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Clarify class declaration syntax #1026

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Jan 14, 2022
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Clarify class declaration syntax #1026

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48cf610
Clarify class declaration syntax.
josh11b Jan 13, 2022
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josh11b Jan 14, 2022
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11 changes: 8 additions & 3 deletions docs/design/classes.md
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Original file line number Diff line number Diff line change
Expand Up @@ -676,9 +676,10 @@ for argument passing were decided in

The declarations for nominal class types will have:

- an optional `abstract` or `base` prefix
- `class` introducer
- the name of the class
- in the future, we will have optional modifiers for inheritance
- an optional `extends` followed by the name of the immediate base class
- `{`, an open curly brace
- a sequence of declarations
- `}`, a close curly brace
Expand All @@ -705,10 +706,13 @@ determined at compile time.

To support circular references between class types, we allow
[forward declaration](https://en.wikipedia.org/wiki/Forward_declaration) of
types. A type that is forward declared is considered incomplete until the end of
a definition with the same name.
types. Forward declarations end with semicolon `;` after the name of the class,
instead of any `extends` clause and the block of declarations in curly braces
`{`...`}`. A type that is forward declared is considered incomplete until the
end of a definition with the same name.

```
// Forward declaration of `GraphNode`.
class GraphNode;

class GraphEdge {
Expand All @@ -719,6 +723,7 @@ class GraphEdge {
class GraphNode {
var edges: Vector(GraphEdge*);
}
// `GraphNode` is first complete here.
```

**Open question:** What is specifically allowed and forbidden with an incomplete
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