[plt-dev] `unsafe-fl' and unboxing

From: Dave Herman (dherman at ccs.neu.edu)
Date: Sun Oct 4 16:55:15 EDT 2009

> ;;; (real->float x) -> inexact-real?
> ;;;   x : real?
> ;;; Returns an inexact real (i.e., a float) given real x. Raises an  
> error if x
> ;;; is not a real. This can be used to ensure a real value is a  
> float, even in
> ;;; unsafe code.
> (define (real->float x)
>   (if (real? x)
>       (exact->inexact x)
>       (error "expected real, given" x)))
> I'll use it to protect unsafe code. I'm sure it's more overhead than  
> putting it in-line, but hopefully not too much. Putting a contract  
> on it would probably not make much sense.

I feel a little dirty suggesting it, but you could also do:

(define-syntax-rule (real->float exp)
   (let ([x exp])
     (if (real? x)
         (exact->inexact x)
         (error "expected real, given" x))))

I'm not sure whether the mzscheme compiler obeys the Macro Writer's  
Bill of Rights in optimizing the case where exp is just a variable  
reference. If not, you could do the optimization yourself:

(define-syntax (real->float stx)
   (syntax-case stx ()
     [(_ x)
      (identifier? #'x)
      #'(if (real? x)
            (exact->inexact x)
            (error "expected real, given" x))]
     [(_ exp)
      #'(let ([x exp])
          (real->float x))]))


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