Added String#presence (#8345)

spec/std/string_spec.cr

@@ -730,6 +730,12 @@ describe "String" do
     it { "hello".blank?.should be_false }
   end
 
+  describe "presence" do
+    it { " \t\n".presence.should be_nil }
+    it { "\u{1680}\u{2029}".presence.should be_nil }
+    it { "hello".presence.should eq("hello") }
+  end
+
   describe "index" do
     describe "by char" do
       it { "foo".index('o').should eq(1) }

src/nil.cr

@@ -106,6 +106,19 @@ struct Nil
     raise NilAssertionError.new
   end
 
+  # Returns `self`.
+  # This method enables to call the `presence` method (see `String#presence`) on a union with `Nil`.
+  # The idea is to return `nil` when the value is `nil` or empty.
+  #
+  # ```
+  # config = {"empty" => ""}
+  # config["empty"]?.presence   # => nil
+  # config["missing"]?.presence # => nil
+  # ```
+  def presence
+    self
+  end
+
   def clone
     self
   end

src/string.cr

@@ -2402,6 +2402,25 @@ class String
     true
   end
 
+  # Returns `self` unless `#blank?` is `true` in which case it returns `nil`.
+  #
+  # ```
+  # "a".presence         # => "a"
+  # "".presence          # => nil
+  # "   ".presence       # => nil
+  # "    a    ".presence # => "    a    "
+  # nil.presence         # => nil
+  #
+  # config = {"empty" => ""}
+  # config["empty"]?.presence || "default"   # => "default"
+  # config["missing"]?.presence || "default" # => "default"
+  # ```
+  #
+  # See also: `Nil#presence`.
+  def presence : self?
+    self if !blank?
+  end
+
   def ==(other : self)
     return true if same?(other)
     return false unless bytesize == other.bytesize