137 lines
4.0 KiB
Markdown
137 lines
4.0 KiB
Markdown
Multi-Layered Functions
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=======================
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{{#include ../links.md}}
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```admonish info "Usage scenario"
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* A system is divided into separate _layers_, each providing logic in terms of scripted [functions].
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* A lower layer provides _default implementations_ of certain [functions].
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* Higher layers each provide progressively more specific implementations of the same [functions].
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* A more specific [function], if defined in a higher layer, always overrides the implementation
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in a lower layer.
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* This is akin to object-oriented programming but with [functions].
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* This type of system is extremely convenient for dynamic business rules configuration, setting
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corporate-wide policies, granting permissions for specific roles etc. where specific, local rules
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need to override corporate-wide defaults.
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```
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```admonish tip "Practical scenario"
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Assuming a LOB (line-of-business) system for a large MNC (multi-national corporation) with branches,
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facilities and offices across the globe.
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The system needs to provide basic, corporate-wide policies to be enforced through the worldwide
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organization, but also cater for country- or region-specific rules, practices and regulations.
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| Layer | Description |
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| :---------: | --------------------------------------------------- |
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| `corporate` | corporate-wide policies |
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| `regional` | regional policy overrides |
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| `country` | country-specific modifications for legal compliance |
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| `office` | special treatments for individual office locations |
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```
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```admonish abstract "Key concepts"
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* Each layer is a separate script.
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* The lowest layer script is compiled into a base [`AST`].
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* Higher layer scripts are also compiled into [`AST`] and _combined_ into the base using
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[`AST::combine`]({{rootUrl}}/engine/ast.md) (or the `+=` operator), overriding any existing [functions].
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```
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Examples
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--------
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Assume the following four scripts, one for each layer:
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```rust
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┌────────────────┐
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│ corporate.rhai │
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└────────────────┘
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// Default implementation of 'foo'.
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fn foo(x) { x + 1 }
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// Default implementation of 'bar'.
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fn bar(x, y) { x + y }
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// Default implementation of 'no_touch'.
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fn no_touch() { throw "do not touch me!"; }
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┌───────────────┐
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│ regional.rhai │
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└───────────────┘
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// Specific implementation of 'foo'.
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fn foo(x) { x * 2 }
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// New implementation for this layer.
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fn baz() { print("hello!"); }
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┌──────────────┐
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│ country.rhai │
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└──────────────┘
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// Specific implementation of 'bar'.
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fn bar(x, y) { x - y }
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// Specific implementation of 'baz'.
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fn baz() { print("hey!"); }
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┌─────────────┐
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│ office.rhai │
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└─────────────┘
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// Specific implementation of 'foo'.
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fn foo(x) { x + 42 }
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```
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Load and combine them sequentially:
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```rust
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let engine = Engine::new();
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// Compile the baseline layer.
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let mut ast = engine.compile_file("corporate.rhai".into())?;
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// Combine the first layer.
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let lowest = engine.compile_file("regional.rhai".into())?;
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ast += lowest;
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// Combine the second layer.
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let middle = engine.compile_file("country.rhai".into())?;
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ast += middle;
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// Combine the third layer.
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let highest = engine.compile_file("office.rhai".into())?;
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ast += highest;
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// Now, 'ast' contains the following functions:
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//
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// fn no_touch() { // from 'corporate.rhai'
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// throw "do not touch me!";
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// }
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// fn foo(x) { x + 42 } // from 'office.rhai'
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// fn bar(x, y) { x - y } // from 'country.rhai'
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// fn baz() { print("hey!"); } // from 'country.rhai'
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```
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```admonish failure.small "No super call"
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Unfortunately, there is no `super` call that calls the base implementation (i.e. no way for a
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higher-layer function to call an equivalent lower-layer function).
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```
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