Runtime evaluation/In an environment: Difference between revisions

In TXR's embedded Lisp dialect, we can implement the same solution as Lisp or Scheme: transform the code fragment by wrapping a <code>let</code> around it which binds a variable, and then evaluating the whole thing:

(- (eval ^(let ((x ,x1)) ,code-fragment))

(eval ^(let ((x ,x0)) ,code-fragment))))

Cutting edge TXR code provides access to the environment manipulation functions, making this possible:

(let ((e1 (make-env (list (cons 'x x1)))) ;; create two environments stuffed with binding for x

(e0 (make-env (list (cons 'x x0)))))

Alternatively, empty environments can be made and extended with bindings:

(let ((e1 (make-env))

(e0 (make-env)))

However, our two <code>let</code> constructs carefully save and restore the dynamic environment (and therefore any prior value of <code>x</code>), even in the face of exceptions, and

(defun eval-subtract-for-two-values-of-x (code-fragment x1 x0)