handle improper lists in variable inspector

Fixes #1028

Author: lukaszsamson

apps/elixir_ls_debugger/lib/debugger/variables.ex

@@ -13,6 +13,16 @@ defmodule ElixirLS.Debugger.Variables do
       :named
     else
       :indexed
+
+      try do
+        # this call will raise ArgumentError for improper list, no better way to check it
+        _ = length(var)
+        :indexed
+      rescue
+        ArgumentError ->
+          # improper list has head and tail
+          :named
+      end
     end
   end
 
@@ -36,17 +46,36 @@ defmodule ElixirLS.Debugger.Variables do
 
   def children(var, start, count) when is_list(var) do
     start = start || 0
-    count = count || Enum.count(var)
-
-    sliced =
-      var
-      |> Enum.slice(start, count)
 
-    if Keyword.keyword?(var) do
-      sliced
-    else
-      sliced
-      |> with_index_as_name(start)
+    try do
+      # this call will raise ArgumentError for improper list, no better way to check it
+      max_count = length(var)
+      count = count || max_count
+
+      sliced =
+        var
+        |> Enum.slice(start, count)
+
+      if Keyword.keyword?(var) do
+        sliced
+      else
+        sliced
+        |> with_index_as_name(start)
+      end
+    rescue
+      ArgumentError ->
+        count = count || 2
+
+        if start == 0 and count >= 1 do
+          [{"hd", hd(var)}]
+        else
+          []
+        end ++
+          if start + count >= 2 do
+            [{"tl", tl(var)}]
+          else
+            []
+          end
     end
   end
 
@@ -107,7 +136,13 @@ defmodule ElixirLS.Debugger.Variables do
   end
 
   def num_children(var) when is_list(var) do
-    Enum.count(var)
+    try do
+      length(var)
+    rescue
+      ArgumentError ->
+        # improper list has head and tail
+        2
+    end
   end
 
   def num_children(var) when is_bitstring(var) do
@@ -167,7 +202,14 @@ defmodule ElixirLS.Debugger.Variables do
     if Keyword.keyword?(var) and var != [] do
       "keyword"
     else
-      "list"
+      try do
+        # this call will raise ArgumentError for improper list, no better way to check it
+        _ = length(var)
+        "list"
+      rescue
+        ArgumentError ->
+          "improper list"
+      end
     end
   end
 

apps/elixir_ls_debugger/test/variables_test.exs

@@ -39,6 +39,8 @@ defmodule ElixirLS.Debugger.VariablesTest do
     assert Variables.type([1]) == "list"
     assert Variables.type(~c"asd") == "list"
 
+    assert Variables.type([[] | "data"]) == "improper list"
+
     assert Variables.type(abc: 123) == "keyword"
 
     assert Variables.type(%{}) == "map"
@@ -47,6 +49,8 @@ defmodule ElixirLS.Debugger.VariablesTest do
 
     assert Variables.type(%Date{year: 2022, month: 1, day: 1}) == "%Date{}"
     assert Variables.type(%ArgumentError{}) == "%ArgumentError{}"
+    # TODO MapSet
+    # TODO :array
   end
 
   test "num_children" do
@@ -92,6 +96,8 @@ defmodule ElixirLS.Debugger.VariablesTest do
     assert Variables.num_children([1]) == 1
     assert Variables.num_children(~c"asd") == 3
 
+    assert Variables.num_children([[] | "data"]) == 2
+
     assert Variables.num_children(abc: 123) == 1
 
     assert Variables.num_children(%{}) == 0
@@ -111,6 +117,11 @@ defmodule ElixirLS.Debugger.VariablesTest do
       assert Variables.children(~c"asd", 0, 10) == [{"0", 97}, {"1", 115}, {"2", 100}]
     end
 
+    test "improper list" do
+      assert Variables.children([1, 2, 3 | "data"], 0, 10) == [{"hd", 1}, {"tl", [2, 3 | "data"]}]
+      assert Variables.children([1, 2, 3 | "data"], 1, 10) == [{"tl", [2, 3 | "data"]}]
+    end
+
     test "keyword" do
       assert Variables.children([abc: 123], 0, 10) == [abc: 123]