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.


   EvalWithX=. monad : 0
    'CODE V0 V1'=. y
    (". CODE [ x=. V1) - (". CODE [ x=. V0)
   )
 
   EvalWithX '^x';0;1
1.71828183