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#lang eopl
;; scanner spec
(define scanner-spec
'(
(whitespace (whitespace) skip)
(number (digit (arbno digit)) make-number)
(operator ((or "+" "-" "*" "/" "(" ")")) make-symbol)))
;; grammar
(define grammar
'(
;; expression - 2+4*2*(1+2+3)
(expression (term (arbno add-op term)) arith-expr)
;; term - 1*2*3
(term (factor (arbno mult-op factor)) arith-factor)
;; factors - 2, (1+2+3)
(factor (number) number-factor)
(factor ("(" expression ")") expr-factor)
;; addition and subtraction
(add-op ("+") plus-op)
(add-op ("-") minus-op)
;; multiplication and division
(mult-op ("*") times-op)
(mult-op ("/") divide-op)
))
;; make the datatypes
(sllgen:make-define-datatypes scanner-spec grammar)
(define dump-datatypes
(lambda ()
(sllgen:list-define-datatypes scanner-spec grammar)))
;; top level expression evaluator
(define execute-expression
(lambda (expr)
(cases expression expr
;; use a helper function to keep code out of here
(arith-expr (term ops terms) (expr-helper term ops terms))
)))
;; helper function to evaluate an expression
(define expr-helper
(lambda (term ops terms)
(if (null? ops)
;; single term
(eval-term term)
;; else, check if there's just one term
(if (equal? (length ops) 1)
(eval-add-op (car ops) term (car terms))
;; else, handle multiple terms
'a
))))
;; evaluates an add operator
(define eval-add-op
(lambda (op term1 term2)
(cases add-op op
;; perform addition
(plus-op () (+ (eval-term term1) (eval-term term2)))
;; perform subtraction
(minus-op () (- (eval-term term1) (eval-term term2)))
)))
;; evaluate a term
(define eval-term
(lambda (term1)
(cases term term1
;; use a helper function to keep this one clean
(arith-factor (factor ops factors) (term-helper factor ops factors))
)))
;; helper function for evaluating terms
(define term-helper
(lambda (factor ops factors)
;; if there is a single factor...
(if (null? ops)
;; ... evaluate and return it
(eval-factor factor)
;; otherwise, if there are 2 factors...
(if (equal? (length ops) 1)
;; evaluate and return them.
(eval-mult-op (car ops) factor (car factors))
;; otherwise, handle multiple factors
'b
))))
;; handles multiplication and division
(define eval-mult-op
(lambda (op fac1 fac2)
(cases mult-op op
;; perform multiplication
(times-op () (* (eval-factor fac1) (eval-factor fac2)))
;; perform division
(divide-op () (/ (eval-factor fac1) (eval-factor fac2)))
)))
;; evaluate a factor
(define eval-factor
(lambda (fac)
(cases factor fac
;; if it's a number, give the number
(number-factor (num) num)
;; if it's an expression, evaluate the expression
(expr-factor (expr) (execute-expression expr))
)))
;; SLLGEN things
(define scan
(sllgen:make-string-scanner scanner-spec grammar))
(define scan&parse
(sllgen:make-string-parser scanner-spec grammar))
(define read-dump
(sllgen:make-rep-loop "> " (lambda (tree) tree) (sllgen:make-stream-parser scanner-spec grammar)))
(define scan&parse&eval
(lambda (s) (execute-expression (scan&parse s))))
(define read-eval-print
(sllgen:make-rep-loop "> " execute-expression (sllgen:make-stream-parser scanner-spec grammar)))
;; shortcut for quicker testing
(define spe scan&parse&eval)
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