eval Function
Or “How to Shoot Yourself in the Foot even Easier”
Saving the best for last, there is the ever-dreaded… eval function!
let x = 10;
fn foo(x) { x += 12; x }
let script =
"
let y = x;
y += foo(y);
x + y
";
let result = eval(script); // <- look, JavaScript, we can also do this!
result == 42;
x == 10; // prints 10 - arguments are passed by value
y == 32; // prints 32 - variables defined in 'eval' persist!
eval("{ let z = y }"); // to keep a variable local, use a statements block
print(z); // <- error: variable 'z' not found
"print(42)".eval(); // <- nope... method-call style doesn't work with 'eval'Script segments passed to eval execute inside the current Scope, so they can access and modify
everything, including all variables that are visible at that position in code!
let script = "x += 32";
let x = 10;
eval(script); // variable 'x' is visible!
print(x); // prints 42
// The above is equivalent to:
let script = "x += 32";
let x = 10;
x += 32;
print(x);eval can also be used to define new variables and do other things normally forbidden inside
a function call.
let script = "let x = 42";
eval(script);
print(x); // prints 42Treat it as if the script segments are physically pasted in at the position of the eval call.
For those who subscribe to the (very sensible) motto of “eval is evil”,
disable eval via Engine::disable_symbol.
// Disable usage of 'eval'
engine.disable_symbol("eval");Or course we do.
Having the possibility of an eval call disrupts any predictability in the Rhai script,
thus disabling a large number of optimizations.