Document initial profile syntax

This PR sketches out the initial format for WebAssembly profiles,
i.e. world files.

Signed-off-by: Brian H <[email protected]>

Author: fibonacci1729

design/mvp/WORLD.md

@@ -0,0 +1,129 @@
+# The `*.world` format
+
+This is intended to document the `*.world` format as it currently exists. The goal is to provide an overview to understand what features `world` files give you and how they're structured. This isn't intended to be a formal grammar, although it's expected that one day we'll have a formal grammar for `*.world` files.
+
+Conceptually `world` documents act as a contract between runtimes and components. I.e., hosts implement worlds and components target them.
+
+## Lexical structure
+
+> TODO: Mention lexical additions relative to WIT.md
+
+## Top-level items
+
+A `world` document is a sequence of items specified at the top level. These items come one after another and it's recommended to separate them with newlines for readability but  this isn't required.
+
+Conceretely, the structure of a `world` file is:
+
+```
+top-level ::= (import | export | extend | use-item | type-item)*
+```
+
+### Item: `import`
+
+An `import` statement imports an instance, function, or value.
+
+The structure of an import statment is:
+
+```
+import ::= 'import' id ':' type-use
+```
+
+Example:
+
+```world
+import backends: { *: "wasi:http/Handler" }
+import console: "wasi:logging/Logger"
+```
+
+### Item: `export`
+
+An `export` statement exports an instance, function, or value.
+
+The structure of an export statment is:
+
+```
+export ::= 'export' id ':' type-use
+```
+
+Example:
+
+```world
+export backends: { +: "wasi:http/Handler" }
+export handler: "wasi:http/Handler"
+```
+
+### Item: `extends`
+
+An `extends` statement defines a subtype relationship with the referenced profile as the super type.
+
+The structure of an extend statement is:
+
+```
+extends ::= 'extends' pathlit
+```
+
+Example:
+
+```world
+// Service.world -- Common service profile
+
+import console: "wasi:logging/Logger"
+import logs: { *: "wasi:logging/Logger" }
+import config: { *: "wasi:config/Value" }
+import secrets: { *: "wasi:config/Secret" }
+import metrics: { *: "wasi:metrics/Counter" }
+
+```
+
+```world
+// http/Service.world -- An HTTP service profile
+
+extends "wasi:Service"
+export "wasi:http/Handler"
+```
+
+### Item: `use-item`
+
+> TODO: Does `use` apply to `wit` files?
+
+### Item: `type-item`
+
+A `type-item` statement declares a new named type in the `world` document. This name can be later referred to when defining `import` and `export` items. 
+
+The structure of a type-item statement is:
+
+```
+type-item ::= 'type' id '=' extern-type
+```
+
+Example:
+
+```world
+type message = record { x: string }
+type greeter = func(msg: message) -> expected<unit, string>
+```
+
+> TODO: Discuss semantics of type item
+
+# Grammar
+```
+    top-level ::= (extend | import | export  | use-item | type-item)*
+      extends ::= 'extends' worldfile
+       import ::= 'import' id ':' type-use
+       export ::= 'export' id ':' type-use
+    type-item ::= 'type' id '=' extern-type
+     type-use ::= id | extern-type
+    func-type ::= 'async'? 'func' '(' func-args? ')' func-ret?
+    func-args ::= func-arg | func-arg ',' func-args?
+     func-arg ::= id ':' val-type
+     func-ret ::= '->' val-type
+     val-type ::= ty | ... | 'any'
+  extern-type ::= instance-type | func-type | val-type
+instance-type ::= '{' export* '}' | '{' ('*' | '+') extern-type '}' | witfile
+```
+
+> NOTE: `use-def` as defined in [WIT.md](https://github.com/bytecodealliance/wit-bindgen/blob/main/WIT.md#item-use)
+
+> NOTE: `ty` as defined in [WIT.md](https://github.com/bytecodealliance/wit-bindgen/blob/main/WIT.md#item-ty)
+
+> TODO: Describe `*` & `+` instance types.