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Nested interfaces #372

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nested interfaces
macovedj Jun 24, 2024
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75 changes: 73 additions & 2 deletions design/mvp/WIT.md
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Expand Up @@ -116,7 +116,7 @@ would correspond to:
)
```

An `interface` can contain [`use`][use] statements, [type][types] definitions,
An `interface` can contain [`use`][use] and [`nest`][nest] statements, [type][types] definitions,
and [function][functions] definitions. For example:

```wit
Expand All @@ -136,7 +136,7 @@ interface types {
}
```

More information about [`use`][use] and [types] are described below, but this
More information about [`use`][use], [`nest`][nest], and [types] are described below, but this
is an example of a collection of items within an `interface`. All items defined
in an `interface`, including [`use`][use] items, are considered as exports from
the interface. This means that types can further be used from the interface by
Expand Down Expand Up @@ -660,6 +660,73 @@ world w2 {
> configure that a `use`'d interface is a particular import or a particular
> export.

## Nesting WIT interfaces
[nest]: #nesting-wit-interfaces
Interfaces can also contain other interfaces via the `nest` keyword.
With the `nest` keyword, one can reference other interfaces in the same package, foreign packages, or define anonymous interfaces inline.

```wit
package local:example;

interface foo {
...
}

interface top {
nest foo;
nest foreign:pkg/bar;
baz: interface {
...
}
}
```

Each of these forms of nesting interfaces are encoded as:

```wasm
(component
(type (;0;)
(component
(type (;0;)
(instance
... `types from foo`
)
)
(export "local:example/foo" (type 0))
)
)
(export (;1;) "foo" (type 0))
(type (;2;)
(component
(type (;0;)
(instance
(type (;0;)
(instance
... `types from foo`
)
)
(export "local:example/foo" (instance (type 0)))
(type (;1;)
(instance
... `types from foreign:pkg/bar`
)
)
(export "foreign:pkg/bar" (instance (type 1)))
(type (;2;)
(instance
... `types from baz`
)
)
(export "baz" (instance (type 2)))
)
)
(export "local:example/top (type 1))
)
)
(export (;3;) "top" (type 2))
)
```

## WIT Functions
[functions]: #wit-functions

Expand Down Expand Up @@ -1106,6 +1173,7 @@ interface-items ::= gate interface-definition
interface-definition ::= typedef-item
| use-item
| func-item
| nest-item

typedef-item ::= resource-item
| variant-items
Expand All @@ -1128,6 +1196,9 @@ named-type-list ::= ϵ
| named-type ( ',' named-type )*

named-type ::= id ':' ty

nest-item ::= gate nest use-path ';'
| id: interface-item
```


Expand Down