Add 'optional' imports/exports

Author: lukewagner

design/mvp/Binary.md

@@ -232,7 +232,9 @@ instancedecl  ::= 0x00 t:<core:type>                      => t
                 | 0x04 ed:<exportdecl>                    => ed
 importdecl    ::= na:<nameattributes> et:<externtype>     => (import na et)
 exportdecl    ::= na:<nameattributes> et:<externtype>     => (export na et)
-externtype    ::= 0x00 0x11 i:<core:typeidx>              => (core module (type i))
+externtype    ::= 0x10 t:<someexterntype>                 => (optional t)
+                | t:<someexterntype>                      => t
+someexterntype ::= 0x00 0x11 i:<core:typeidx>             => (core module (type i))
                 | 0x01 i:<typeidx>                        => (func (type i))
                 | 0x02 b:<valuebound>                     => (value b) 🪙
                 | 0x03 b:<typebound>                      => (type b)
@@ -541,6 +543,7 @@ named once.
   subset of `canonopt`s.
 * Add optional `shared` immediate to all canonical definitions (explicitly or
   via `<canonopt>`) when shared-everything-threads (🧵②) is added.
+* Improve `externtype` opcodes for `optional`
 
 
 [`core:byte`]: https://webassembly.github.io/spec/core/binary/values.html#binary-byte

design/mvp/CanonicalABI.md

@@ -68,6 +68,7 @@ specified here.
   * [`canon thread.yield-then-resume`](#-canon-threadyield-then-resume) 🧵
   * [`canon thread.suspend-then-promote`](#-canon-threadsuspend-then-promote) 🧵
   * [`canon thread.yield-then-promote`](#-canon-threadyield-then-promote) 🧵
+  * [`canon optional.test`](#-canon-optionaltest) ❓
   * [`canon error-context.new`](#-canon-error-contextnew) 📝
   * [`canon error-context.debug-message`](#-canon-error-contextdebug-message) 📝
   * [`canon error-context.drop`](#-canon-error-contextdrop) 📝
@@ -5097,6 +5098,20 @@ prepared to propagate cancellation, they can omit `cancellable` so that
 cancellation is instead delivered at a later `cancellable` call.
 
 
+### ❓ `canon optional.test`
+
+For a canonical definition:
+```wat
+(canon optional.test (instance $instanceidx) (core global $present))
+```
+validation specifies:
+* `$instanceidx` must refer to an instance in the instance index space with
+  type `optional`
+* `$optional.test` is given type `(global i32)`
+
+TODO: describe
+
+
 ### 📝 `canon error-context.new`
 
 For a canonical definition:

design/mvp/Explainer.md

@@ -21,12 +21,14 @@ more user-focused explanation, take a look at the
     * [Specialized value types](#specialized-value-types)
     * [Definition types](#definition-types)
     * [Declarators](#declarators)
+    * [Optional types](#optional-types)
     * [Type checking](#type-checking)
   * [Canonical definitions](#canonical-definitions)
     * [Canonical ABI](#canonical-abi)
     * [Canonical built-ins](#canonical-built-ins)
       * [Resource built-ins](#resource-built-ins)
       * [Concurrency built-ins](#-concurrency-built-ins)
+      * [Feature-test built-in](#-feature-test-built-in)
       * [Error Context built-ins](#-error-context-built-ins)
   * [Value definitions](#-value-definitions)
   * [Start definitions](#-start-definitions)
@@ -69,6 +71,7 @@ subsequent WASI Developer Preview minor releases:
 * 🐘: [memory64]
 * 🗺️: the `map` type
 * 🏷️: `implements` annotations for plain-named interface imports/exports
+* ❓: `optional` imports and exports
 
 
 ## Grammar
@@ -618,7 +621,9 @@ instancedecl  ::= core-prefix(<core:type>)
                 | <exportdecl>
 importdecl    ::= (import <externnamelit> <attribute>* bind-id(<externtype>))
 exportdecl    ::= (export <externnamelit> <attribute>* bind-id(<externtype>))
-externtype    ::= (<sort> (type <u32>) )
+externtype    ::= (optional <someexterntype>)
+                | <someexterntype>
+someexterntype ::= (<sort> (type <u32>) )
                 | core-prefix(<core:moduletype>)
                 | <functype>
                 | <componenttype>
@@ -1009,6 +1014,10 @@ definitions:
 Note that the inline use of `$G` and `$U` are syntactic sugar for `outer`
 aliases.
 
+#### Optional types
+
+TODO
+
 #### Type Checking
 
 Like core modules, components have an up-front validation phase in which the
@@ -1091,6 +1100,8 @@ and `$C1` is a subtype of `$C2`:
 checking and so any `external-id`, `implements` or `versionsuffix` attribute can
 be added to any of the `func`s above without changing the result.
 
+TODO: also mention `optional`
+
 When we next consider type imports and exports, there are two distinct
 subcases of `typebound` to consider: `eq` and `sub`.
 
@@ -1545,6 +1556,7 @@ canon ::= ...
         | (canon thread.yield-then-resume cancellable? (core func <id>?)) 🧵
         | (canon thread.suspend-then-promote cancellable? (core func <id>?)) 🧵
         | (canon thread.yield-then-promote cancellable? (core func <id>?)) 🧵
+        | (canon optional.test (instance <instanceidx>) (core global <id>?)) ❓
         | (canon error-context.new <canonopt>* (core func <id>?)) 📝
         | (canon error-context.debug-message <canonopt>* (core func <id>?)) 📝
         | (canon error-context.drop (core func <id>?)) 📝
@@ -2331,6 +2343,11 @@ For details, see [`canon_thread_available_parallelism`] in the Canonical ABI
 explainer.
 
 
+##### ❓ Feature-test built-in
+
+TODO: describe `canon optional.test`
+
+
 ##### 📝 Error Context built-ins
 
 ###### 📝 `error-context.new`

design/mvp/WIT.md

@@ -308,6 +308,8 @@ world union-my-world {
 }
 ```
 
+TODO: add `optional` to examples
+
 ### De-duplication of interfaces
 
 If two worlds share an imported or exported [interface name], then the union of
@@ -339,6 +341,8 @@ world union-my-world-b {
 }
 ```
 
+TODO: mention merging of `optional` and non-`optional`
+
 ### Name Conflicts and `with`
 
 When two or more included Worlds have the same name for an import or export
@@ -438,6 +442,7 @@ world invalid-union-world {
 
 ### A Note on Subtyping
 
+TODO: update this
 In the future, when `optional` export is supported, the world author may explicitly mark exports as optional to make a component targeting an included World a subtype of the union World.
 
 For now, we are not following the subtyping rules for the `include` statement. That is, the `include` statement does not imply any subtyping relationship between the included worlds and the union world.
@@ -563,6 +568,9 @@ interface my-interface {
 }
 ```
 
+TODO: mention `use` from `optional` import/export on world
+TODO: we'll need `optional` dependencies... (and if we have that, maybe can have optional on interface items...)
+
 #### Top-level `use`
 
 If a package being referred to has a version number, then using the above syntax
@@ -1448,9 +1456,11 @@ import-item ::= external-id? 'import' id ':' extern-type
 
 external-id ::= '@external-id' '(' string-literal ')' 🏷️
 
-extern-type ::= func-type ';'
-              | 'interface' '{' interface-items* '}'
-              | use-path ';' 🏷️
+extern-type      ::= 'optional' some-extern-type
+                   | some-extern-type
+some-extern-type ::= func-type ';'
+                   | 'interface' '{' interface-items* '}'
+                   | use-path ';' 🏷️
 ```
 
 🏷️ While `id`s are restricted to kebab-case (to enable idiomatic bindings for
@@ -1508,6 +1518,8 @@ world my-world {
 }
 ```
 
+TODO: note `optional` and what it means
+
 [`componenttype`]: Explainer.md#type-definitions
 
 ## Item: `include`