Add a short doc explaining some of the current extensions above-and-b…

…eyond LambdaMOO 1.8

doc/extensions.md

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+## Extensions
+
+`moor` adds the following notable extensions to the LambdaMOO 1.8.x language and runtime. Note that this list
+isn't fully complete as some features are still in development, and some are not so easily categorized
+as extensions and more "architectural differences" which will be described elsewhere.
+
+### Lexical variable scoping
+
+Adds block-level lexical scoping to the LambdaMOO language.
+
+Enabled by default, can be disabled with command line option `--lexical-scopes=false`
+
+In LambdaMOO all variables are global to the verb scope, and are bound at the first assignment.
+`moor` adds optional lexical scoping, where variables are bound to the scope in which they are declared,
+and leave the scope when it is exited.
+This is done by using the `let` keyword to declare variables that will exist only in the current scope.
+
+```moo
+while (y < 10)
+  let x = 1;
+  ...
+  if (x == 1000)
+    return x;
+  endif
+endwhile
+```
+
+The block scopes for lexical variables apply to `for`, `while`, `if`/`elseif`, `try`/`except`
+... and a new ad-hoc block type declared with `begin` / `end`:
+
+```moo
+begin
+  let x = 1;
+  ...
+end
+```
+
+Where a variable is declared in a block scope, it is not visible outside of that block.
+Global variables are still available, and can be accessed from within a block scope.
+In the future a `strict` mode may be added to require all variables to be declared lexically before use,
+but this would break compatibility with existing code.
+
+### Primitive type dispatching
+
+Adds the ability to call verbs on primitive types, such as numbers and strings.
+
+Enabled by default, can be disabled with command line option `--type-dispatch=false`
+
+In LambdaMOO, verbs can only be called on objects. `moor` adds the ability to indirectly dispatch verbs for primitive
+types by interpreting the verb name as a verb on a special object that represents the primitive type and passing
+the value itself as the first argument to the verb. Where other arguments are passed, they are appended to the argument
+list.
+
+```moo
+"hello":length() => 5
+```
+
+is equivalent to
+
+```moo
+$string:length("hello") => 5
+```
+
+The names of the type objects to the system objects `$string`, `$float`, `$integer`, `$list`, `$map`, and `$error`
+
+### Map type
+
+`moor` adds a dictionary or map type mostly equivalent to the one present in `stunt`/`toaststunt`.
+
+Enabled by default, can be disabled with command line option `--map-type=false`
+
+This is a set of immutable sorted key-value pairs with convenient syntax for creating and accessing them and
+can be used as an alternative to traditional MOO `alists` / "associative lists".
+
+```moo
+let my_map = [ "a" => 1, "b" => 2, "c" => 3 ];
+my_map["a"] => 1
+```
+
+Wherever possible, the Map type semantics are made to be compatible with those from `stunt`, so
+most existing code from those servers should work with `moor` with no changes.
+
+Values are stored sorted, and lookup uses a binary search. The map is immutable, and modification is done through
+copy and update like other MOO primitive types.
+
+Performance wise, construction and lookup are O(log n) operations, and iteration is O(n).
+
+### "Rich" output via `notify`
+
+The `notify` builtin in MOO is used to output a line of text over the telnet connection to the player.
+
+In `moor`, the connection may not be `telnet`, and the output may be more than just text.
+
+Enabled by default, can be disabled with command line option `--rich-notify=false`
+
+Moor ships with a `web-host` that can serve HTTP and websocket connections, and the `notify` verb can be used to
+output JSON values over the websocket connection.
+
+When this feature is enabled, the second (value) argument to `notify` can be any MOO value, and it will be
+serialized to JSON and sent to the client.
+
+If a third argument is present, it is expected to be a "content-type" string, and will be places in the websocket
+JSON message as such.
+
+```moo
+notify(player, [ "type" => "message", "text" => "Hello, world!" ], "application/json");
+```
+
+becomes, on the websocket:
+
+```json
+{
+  "content-type": "application/json",
+  "message": {
+    "map_pairs": [
+      [
+        "type",
+        "message"
+      ],
+      [
+        "text",
+        "Hello, world!"
+      ]
+    ]
+  },
+  "system-time": 1234567890
+}
+```
+
+The `telnet` host will still output the text value of the second argument as before, and ignore anything which
+is not a string.