Allow choice fallback after traversal errors (#154)

Fixes #146

Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>

Author: mvanhorn

lib/nimble_parsec/compiler.ex

@@ -365,7 +365,7 @@ defmodule NimbleParsec.Compiler do
     end
   end
 
-  defp traverse(traversal, next, rest, user_acc, context, line, offset, _) do
+  defp traverse(traversal, next, rest, user_acc, context, line, offset, config) do
     case apply_traverse(traversal, rest, user_acc, context, line, offset) do
       {:{}, _, [rest, expanded_acc, context]} ->
         quote do
@@ -382,23 +382,34 @@ defmodule NimbleParsec.Compiler do
         end
 
       {:error, reason} ->
-        quote do
-          {:error, unquote(reason), rest, context, line, offset}
-        end
+        traverse_error(reason, config)
 
       quoted ->
+        error = traverse_error(quote(do: reason), config)
+
         quote generated: true do
           case unquote(quoted) do
             {rest, user_acc, context} when is_list(user_acc) ->
               unquote(next)(rest, user_acc ++ acc, stack, context, line, offset)
 
             {:error, reason} ->
-              {:error, reason, rest, context, line, offset}
+              unquote(error)
           end
         end
     end
   end
 
+  defp traverse_error(reason, %{catch_all: nil}) do
+    quote do
+      {:error, unquote(reason), rest, context, line, offset}
+    end
+  end
+
+  defp traverse_error(_reason, %{catch_all: catch_all, acc_depth: n}) do
+    {_, _, _, body} = build_proxy_to(:unused, catch_all, n)
+    body
+  end
+
   defp apply_traverse(mfargs, rest, acc, context, line, offset) do
     apply_traverse(Enum.reverse(mfargs), {:{}, [], [rest, acc, context]}, line, offset)
   end

test/nimble_parsec_test.exs

@@ -1,3 +1,7 @@
+defmodule NimbleParsecTest.QuotedTraversal do
+  def error(_rest, _acc, _context, _line, _offset), do: {:error, "quoted traversal error"}
+end
+
 defmodule NimbleParsecTest do
   use ExUnit.Case, async: true
 
@@ -1252,9 +1256,44 @@ defmodule NimbleParsecTest do
   end
 
   describe "choice/2 combinator" do
+    three_ascii_chars = times(ascii_char([?a..?z, ?\n]), 3)
+
     defparsecp :simple_choice,
                choice([ascii_char([?a..?z]), ascii_char([?A..?Z]), ascii_char([?0..?9])])
 
+    defparsecp :choice_post_traverse,
+               choice([
+                 post_traverse(three_ascii_chars, {__MODULE__, :error_when_last_is_z, []}),
+                 replace(three_ascii_chars, nil)
+               ])
+
+    defparsecp :choice_pre_traverse,
+               choice([
+                 pre_traverse(three_ascii_chars, {__MODULE__, :error_when_last_is_z, []}),
+                 replace(three_ascii_chars, nil)
+               ])
+
+    defparsecp :choice_quoted_post_traverse,
+               choice([
+                 quoted_post_traverse(
+                   three_ascii_chars,
+                   {NimbleParsecTest.QuotedTraversal, :error, []}
+                 ),
+                 replace(three_ascii_chars, nil)
+               ])
+
+    defparsecp :standalone_quoted_post_traverse,
+               quoted_post_traverse(
+                 three_ascii_chars,
+                 {NimbleParsecTest.QuotedTraversal, :error, []}
+               )
+
+    defparsecp :choice_post_traverse_without_matching_fallback,
+               choice([
+                 post_traverse(three_ascii_chars, {__MODULE__, :error_when_last_is_z, []}),
+                 string("xyz")
+               ])
+
     defparsecp :choice_label,
                choice([ascii_char([?a..?z]), ascii_char([?A..?Z]), ascii_char([?0..?9])])
                |> label("something")
@@ -1304,6 +1343,31 @@ defmodule NimbleParsecTest do
       assert simple_choice("+=") == {:error, @error, "+=", %{}, {1, 0}, 0}
     end
 
+    test "falls back after a post-traversal error" do
+      assert choice_post_traverse("a\nc!") == {:ok, ~c"a\nc", "!", %{}, {2, 2}, 3}
+
+      assert choice_post_traverse("a\nz!", context: %{source: :initial}) ==
+               {:ok, [nil], "!", %{source: :initial}, {2, 2}, 3}
+    end
+
+    test "falls back after a pre-traversal error" do
+      assert choice_pre_traverse("a\nz!") == {:ok, [nil], "!", %{}, {2, 2}, 3}
+    end
+
+    test "falls back after a quoted traversal error" do
+      assert choice_quoted_post_traverse("a\nc!") == {:ok, [nil], "!", %{}, {2, 2}, 3}
+
+      assert standalone_quoted_post_traverse("a\nc!") ==
+               {:error, "quoted traversal error", "!", %{}, {2, 2}, 3}
+    end
+
+    test "returns a parser error when no branch succeeds after a traversal error" do
+      assert {:error, reason, "a\nz!", %{}, {1, 0}, 0} =
+               choice_post_traverse_without_matching_fallback("a\nz!")
+
+      assert is_binary(reason)
+    end
+
     @error "expected something"
 
     test "returns ok/error with wrapping label" do