Add ? to the list of things to quote when it's an exact value.

CHANGELOG.md

@@ -15,6 +15,10 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 - Support for escape and unicode characters [#98](https://github.com/ufirstgroup/ymlr/pull/98)
 - Refactor `Ymlr.Encode` to make it faster.
 
+### Fixed
+
+- Put single quotes around '?' when it is the entire value.
+
 <!--------------------- Don't add new entries after this line --------------------->
 
 ### Added

lib/ymlr/encode.ex

@@ -106,6 +106,8 @@ defmodule Ymlr.Encode do
   @single_quote_when_exact [
     "",
     "~",
+    "?",
+    "-",
     "null",
     "yes",
     "no",

test/ymlr/encode_test.exs

@@ -18,6 +18,14 @@ defmodule Ymlr.EncodeTest do
     test "plain strings" do
       assert_identity_and_output("hello world", "hello world")
       assert_identity_and_output("that's it", "that's it")
+
+      # Strings where the ":", "?", or "-" indicator character is used as the
+      # first character may be encoded in Plain Style (without being quoted)
+      # if followed by a non-space “safe” character.
+      # https://yaml.org/spec/1.2.2/#733-plain-style
+      assert_identity_and_output("?x", ~S(?x))
+      assert_identity_and_output(":x", ~S(:x))
+      assert_identity_and_output("-x", ~S(-x))
     end
 
     # see http://blogs.perl.org/users/tinita/2018/03/strings-in-yaml---to-quote-or-not-to-quote.html
@@ -48,6 +56,30 @@ defmodule Ymlr.EncodeTest do
       assert_identity_and_output("hello #world", ~S('hello #world'))
     end
 
+    test "quoted strings - indicator characters on their own" do
+      assert_identity_and_output("-", ~S('-'))
+      assert_identity_and_output("?", ~S('?'))
+      assert_identity_and_output(":", ~S(':'))
+      assert_identity_and_output("[", ~S('['))
+      assert_identity_and_output("]", ~S(']'))
+      assert_identity_and_output("{", ~S('{'))
+      assert_identity_and_output("}", ~S('}'))
+      assert_identity_and_output("#", ~S('#'))
+      assert_identity_and_output("&", ~S('&'))
+      assert_identity_and_output("&", ~S('&'))
+      assert_identity_and_output("*", ~S('*'))
+      assert_identity_and_output("*", ~S('*'))
+      assert_identity_and_output("!", ~S('!'))
+      assert_identity_and_output("!", ~S('!'))
+      assert_identity_and_output("|", ~S('|'))
+      assert_identity_and_output("|", ~S('|'))
+      assert_identity_and_output(">", ~S('>'))
+      assert_identity_and_output(">", ~S('>'))
+      assert_identity_and_output("%", ~S(%))
+      assert_identity_and_output("@", ~S('@'))
+      assert_identity_and_output("`", ~S('`'))
+    end
+
     test "quoted strings - starts with special char" do
       assert_identity_and_output("!tag", ~S('!tag'))
       assert_identity_and_output("&anchor", ~S('&anchor'))