75 lines
1.9 KiB
Racket
75 lines
1.9 KiB
Racket
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#lang r5rs
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;This is a simple interpreter for arithmetic expressions
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;An expression can be a number, a symbol or a list consisting of an operator and at least one expression
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;Variables can be defined using def (e.g., (def a 1))
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;Assoc containing defined variables
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(define env '())
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;Add a variable to the environment
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(define (add-variable name value)
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(set! env (cons (cons name value) env))
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"Variable added!")
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;Read a variable from the environment
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(define (read-variable name)
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(define pair (assq name env))
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(if pair
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(cdr pair)
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(begin
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(display "Undefined variable")
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(newline))))
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;Load the very secret flag
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(define (load-flag)
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(define flag '())
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(call-with-input-file
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"flag.txt"
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(lambda (input-port)
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(let loop ((x (read-char input-port)))
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(if (not (eof-object? x))
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(begin
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(set! flag (cons x flag))
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(loop (read-char input-port)))))))
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(set! flag (list->string flag))
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(add-variable 'flag flag))
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;Allowed operators
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(define operators '(+ - * /))
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;Evaluate an expression of the form (<op> <exp> ... <exp>)
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(define (evaluate-pair expression)
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(define operator (car expression))
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(define arguments (cdr expression))
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(cond
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((eq? operator 'def)
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(add-variable (cadr expression) (evaluate (caddr expression))))
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((member (car expression) operators)
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(apply (eval operator (scheme-report-environment 5)) (map evaluate arguments)))
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(else
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"Operator not allowed!")))
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;Evaluate any expression
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(define (evaluate expression)
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(cond
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((pair? expression)
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(evaluate-pair expression))
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((symbol? expression)
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(read-variable expression))
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(else
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expression)))
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;Start the read-eval-print loop
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(define (read-eval-print)
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(display ">> ")
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(let*
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((expression (read))
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(result (evaluate expression)))
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(display result)
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(newline)
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(read-eval-print)))
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(load-flag)
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(read-eval-print)
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