Add lazy class

src/spectator/lazy.cr

@@ -0,0 +1,34 @@
+module Spectator
+  # Lazy initialization of a value.
+  # Constructs a value only once by calling a `Proc`.
+  # The value is then stored and reused - the `Proc` is only called once.
+  struct Lazy(T)
+    @value_or_block : Proc(T) | T
+
+    # Creates a lazy instance.
+    # The block provided to this method will be called
+    # when `#value` is invoked.
+    # The block will only be called once,
+    # and the result of the block will be cached.
+    def initialize(&block : -> T)
+      @value_or_block = block
+    end
+
+    # Retrieves the lazy initialized value.
+    # The first call to this method will create the value.
+    # Subsequent calls will return the same value.
+    def value : T
+      if value = @value_or_block.as?(T)
+        return value
+      else
+        @value_or_block = construct
+      end
+    end
+
+    # Calls the block used to construct the value.
+    # This method can only be called once per instance.
+    private def construct : T
+      @value_or_block.as(Proc(T)).call
+    end
+  end
+end