; The function "expand" is a Newlisp version of mathematical operation ; of the substitution. It is very useful function. For example, ; in code ; ; (setf 'x 'new-variable) ; (expand '(lambda(x y)(print x)) 'x) ; ; ===> (lambda (new-variable y) (print new-variable)) ; ; Newlisp "expands" all occurences of the symbol x with symbol ; new-variable. ; ; However, it is not always convenient to apply substitution on ; all occurences. For example, let us assume that you want to ; write interpreter for some other dialect of Lisp in Newlisp. ; That interpreter should be able to compute expressions like ; ; ((lambda(x)(+ x (* 2 x) (let((x 5))(* x x)))) 3). ; ; It can be accompplished by substituting argument of the function (3) ; on place of parameter of the function (x) of the body of the ; function ; (+ x (* 2 x) (let((x 5))(* x x))). ; ; However, the substitution is needed only for first two occurences ; of x, while not for third, fourth and fifth occurence - these ; occurences are not "free", they are "bounded." ; ; I defined expand-free-variable function so it recognizes few most ; important ways for binding of the variables: lambda, lambda-macro, ; local, let, letn and letex. As many of these operations are ; "polymorphic", only the most basic form is supported. It turned ; to be relatively hard to write, because almost every binding ; operator, every form of it, requires slightly different code. (set 'function-parameters (lambda(f)(first f))) (set 'function-body (lambda(f)(rest f))) (set 'expand-free-variables (lambda(E) (let ((vars-to-expand (args))) (cond ((symbol? E) (eval (append '(expand) (map quote (list E)) (map quote vars-to-expand)))) ;------------------------------------------------ ((or (lambda? E) (macro? E)) (letn((new-vars-to-expand (difference vars-to-expand (function-parameters E))) (new-expand-function (append (lambda(expr)) (list (append '(expand-free-variables expr) (map quote new-vars-to-expand)))))) (append (cond ((lambda? E) '(lambda)) ((macro? E) '(lambda-macro))) (list (function-parameters E)) (map new-expand-function (function-body E))))) ;----------------------------------------------- ((and (list? E) (starts-with E 'local)) (letn((new-vars-to-expand (difference vars-to-expand (nth 1 E))) (new-expand-function (append (lambda(expr)) (list (append '(expand-free-variables expr) (map quote new-vars-to-expand)))))) (append '(local) (list (nth 1 E)) (map new-expand-function (rest (rest E)))))) ;----------------------------------------------- ((and (list? E) (or (starts-with E 'let) (starts-with E 'letn) (starts-with E 'letex))) (letn((new-vars-to-expand (difference vars-to-expand (map first (nth 1 E)))) (new-expand-function (append (lambda(expr)) (list (append '(expand-free-variables expr) (map quote new-vars-to-expand)))))) (append (cond ((starts-with E 'let) '(let)) ((starts-with E 'letn) '(letn)) ((starts-with E 'letex) '(letex))) (list (first (rest E))) (map new-expand-function (rest (rest E)))))) ;------------------------------------------------ ((list? E)(let((new-expand-function (append (lambda(expr)) (list (append '(expand-free-variables expr) (map quote vars-to-expand)))))) (map new-expand-function E))) ;------------------------------------------------ ((or (number? E) (string? E)) E) ;------------------------------------------------ ((quote? E) (list 'quote (eval (append '(expand-free-variables) (list (list 'quote (eval E))) (map quote vars-to-expand))))) ;------------------------------------------------ (true (println "Expand for " E " is not defined.\n") (throw-error "expand isn't defined.")))))) ; FEW TESTS (setf x 1 y 2 z 3 v 4 w 5) (println (expand-free-variables '(local(x y z)x y z v w 7) 'x 'v)) ; (local (x y z) ; x y z 4 w 7) (println (expand-free-variables '('('(x)) '('(z)) '''y (local(x)x y)) 'x 'y)) ; ((quote ((quote (1)))) (quote ((quote (z)))) (quote (quote (quote 2))) ; (local (x) ; x 2)) ; (println (expand-free-variables '(lambda(x a y) x b z) 'x 'y 'z 'w)) ; (lambda (x a y) x b 3) (setf x 'new-variable) (println (expand-free-variables (list 'x '(lambda(x y)(print x))) 'x)) ; (new-variable ; (let ((x 3)) ; (x even-newer-variable ; (letex ((y 4)) y)))) ; (setf x 'new-variable y 'even-newer-variable) (println (expand-free-variables '(x (let((x 3)) (x y (letex((y 4))y)))) 'x 'y)) ; (lambda (x a y) x b 3) (exit) |
Expansion of Free Variables.
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