feat: guards for encoded Lua values (#85)

Author: davydog187

CHANGELOG.md

@@ -7,6 +7,14 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## Unreleased
 
+### Added
+- Guards for encoded Lua values in `deflua` functions
+  - `is_table/1`
+  - `is_userdata/1`
+  - `is_lua_func/1`
+  - `is_erl_func/1`
+  - `is_mfa/1`
+
 ### Fixed
 - `deflua` function can now specify guards when using or not using state
 

lib/lua/api.ex

@@ -74,8 +74,22 @@ defmodule Lua.API do
           ~LUA[print("Hello at install time!")]c
         end
       end
+
+  ## Guards
+
+  When doing `use Lua.API`, we also import the guards documented in this API.
+  This can be useful for having different function heads that match on encoded
+  values. E.g.
+
+      deflua say_type(value) when is_table(value), do: "table"
+      deflua say_type(value) when is_userdata(value), do: "table"
+
+  Keep in mind that if you want to work with values passed to `deflua` functions,
+  they still need to be decoded first.
   """
 
+  require Record
+
   defmacro __using__(opts) do
     scope = opts |> Keyword.get(:scope, "") |> String.split(".", trim: true)
 
@@ -85,7 +99,17 @@ defmodule Lua.API do
       @before_compile Lua.API
 
       import Lua.API,
-        only: [runtime_exception!: 1, deflua: 2, deflua: 3, validate_func!: 3]
+        only: [
+          runtime_exception!: 1,
+          deflua: 2,
+          deflua: 3,
+          validate_func!: 3,
+          is_table: 1,
+          is_userdata: 1,
+          is_lua_func: 1,
+          is_erl_func: 1,
+          is_mfa: 1
+        ]
 
       @impl Lua.API
       def scope do
@@ -100,6 +124,32 @@ defmodule Lua.API do
   @callback install(Lua.t(), scope_def(), any()) :: Lua.t() | Lua.Chunk.t() | String.t()
   @optional_callbacks [install: 3]
 
+  @doc """
+  Is the value a reference to a Lua table?
+
+  """
+  defguard is_table(record) when Record.is_record(record, :tref)
+
+  @doc """
+  Is the value a reference to userdata?
+  """
+  defguard is_userdata(record) when Record.is_record(record, :usdref)
+
+  @doc """
+  Is the value a reference to a Lua function?
+  """
+  defguard is_lua_func(record) when Record.is_record(record, :funref)
+
+  @doc """
+  Is the value a reference to an Erlang / Elixir function?
+  """
+  defguard is_erl_func(record) when Record.is_record(record, :erl_func)
+
+  @doc """
+  Is the value a reference to an Erlang / Elixir mfa?
+  """
+  defguard is_mfa(record) when Record.is_record(record, :erl_mfa)
+
   @doc """
   Raises a runtime exception inside an API function, displaying contextual
   information about where the exception was raised.

lib/lua/util.ex

@@ -26,7 +26,7 @@ defmodule Lua.Util do
   # take out when https://github.com/rvirding/luerl/pull/213
   # is released
   def encoded?(number) when is_number(number), do: true
-  def encoded?(table_ref) when Record.is_record(table_ref, :tref), do: true
+  def encoded?(record) when Record.is_record(record, :tref), do: true
   def encoded?(record) when Record.is_record(record, :usdref), do: true
   def encoded?(record) when Record.is_record(record, :funref), do: true
   def encoded?(record) when Record.is_record(record, :erl_func), do: true

test/lua/api_test.exs

@@ -336,4 +336,57 @@ defmodule Lua.APITest do
       assert {"not a int", _} = module.with_state(true, Lua.new())
     end
   end
+
+  describe "guards" do
+    test "can use in functions" do
+      assert [{module, _}] =
+               Code.compile_string("""
+               defmodule GuardCheck do
+                 use Lua.API, scope: "guard"
+
+                 deflua type(value) when is_table(value) do
+                   "table"
+                 end
+
+                 deflua type(value) when is_userdata(value) do
+                   "userdata"
+                 end
+
+                 deflua type(value) when is_lua_func(value) do
+                   "lua function"
+                 end
+
+                 deflua type(value) when is_erl_func(value) do
+                   "erl function"
+                 end
+
+                 deflua type(value) when is_mfa(value) do
+                   "mfa"
+                 end
+
+                 deflua type(_value) do
+                   "other"
+                 end
+               end
+               """)
+
+      lua =
+        Lua.load_api(Lua.new(), module)
+        |> Lua.set!(["foo"], {:userdata, URI.parse("https://tvlabs.ai")})
+
+      assert {["table"], _} = Lua.eval!(lua, "return guard.type({})")
+      assert {["userdata"], _} = Lua.eval!(lua, "return guard.type(foo)")
+
+      assert {["lua function"], _} =
+               Lua.eval!(lua, """
+               return guard.type(function()
+                 return 42
+               end)
+               """)
+
+      assert {["erl function"], _} = Lua.eval!(lua, "return guard.type(guard.type)")
+      assert {["mfa"], _} = Lua.eval!(lua, "return guard.type(string.lower)")
+      assert {["other"], _} = Lua.eval!(lua, "return guard.type(5)")
+    end
+  end
 end