Runtime evaluation/In an environment

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>

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>

Perl

<lang perl>sub eval_with_x

  {my $code = shift;
   my $x = shift;
   my $first = eval $code;
   $x = shift;
   return eval($code) - $first;}

print eval_with_x('3 * $x', 5, 10), "\n"; # Prints "15".</lang>

PHP

<lang php><?php function eval_with_x($code, $a, $b) {

   $x = $a;
   $first = eval($code);
   $x = $b;
   $second = eval($code);
   return $second - $first;

}

echo eval_with_x('return 3 * $x;', 5, 10), "\n"; # Prints "15". ?></lang>

Python

<lang python>>>> def eval_with_x(code, a, b): return eval(code, {'x':b}) - eval(code, {'x':a})

>>> eval_with_x('2 ** x', 3, 5) 24</lang>

Python: for multiple names

A slight change allows the evaluation to take multiple names: <lang python>>>> def eval_with_args(code, **kwordargs): return eval(code, kwordargs)

>>> code = '2 ** x' >>> eval_with_args(code, x=5) - eval_with_args(code, x=3) 24 >>> code = '3 * x + y' >>> eval_with_args(code, x=5, y=2) - eval_with_args(code, x=3, y=1) 7 >>> </lang>

Ruby

<lang ruby>def getBinding(x)

 binding

end

def eval_with_x(code, a, b)

 eval(code, getBinding(b)) - eval(code, getBinding(a))

end

puts eval_with_x('2 ** x', 3, 5) # Prints "24"</lang>

Scheme

Almost identical to the Common Lisp version above. <lang scheme>(define (eval-with-x prog a b)

 (let ((at-a (eval `(let ((x ',a)) ,prog)))
       (at-b (eval `(let ((x ',b)) ,prog))))
   (- at-b at-a)))</lang>