Runtime evaluation/In an environment
You are encouraged to solve this task according to the task description, using any language you may know.
Given a program in the language representing a function, evaluate it with the variable x (or another name if that is not valid) bound to a provided value, then evaluate it again with x bound to another provided value, then subtract the result of the first from the second and return or print it.
Preferably, do so in a way which does not involve string manipulation of source code, and is plausibly extensible to a runtime-chosen set of bindings.
For more general examples and language-specific details, see Eval.
Common Lisp
<lang lisp> (defun eval-with-x (program a b)
(let ((at-a (eval `(let ((x ',a)) ,program)))
(at-b (eval `(let ((x ',b)) ,program))))
(- at-b at-a)))
</lang>
<lang lisp> (eval-with-x '(exp x) 0 1) => 1.7182817 </lang lisp>
This version ensures that the program is compiled, once, for more efficient execution:
<lang lisp> (defun eval-with-x (program a b)
(let* ((f (compile nil `(lambda (x) ,program)))
(at-a (funcall f a))
(at-b (funcall f b)))
(- at-b at-a)))
</lang>