175 lines
5.5 KiB
Markdown
175 lines
5.5 KiB
Markdown
# Angel3 Symbol Table
|
|
|
|
[](https://pub.dartlang.org/packages/angel3_symbol_table)
|
|
[](https://dart.dev/null-safety)
|
|
[](https://gitter.im/angel_dart/discussion)
|
|
|
|
[](https://github.com/dukefirehawk/angel/tree/angel3/packages/symbol_table/LICENSE)
|
|
|
|
**DEPRECATED: Replaced by [`belatuk_symbol_table`](https://pub.dartlang.org/packages/belatuk_symbol_table) package**
|
|
|
|
A generic symbol table implementation in Dart, with support for scopes and constants.
|
|
The symbol tables produced by this package are hierarchical (in this case, tree-shaped),
|
|
and utilize basic memoization to speed up repeated lookups.
|
|
|
|
# Variables
|
|
|
|
To represent a symbol, use `Variable`. I opted for the name
|
|
`Variable` to avoid conflict with the Dart primitive `Symbol`.
|
|
|
|
```dart
|
|
var foo = Variable<String>('foo');
|
|
var bar = Variable<String>('bar', value: 'baz');
|
|
|
|
// Call `lock` to mark a symbol as immutable.
|
|
var shelley = Variable<String>('foo', value: 'bar')..lock();
|
|
|
|
foo.value = 'bar';
|
|
shelley.value = 'Mary'; // Throws a StateError - constants cannot be overwritten.
|
|
|
|
foo.lock();
|
|
foo.value = 'baz'; // Also throws a StateError - Once a variable is locked, it cannot be overwritten.
|
|
```
|
|
|
|
## Visibility
|
|
|
|
Variables are *public* by default, but can also be marked as *private* or *protected*. This can be helpful if you are trying
|
|
to determine which symbols should be exported from a library or class.
|
|
|
|
```dart
|
|
myVariable.visibility = Visibility.protected;
|
|
myVariable.visibility = Visibility.private;
|
|
```
|
|
|
|
# Symbol Tables
|
|
|
|
It's easy to create a basic symbol table:
|
|
|
|
```dart
|
|
var mySymbolTable = SymbolTable<int>();
|
|
var doubles = SymbolTable<double>(values: {
|
|
'hydrogen': 1.0,
|
|
'avogadro': 6.022e23
|
|
});
|
|
|
|
// Create a new variable within the scope.
|
|
doubles.create('one');
|
|
doubles.create('one', value: 1.0);
|
|
doubles.create('one', value: 1.0, constant: true);
|
|
|
|
// Set a variable within an ancestor, OR create a new variable if none exists.
|
|
doubles.assign('two', 2.0);
|
|
|
|
// Completely remove a variable.
|
|
doubles.remove('two');
|
|
|
|
// Find a symbol, either in this symbol table or an ancestor.
|
|
var symbol = doubles.resolve('one');
|
|
|
|
// Find OR create a symbol.
|
|
var symbol = doubles.resolveOrCreate('one');
|
|
var symbol = doubles.resolveOrCreate('one', value: 1.0);
|
|
var symbol = doubles.resolveOrCreate('one', value: 1.0, constant: true);
|
|
```
|
|
|
|
# Exporting Symbols
|
|
|
|
Due to the tree structure of symbol tables, it is extremely easy to
|
|
extract a linear list of distinct variables, with variables lower in the hierarchy superseding their parents
|
|
(effectively accomplishing variable shadowing).
|
|
|
|
```dart
|
|
var allSymbols = mySymbolTable.allVariables;
|
|
```
|
|
|
|
We can also extract symbols which are *not* private. This helps us export symbols from libraries
|
|
or classes.
|
|
|
|
```dart
|
|
var exportedSymbols = mySymbolTable.allPublicVariables;
|
|
```
|
|
|
|
It's easy to extract symbols of a given visibility:
|
|
|
|
```dart
|
|
var exportedSymbols = mySymbolTable.allVariablesWithVisibility(Visibility.protected);
|
|
```
|
|
|
|
# Child Scopes
|
|
|
|
There are three ways to create a new symbol table:
|
|
|
|
## Regular Children
|
|
|
|
This is what most interpreters need; it simply creates a symbol table with the current symbol table
|
|
as its parent. The new scope can define its own symbols, which will only shadow the ancestors within the
|
|
correct scope.
|
|
|
|
```dart
|
|
var child = mySymbolTable.createChild();
|
|
var child = mySymbolTable.createChild(values: {...});
|
|
```
|
|
|
|
### Depth
|
|
|
|
Every symbol table has an associated `depth` attached to it, with the `depth` at the root
|
|
being `0`. When `createChild` is called, the resulting child has an incremented `depth`.
|
|
|
|
## Clones
|
|
|
|
This creates a scope at the same level as the current one, with all the same variables.
|
|
|
|
```dart
|
|
var clone = mySymbolTable.clone();
|
|
```
|
|
|
|
## Forked Scopes
|
|
|
|
If you are implementing a language with closure functions, you might consider looking into this.
|
|
A forked scope is a scope identical to the current one, but instead of merely copying references
|
|
to variables, the values of variables are copied into new ones.
|
|
|
|
The new scope is essentially a "frozen" version of the current one.
|
|
|
|
It is also effectively orphaned - though it is aware of its `parent`, the parent scope is unaware
|
|
that the forked scope is a child. Thus, calls to `resolve` may return old variables, if a parent
|
|
has called `remove` on a symbol.
|
|
|
|
```dart
|
|
var forked = mySymbolTable.fork();
|
|
var forked = mySymbolTable.fork(values: {...});
|
|
```
|
|
|
|
# Creating Names
|
|
|
|
In languages with block scope, oftentimes, identifiers will collide within a global scope.
|
|
To avoid this, symbol tables expose a `uniqueName()` method that simply attaches a numerical suffix to
|
|
an input name. The name is guaranteed to never be repeated within a specific scope.
|
|
|
|
```dart
|
|
var name0 = mySymbolTable.uniqueName('foo'); // foo0
|
|
var name1 = mySymbolTable.uniqueName('foo'); // foo1
|
|
var name2 = mySymbolTable.uniqueName('foo'); // foo2
|
|
```
|
|
|
|
# `this` Context
|
|
|
|
Many languages handle a sort of `this` context that values within a scope may
|
|
optionally be resolved against. Symbol tables can easily set their context
|
|
as follows:
|
|
|
|
```dart
|
|
void foo() {
|
|
mySymbolTable.context = thisContext;
|
|
}
|
|
```
|
|
|
|
Resolution of the `context` getter functions just like a symbol; if none is
|
|
set locally, then it will refer to the parent.
|
|
|
|
```dart
|
|
void bar() {
|
|
mySymbolTable.context = thisContext;
|
|
expect(mySymbolTable.createChild().createChild().context, thisContext);
|
|
}
|
|
```
|