feat: single var filter

lib/wongi/engine/dsl.ex

@@ -134,6 +134,13 @@ defmodule Wongi.Engine.DSL do
   """
   def filter(func), do: Filter.new(Function.new(func))
 
+  @doc """
+  A filter that passes if the value is a unary function that returns true.
+
+  The function will receive the variable as its argument.
+  """
+  def filter(var, func), do: Filter.new(Function.new(var, func))
+
   @doc """
   A matcher that computes some value across all its incoming tokens, across all
   execution paths.

lib/wongi/engine/filter/function.ex

@@ -1,18 +1,26 @@
 defmodule Wongi.Engine.Filter.Function do
   @moduledoc false
-  defstruct [:func]
+  defstruct [:var, :func]
 
   def new(func) do
     %__MODULE__{func: func}
   end
 
+  def new(var, func) do
+    %__MODULE__{var: var, func: func}
+  end
+
   defimpl Wongi.Engine.Filter do
-    def pass?(%@for{func: func}, _token) when is_function(func, 0) do
+    def pass?(%@for{var: nil, func: func}, _token) when is_function(func, 0) do
       func.()
     end
 
-    def pass?(%@for{func: func}, token) when is_function(func, 1) do
+    def pass?(%@for{var: nil, func: func}, token) when is_function(func, 1) do
       func.(token)
     end
+
+    def pass?(%@for{var: var, func: func}, token) when is_function(func, 1) do
+      func.(token[var])
+    end
   end
 end

test/wongi/engine/filter/function_test.exs

@@ -8,7 +8,7 @@ defmodule Wongi.Engine.Filter.FunctionTest do
         rule(
           forall: [
             has(:a, :b, var(:x)),
-            filter(&__MODULE__.true?/1)
+            filter(&__MODULE__.x_is_true?/1)
           ]
         )
       )
@@ -24,7 +24,7 @@ defmodule Wongi.Engine.Filter.FunctionTest do
         rule(
           forall: [
             has(:a, :b, var(:x)),
-            filter(&__MODULE__.true?/1)
+            filter(&__MODULE__.x_is_true?/1)
           ]
         )
       )
@@ -33,5 +33,24 @@ defmodule Wongi.Engine.Filter.FunctionTest do
     assert [] = tokens(rete, ref) |> Enum.to_list()
   end
 
-  def true?(token), do: token[:x]
+  test "accepts a single variable" do
+    {rete, ref} =
+      new()
+      |> compile_and_get_ref(
+        rule(
+          forall: [
+            has(:a, :b, var(:x)),
+            filter(:x, &__MODULE__.true?/1)
+          ]
+        )
+      )
+
+    rete = rete |> assert(:a, :b, false)
+    assert [] = tokens(rete, ref) |> Enum.to_list()
+  end
+
+  def x_is_true?(token), do: token[:x]
+
+  def true?(true), do: true
+  def true?(false), do: false
 end