons (list 'LAMBDA names body)
values)))
;;; Procedure applications -- NO-ARGS? and LAST-OPERAND? added
(define (application? exp)
(pair? exp))
(define (no-args? exp) ;; Added for tail recursion
(and (pair? exp)
(null? (cdr exp))))
(define (args-application? exp) ;; Changed from 5.2.1
(and (pair? exp)
(not (null? (cdr exp)))))
(define (operator app) (car app))
(define (operands app) (cdr app))
(define (last-operand? args) ;; Added for tail recursion
(null? (cdr args)))
(define (no-operands? args) (null? args))
(define (first-operand args) (car args))
(define (rest-operands args) (cdr args))
;;; Another special form that will be needed later.
(define (amb? exp)
(and (pair? exp) (eq? (car exp) 'amb)))
(define (amb-alternatives exp) (cdr exp))
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