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One other way to do the same is use the coersion table:

(define (integer->rational integer) (make-rational integer 1)) (define (rational->real rational) (define (integer->floating-point integer) (* integer 1.0)) (make-real (/ (integer->floating-point (numer rational)) (denom rational)))) (define (real->complex real) (make-complex-from-real-imag real 0)) >>> (put-coersion 'integer 'rational integer->rational) (put-coersion 'rational 'real rational->real) (put-coersion 'real 'complex real->complex) (define (raise number) (define tower '(integer rational real complex)) (define (try tower) (if (< (length tower) 2) (error "Couldn't raise type" number) (let ((current-type (car tower)) (next-types (cdr tower)) (next-type (car next-types))) (if (eq? (type-tag number) current-type) ((get-coersion current-type next-type) number) (try next-types))))) (try tower))

I have to say, these exercises are rather difficult because it's not clear to what extent we should be creating a working system. I see that a lot of people here have been implementing everything so that all the code runs, but the feeling I get from the book is that we only need to write procedures which work in theory and y'know leave everything else up to ol' George. So anyway, I have sort of been skimming these exercises and looking at them as puzzles rather than an actual program. Anyway, this procedure doesn't really do anything because I don't have a table and can't look up any procedures, so I decided to just not bother, it is trivial to get the actual procedures, this is just the raising mechanism.

(define tower '(integer rational real complex)) (define (raise type) (let ((position (memq type tower))) (if (eq? position #f) (error "No such datatype: " type) (let ((this-type (car position))) (let ((rest (cdr position))) (if (null? rest) this-type (let ((next-type (cadr position))) next-type)))))))

meteorgan