x
x
x


Do so in a way which:

 does not involve string manipulation of the input source code
 is plausibly extensible to a runtime-chosen set of bindings rather than just x
 does not make x a global variable


or note that these are impossible.



See also

 For more general examples and language-specific details, see Eval.
 Dynamic variable names is a similar task.


declare
   fun {EvalWithX Program A B}
      {Compiler.evalExpression Program env('X':B) _}
      -
      {Compiler.evalExpression Program env('X':A) _}   
   end
in
   {Show {EvalWithX "{Exp X}" 0.0 1.0}}