Added discussion

rfcs/function-and-table-types.md

@@ -7,8 +7,39 @@ Add types for "real" functions and tables.
 ## Motivation
 
 Some APIs require "real" functions and tables, not just things that
-"look functiony" (e.g. tables with a `__call__` metamethod) or "look
-tabley" (e.g. instances of classes). This RFC adds types for those.
+"look functiony" (e.g. tables with a `__call` metamethod) or "look
+tably" (e.g. instances of classes). This RFC adds types for those.
+
+For example, the function:
+
+```lua
+  function succ(x)
+    if type(x) == "function"
+      return x() + 1
+    else
+      assert(type(x) == number)
+      return x + 1
+    end
+  end
+```
+
+cannot quite be given an accurate Luau type. The nearest is `(number |
+()->number) -> number` but this is slightly too generous, since it
+allows "functiony" types such as tables with a `__call` metamethod:
+
+```lua
+  local t = setmetatable({}, {__call = function(self) return 5 end})
+  succ(t)
+```
+
+This will typecheck but produce a runtime error.
+
+A similar issue affects tables.
+
+The common cases in practice are built-in APIs such as the Luau
+standard library, or APIs implemented using the C++ FFI. For example,
+`pairs` takes a table argument, and gives a runtime error if it is called
+with a class instance.
 
 ## Design
 
@@ -18,14 +49,16 @@ Add:
 * a type `function`, inhabited by Luau functions (but not class methods or
 tables with metamethods).
 
-Luau functions with known source and targe types are now an intersection type `function & (T) -> U`.
+Luau functions with known source and target types are now an intersection type `function & (T) -> U`.
 
-Luau tables with known properties are now an intersection type `table & { p : T }`.
+Luau tables with known shape are now an intersection type `table & { p : T }`.
 
 We may want to provide syntax sugar `function(T) -> U` and `table{ p : T }` for these, since they will probably be quite common.
 
 We should audit APIs to see which ones accept functiony or tably arguments, and which ones want real functions and tables.
 
+The use of intersecton types `table & { p: T }` depends on the [Shape types RFC](shape-types).
+
 ## Drawbacks
 
 Another bit of complexity budget spent.