eval Function

Or “How to Shoot Yourself in the Foot even Easier”

Saving the best for last, there is the ever-dreaded... eval function!


#![allow(unused)]
fn main() {
let x = 10;

fn foo(x) { x += 12; x }

let script = "let y = x;";      // build a script
script +=    "y += foo(y);";
script +=    "x + y";

let result = eval(script);      // <- look, JavaScript, we can also do this!

result == 42;

x == 10;                        // prints 10: functions call 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 statement 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! It is almost as if the script segments were physically pasted in at the position of the eval call.

Cannot Define New Functions

New functions cannot be defined within an eval call, since functions can only be defined at the global level, not inside another function call!


#![allow(unused)]
fn main() {
let script = "x += 32";
let x = 10;
eval(script);                   // variable 'x' in the current scope is visible!
print(x);                       // prints 42

// The above is equivalent to:
let script = "x += 32";
let x = 10;
x += 32;
print(x);
}

eval is Evil

For those who subscribe to the (very sensible) motto of eval is evil”, disable eval using Engine::disable_symbol:


#![allow(unused)]
fn main() {
engine.disable_symbol("eval");  // disable usage of 'eval'
}