Merge branch 'let-assignment-variant' into 'release/0.8'

Implement assignment variant of let keyword

See merge request arctic-fox/spectator!11

src/spectator/dsl/structure_dsl.cr

@@ -789,7 +789,54 @@ module Spectator::DSL
     # The name can be used in examples to retrieve the value (basically a method).
     # This can be used to define a value once and reuse it in multiple examples.
     #
-    # This macro expects a name and a block.
+    # There are two variants - assignment and block.
+    # Both must be given a name.
+    #
+    # For the assignment variant:
+    # ```
+    # let string = "foobar"
+    #
+    # it "isn't empty" do
+    #   expect(string.empty?).to be_false
+    # end
+    #
+    # it "is six characters" do
+    #   expect(string.size).to eq(6)
+    # end
+    # ```
+    #
+    # The value is evaluated and stored immediately.
+    # This is different from other `#let` variants that lazily-evaluate.
+    #
+    # ```
+    # let current_time = Time.utc
+    # let(lazy_time) { Time.utc }
+    #
+    # it "lazy evaluates" do
+    #   now = current_time
+    #   sleep 5
+    #   expect(lazy_time).to_not eq(now)
+    # end
+    # ```
+    #
+    # However, the value is not reused across tests.
+    # It will be reconstructed when the next test starts.
+    #
+    # ```
+    # let array = [0, 1, 2]
+    #
+    # it "modifies the array" do
+    #   array[0] = 42
+    #   expect(array).to eq([42, 1, 2])
+    # end
+    #
+    # it "doesn't carry across tests" do
+    #   array[1] = 777
+    #   expect(array).to eq([0, 777, 2])
+    # end
+    # ```
+    #
+    # The block variant expects a name and a block.
     # The name can be a symbol or a literal - same as `Object#getter`.
     # The block should return the value.
     #
@@ -838,6 +885,16 @@ module Spectator::DSL
     # end
     # ```
     macro let(name, &block)
+      {% if block.is_a?(Nop) %}
+        # Assignment variant.
+        @%value = {{name.value}}
+
+        def {{name.target}}
+          @%value
+        end
+      {% else %}
+        # Block variant.
+
         # Create a block that returns the value.
         let!(%value) {{block}}
 
@@ -864,6 +921,7 @@ module Spectator::DSL
             end
           end
         end
+      {% end %}
     end
 
     # The noisier sibling to `#let`.