From: Brian Candler <B.Candler@...>
Date: 2002-10-17T06:20:04+09:00
Subject: Precompiling eval expressions

What ways are there to pre-compile an "eval" expression which is going to be
used multiple times, so that it does not have to be parsed every time?
This would be similar to how a Regexp object can be used repeatedly.

So far the best I can come up with is to eval Proc.new, although the
resulting object is only able to access objects which exist at the time of
definition:

myexpr = '"The time is #{t}"'
myproc = eval "Proc.new { #{myexpr} }"
t = 123
myproc.call
 => NameError: undefined local variable or method `t' for #<Object:0x810dcd8>

But if it did exist at that time, it _is_ able to access the 'current'
value, not the value at definition time:

t = 0
myexpr = '"The time is #{t}"'
myproc = eval "Proc.new { #{myexpr} }"
t = 123
myproc.call
 => "The time is 123"
t = 456
myproc.call
 => "The time is 456"

(Can anyone point me to a good explanation of what is going on underneath?)

Anyway, the idea is that I want to load in a number of expressions from a
database (for example containing rules for validating fields or providing
default values); those rules may be used many times but only read in once.
Precompiling them also gives an opportunity to catch syntax errors before
they are actually used.

Regards,

Brian.