65 lines
1.9 KiB
Markdown
65 lines
1.9 KiB
Markdown
Constants
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=========
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{{#include ../links.md}}
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Constants can be defined using the `const` keyword and are immutable.
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Constants follow the same naming rules as [variables].
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```rust
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const x = 42;
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print(x * 2); // prints 84
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x = 123; // <- syntax error: cannot assign to constant
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```
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```rust
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const x; // 'x' is a constant '()'
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const x = 40 + 2; // 'x' is a constant 42
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```
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Manually Add Constant into Custom Scope
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--------------------------------------
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It is possible to add a constant into a custom [`Scope`] so it'll be available to scripts
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running with that [`Scope`].
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When added to a custom [`Scope`], a constant can hold any value, not just a literal value.
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It is very useful to have a constant value hold a [custom type], which essentially acts
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as a [_singleton_](../patterns/singleton.md).
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```rust
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use rhai::{Engine, Scope};
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struct TestStruct(i64); // custom type
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let engine = Engine::new()
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.register_type_with_name::<TestStruct>("TestStruct") // register custom type
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.register_get_set("value",
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|obj: &mut TestStruct| obj.0, // property getter
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|obj: &mut TestStruct, value: i64| obj.0 = value // property setter
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);
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let mut scope = Scope::new(); // create custom scope
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scope.push_constant("MY_NUMBER", TestStruct(123_i64)); // add constant variable
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engine.consume_with_scope(&mut scope, r"
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MY_NUMBER.value = 42; // constant objects can be modified
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print(MY_NUMBER.value); // prints 42
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")?;
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```
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Constants Can be Modified, Just Not Reassigned
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---------------------------------------------
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A custom type stored as a constant can be modified via its registered API -
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being a constant only prevents it from being re-assigned or operated upon by Rhai;
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mutating it via a Rust function is still allowed.
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