97 lines
2.7 KiB
Markdown
97 lines
2.7 KiB
Markdown
`eval` Statement
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===============
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{{#include ../links.md}}
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Or "How to Shoot Yourself in the Foot even Easier"
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------------------------------------------------
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Saving the best for last: in addition to script optimizations, there is the ever-dreaded... `eval` function!
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```rust
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let x = 10;
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fn foo(x) { x += 12; x }
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let script = "let y = x;"; // build a script
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script += "y += foo(y);";
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script += "x + y";
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let result = eval(script); // <- look, JS, we can also do this!
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print("Answer: " + result); // prints 42
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print("x = " + x); // prints 10: functions call arguments are passed by value
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print("y = " + y); // prints 32: variables defined in 'eval' persist!
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eval("{ let z = y }"); // to keep a variable local, use a statement block
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print("z = " + z); // <- error: variable 'z' not found
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"print(42)".eval(); // <- nope... method-call style doesn't work
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```
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Script segments passed to `eval` execute inside the current [`Scope`], so they can access and modify _everything_,
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including all variables that are visible at that position in code! It is almost as if the script segments were
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physically pasted in at the position of the `eval` call.
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Cannot Define New Functions
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--------------------------
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New functions cannot be defined within an `eval` call, since functions can only be defined at the _global_ level,
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not inside another function call!
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```rust
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let script = "x += 32";
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let x = 10;
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eval(script); // variable 'x' in the current scope is visible!
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print(x); // prints 42
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// The above is equivalent to:
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let script = "x += 32";
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let x = 10;
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x += 32;
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print(x);
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```
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`eval` is Evil
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--------------
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For those who subscribe to the (very sensible) motto of ["`eval` is evil"](http://linterrors.com/js/eval-is-evil),
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disable `eval` by overloading it, probably with something that throws.
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```rust
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fn eval(script) { throw "eval is evil! I refuse to run " + script }
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let x = eval("40 + 2"); // 'eval' here throws "eval is evil! I refuse to run 40 + 2"
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```
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Or overload it from Rust:
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```rust
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fn alt_eval(script: String) -> Result<(), Box<EvalAltResult>> {
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Err(format!("eval is evil! I refuse to run {}", script).into())
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}
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engine.register_result_fn("eval", alt_eval);
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```
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`EvalPackage`
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-------------
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There is even a package named [`EvalPackage`]({{rootUrl}}/rust/packages.md) which implements the disabling override:
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```rust
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use rhai::Engine;
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use rhai::packages::Package // load the 'Package' trait to use packages
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use rhai::packages::EvalPackage; // the 'eval' package disables 'eval'
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let mut engine = Engine::new();
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let package = EvalPackage::new(); // create the package
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engine.load_package(package.get()); // load the package
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```
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