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Add dynamic dispatch.

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Stephen Chung 2020-06-25 19:22:14 +08:00
parent 580a62daaf
commit 259b6d0fcf
3 changed files with 81 additions and 28 deletions
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2
doc/src/about/features.md
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@ -35,7 +35,7 @@ Dynamic
* Organize code base with dynamically-loadable [modules].
* [Function pointers].
* Dynamic dispatch via [function pointers].
Safe
----

52
doc/src/appendix/keywords.md
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@ -3,28 +3,30 @@ Keywords List
{{#include ../links.md}}
| Keyword | Description | Not available under |
| :--------: | ------------------------------------- | ------------------- |
| `true` | Boolean true literal | |
| `false` | Boolean false literal | |
| `let` | Variable declaration | |
| `const` | Constant declaration | |
| `if` | If statement | |
| `else` | else block of if statement | |
| `while` | While loop | |
| `loop` | Infinite loop | |
| `for` | For loop | |
| `in` | Containment test, part of for loop | |
| `continue` | Continue a loop at the next iteration | |
| `break` | Loop breaking | |
| `return` | Return value | |
| `throw` | Throw exception | |
| `private` | Mark function private | [`no_function`] |
| `import` | Import module | [`no_module`] |
| `export` | Export variable | [`no_module`] |
| `as` | Alias for variable export | [`no_module`] |
| `fn` | Function definition | [`no_function`] |
| `type_of` | Get type name of value | |
| `print` | Print value | |
| `debug` | Print value in debug format | |
| `eval` | Evaluate script | |
| Keyword | Description | Not available under |
| :-------------------: | --------------------------------------- | ------------------- |
| `true` | Boolean true literal | |
| `false` | Boolean false literal | |
| `let` | Variable declaration | |
| `const` | Constant declaration | |
| `if` | If statement | |
| `else` | else block of if statement | |
| `while` | While loop | |
| `loop` | Infinite loop | |
| `for` | For loop | |
| `in` | Containment test, part of for loop | |
| `continue` | Continue a loop at the next iteration | |
| `break` | Loop breaking | |
| `return` | Return value | |
| `throw` | Throw exception | |
| `private` | Mark function private | [`no_function`] |
| `import` | Import module | [`no_module`] |
| `export` | Export variable | [`no_module`] |
| `as` | Alias for variable export | [`no_module`] |
| `fn` (lower-case `f`) | Function definition | [`no_function`] |
| `Fn` (capital `F`) | Function to create a [function pointer] | [`no_function`] |
| `call` | Call a [function pointer] | [`no_function`] |
| `type_of` | Get type name of value | |
| `print` | Print value | |
| `debug` | Print value in debug format | |
| `eval` | Evaluate script | |

55
doc/src/language/fn-ptr.md
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@ -7,8 +7,8 @@ In fact, internally a function pointer simply stores the _name_ of the function
Call a function pointer using the `call` method, which needs to be called in method-call style.
Built-in Functions
------------------
Built-in methods
----------------
The following standard methods (mostly defined in the [`BasicFnPackage`]({{rootUrl}}/rust/packages.md) but excluded if
using a [raw `Engine`]) operate on [strings]:
@ -54,3 +54,54 @@ let hello = Fn(fn_name + "_world");
hello.call(0); // error: function not found - "hello_world (i64)"
```
Dynamic Dispatch
----------------
The purpose of function pointers is to enable rudimentary _dynamic dispatch_, meaning to determine,
at runtime, which function to call among a group.
Although it is possible to simulate dynamic dispatch via a number and a large `if-then-else-if` statement,
using function pointers significantly simplifies the code.
```rust
let x = some_calculation();
// These are the functions to call depending on the value of 'x'
fn method1(x) { ... }
fn method2(x) { ... }
fn method3(x) { ... }
// Traditional - using decision variable
let func = sign(x);
// Dispatch with if-statement
if func == -1 {
method1(42);
} else if func == 0 {
method2(42);
} else if func == 1 {
method3(42);
}
// Using pure function pointer
let func = if x < 0 {
Fn("method1")
} else if x == 0 {
Fn("method2")
} else if x > 0 {
Fn("method3")
}
// Dynamic dispatch
func.call(42);
// Using functions map
let map = [ Fn("method1"), Fn("method2"), Fn("method3") ];
let func = sign(x) + 1;
// Dynamic dispatch
map[func].call(42);
```