Signature fields named nil, true or false keep their names

lib/imp/signature/field.ex

@@ -69,6 +69,7 @@ defmodule Imp.Signature.Field do
   end
 
   def new(name, kind) when is_atom(name) do
+    name = normalize_name(name)
     %__MODULE__{name: name, kind: normalize_kind(kind), prefix: infer_prefix(name)}
   end
 
@@ -172,10 +173,22 @@ defmodule Imp.Signature.Field do
     end
   end
 
+  # `nil`, `true` and `false` are atoms but not names: a field given one of
+  # them as an atom is refused, and one written as text keeps its text, so
+  # `"nil: string -> a"` has a field named "nil" that saves and loads as "nil".
+  defp normalize_name(name) when name in [nil, true, false] do
+    raise ArgumentError,
+          "Imp.Signature.Field.new/2 expects a field name, got: #{inspect(name)}"
+  end
+
   defp normalize_name(name) when is_atom(name), do: name
 
-  defp normalize_name(name) when is_binary(name),
-    do: name |> String.trim() |> existing_atom_or_string()
+  defp normalize_name(name) when is_binary(name) do
+    case String.trim(name) do
+      literal when literal in ["nil", "true", "false"] -> literal
+      trimmed -> existing_atom_or_string(trimmed)
+    end
+  end
 
   defp normalize_name(name) do
     raise ArgumentError,

test/property_invariants_test.exs

@@ -43,6 +43,22 @@ defmodule PropertyInvariantsTest do
     end
   end
 
+  test "fields named nil, true and false keep their names through dump and load" do
+    signature = Imp.signature("nil: string, true: string -> false: string")
+    loaded = signature |> Imp.Signature.dump() |> Jason.encode!() |> Jason.decode!()
+    loaded = Imp.Signature.load!(loaded)
+
+    assert Imp.Signature.input_names(signature) == ["nil", "true"]
+    assert Imp.Signature.input_names(loaded) == ["nil", "true"]
+    assert Imp.Signature.output_names(loaded) == ["false"]
+  end
+
+  test "a field name given as the atom nil is refused" do
+    assert_raise ArgumentError, ~r/expects a field name, got: nil/, fn ->
+      Imp.Signature.Field.new(nil, :input)
+    end
+  end
+
   property "signature dump/load round-trips the serialized contract" do
     check all(spec <- signature_spec(), max_runs: 100) do
       signature = Imp.signature(spec)