11B. Eval()

simply evaluates a quoted expression (exp = Eval(`exp)).

NOTE: because E is a somewhat typeless language, accidentally writing
"Eval(x*x)"  instead of  "Eval(`x*x)"  will go unnoticed by the
compiler, and will give you big runtime problems: the value of x*x
will be used as a pointer to code.

To understand why 'quoted expressions' is a powerful feature think of the
following cases: if you were to perform a set of actions on a set of different
variables, you'd normally write a function, and call that function with
different arguments. But what happens when the element that you want to give
as argument is a piece of code? in traditional languages this would not be
possible, so you would have to 'copy' the blocks of code representing your
function, and put the expression in it. Not in E. say you wanted to write
a program that times the execution time of different expressions. In E you
would simply write:

PROC timing(func,title)
  /* do all sorts of things to initialise time */
  /* and the rest */
  WriteF('time measured for \s was \d\n',title,t)

and then call it with:

timing(`sizycalc(),'large calculation')

in any other imperative language, you would have to write out
copies of timing() for every call to it, or you would have to
put each expression in a separate function. This is just a simple
example: think about what you could do with data structures (LISTs)
filled with unevaluated code:


Note that this idea of functions as normal variables/values is not new
in E, quoted expressions are literally from LISP, which also has the
somewhat more powerful so-called Lambda function, which can also be
given as argument to functions; E's quoted expressions can also be
seen as parameterless (or global parameter only) lambdas.