From f9b5ae545f23f41c2087117584f3f78130ed573b Mon Sep 17 00:00:00 2001
From: Josh Price <josh@alembic.com.au>
Date: Tue, 10 Oct 2017 15:36:41 +1100
Subject: [PATCH 1/4] Fix compile warnings

---
 mix.exs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/mix.exs b/mix.exs
index 29d1c6e..50ba596 100644
--- a/mix.exs
+++ b/mix.exs
@@ -7,9 +7,9 @@ defmodule Voxpop.Mixfile do
      elixir: "~> 1.2",
      build_embedded: Mix.env == :prod,
      start_permanent: Mix.env == :prod,
-     deps: deps,
-     package: package,
-     description: description
+     deps: deps(),
+     package: package(),
+     description: description()
     ]
   end
 

From d4b2890ae6fe79acd46a9ea18ffa756d1f76b33b Mon Sep 17 00:00:00 2001
From: Josh Price <josh@alembic.com.au>
Date: Tue, 10 Oct 2017 15:36:55 +1100
Subject: [PATCH 2/4] Failing test for missing rules

---
 test/voxpop_test.exs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/test/voxpop_test.exs b/test/voxpop_test.exs
index 9815702..9e0bce5 100644
--- a/test/voxpop_test.exs
+++ b/test/voxpop_test.exs
@@ -21,4 +21,9 @@ defmodule VoxpopTest do
     concat_grammar = %Voxpop.Grammar.Definition{start: "{greeting} world", rules: %{greeting: "Hello"}}
     assert Voxpop.generate(concat_grammar) == "Hello world"
   end
+
+  test "gracefully handles missing rules" do
+    concat_grammar = %Voxpop.Grammar.Definition{start: "{greeting} world", rules: %{greeting: "Hello {invalid}"}}
+    assert Voxpop.generate(concat_grammar) == "Hello "
+  end
 end

From 27beac3323b1fea2383695729675bf0ac3fb0b65 Mon Sep 17 00:00:00 2001
From: Josh Price <josh@alembic.com.au>
Date: Tue, 10 Oct 2017 16:25:13 +1100
Subject: [PATCH 3/4] Show error inline perhaps?

---
 lib/voxpop/non_terminal.ex | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/lib/voxpop/non_terminal.ex b/lib/voxpop/non_terminal.ex
index 8e7d2fb..7048c03 100644
--- a/lib/voxpop/non_terminal.ex
+++ b/lib/voxpop/non_terminal.ex
@@ -5,6 +5,8 @@ end
 
 defimpl Voxpop.Production, for: Voxpop.NonTerminal do
   def evaluate(non_terminal, registry) do
-    registry.rules |> Map.get(non_terminal.rule) |> Voxpop.Production.evaluate(registry)
+    registry.rules
+    |> Map.get(non_terminal.rule, %Voxpop.Terminal{atom: "<missing rule `#{non_terminal.rule}`>"})
+    |> Voxpop.Production.evaluate(registry)
   end
 end

From 3abe05724e109812eaa6138bb51b429f0b10f819 Mon Sep 17 00:00:00 2001
From: Josh Price <josh@alembic.com.au>
Date: Tue, 10 Oct 2017 16:36:30 +1100
Subject: [PATCH 4/4] Raise error on missing rule

---
 lib/voxpop/non_terminal.ex | 10 +++++++---
 test/voxpop_test.exs       |  4 +++-
 2 files changed, 10 insertions(+), 4 deletions(-)

diff --git a/lib/voxpop/non_terminal.ex b/lib/voxpop/non_terminal.ex
index 7048c03..1f26960 100644
--- a/lib/voxpop/non_terminal.ex
+++ b/lib/voxpop/non_terminal.ex
@@ -5,8 +5,12 @@ end
 
 defimpl Voxpop.Production, for: Voxpop.NonTerminal do
   def evaluate(non_terminal, registry) do
-    registry.rules
-    |> Map.get(non_terminal.rule, %Voxpop.Terminal{atom: "<missing rule `#{non_terminal.rule}`>"})
-    |> Voxpop.Production.evaluate(registry)
+    value = Map.get(registry.rules, non_terminal.rule)
+
+    if value do
+      Voxpop.Production.evaluate(value, registry)
+    else
+      raise "Invalid rule: `#{non_terminal.rule}`"
+    end
   end
 end
diff --git a/test/voxpop_test.exs b/test/voxpop_test.exs
index 9e0bce5..c887478 100644
--- a/test/voxpop_test.exs
+++ b/test/voxpop_test.exs
@@ -24,6 +24,8 @@ defmodule VoxpopTest do
 
   test "gracefully handles missing rules" do
     concat_grammar = %Voxpop.Grammar.Definition{start: "{greeting} world", rules: %{greeting: "Hello {invalid}"}}
-    assert Voxpop.generate(concat_grammar) == "Hello "
+    assert_raise RuntimeError, "Invalid rule: `invalid`", fn ->
+      Voxpop.generate(concat_grammar)
+    end
   end
 end