update

crystal/examples/code_executed.cr

@@ -0,0 +1,9 @@
+puts "Hello Crystal!"
+
+class Hello
+  puts "Hello from class"
+end
+
+struct My
+  puts "Hello from struct"
+end

crystal/examples/intro/gets_number.cr

@@ -0,0 +1,4 @@
+print "Give me a number! "
+number = gets
+puts number
+puts number + 1

crystal/examples/other/gets_not_nil.cr

@@ -1,5 +1,5 @@
-x = gets
-puts x.nil?
+# x = gets
+# puts x.nil?
 
 x = gets.not_nil!
 puts x.nil?

crystal/examples/restrict_type_in_code.cr

@@ -0,0 +1,12 @@
+if Random.rand < 0.5
+  x = "abc"
+else
+  x = nil
+end
+
+if x.nil?
+  puts "nil"
+else
+  puts x
+  puts x.size
+end

crystal/examples/restrict_type_in_code_for_hash_broken.cr

@@ -0,0 +1,20 @@
+if Random.rand < 0.5
+  prs = {
+    "name"  => "Foo",
+    "title" => "Manager",
+  }
+else
+  prs = {
+    "name"  => "Foo",
+    "title" => nil,
+  }
+end
+
+p! prs
+
+if prs["title"].nil?
+  puts "nil"
+else
+  puts "not nil"
+  puts prs["title"].size
+end

crystal/examples/restrict_type_in_code_for_hash_fixed.cr

@@ -0,0 +1,28 @@
+if Random.rand < 0.5
+  prs = {
+    "name"  => "Foo",
+    "title" => "Manager",
+  }
+else
+  prs = {
+    "name"  => "Foo",
+    "title" => nil,
+  }
+end
+
+p! prs
+
+title = prs["title"]
+if title.nil?
+  puts "nil"
+else
+  puts "not nil"
+  puts title.size
+end
+
+if title = prs["title"]
+  puts "not nil"
+  puts title.size
+else
+  puts "nil"
+end

crystal/examples/restrict_type_in_code_for_struct_attribute_broken.cr

@@ -0,0 +1,23 @@
+struct Person
+  getter first : String
+  getter family : String?
+
+  def initialize(@first)
+  end
+
+  def initialize(@first, @family)
+  end
+end
+
+if Random.rand < 0.5
+  prs = Person.new("Foo")
+else
+  prs = Person.new("Foo", "Bar")
+end
+
+if prs.family.nil?
+  puts "nil"
+else
+  puts "not nil"
+  puts prs.family.size
+end

crystal/examples/restrict_type_in_code_for_struct_attribute_fixed.cr

@@ -0,0 +1,31 @@
+struct Person
+  getter first : String
+  getter family : String?
+
+  def initialize(@first)
+  end
+
+  def initialize(@first, @family)
+  end
+end
+
+if Random.rand < 0.5
+  prs = Person.new("Foo")
+else
+  prs = Person.new("Foo", "Bar")
+end
+
+family = prs.family
+if family.nil?
+  puts "nil"
+else
+  puts "not nil"
+  puts family.size
+end
+
+if family = prs.family
+  puts "not nil"
+  puts family.size
+else
+  puts "nil"
+end

crystal/examples/struct/struct_from_json_upper.cr

@@ -3,7 +3,6 @@ require "json"
 struct Person
   include JSON::Serializable
 
-
   getter name : String
 
   @[JSON::Field(key: "Email")]

crystal/examples/struct/struct_from_json_upper.out

@@ -0,0 +1,3 @@
+prs # => Person(@name="Bar", @email="[email protected]")
+prs.name # => "Bar"
+prs.email # => "[email protected]"

crystal/examples/type_inference_from_struct.cr

@@ -0,0 +1,47 @@
+struct Dog
+  getter name : String
+  getter owner : String
+
+  def initialize(@name, @owner)
+  end
+end
+
+struct Cat
+  getter name : String
+  getter staff : String
+
+  def initialize(@name, @staff)
+  end
+end
+
+if Random.rand < 0.5
+  animal = Dog.new("Gin", "Foo")
+else
+  animal = Cat.new("Tonic", "Bar")
+end
+
+p! animal
+p! animal.class
+
+# if animal.class == Cat
+#  p! animal.staff
+# else
+#  p! animal.owner
+# end
+
+if animal.is_a?(Cat)
+  p! animal.staff
+else
+  p! animal.owner
+end
+
+case animal
+when Cat
+  puts "cat"
+  p! animal.staff
+when Dog
+  puts "dog"
+  p! animal.owner
+else
+  puts "other"
+end

crystal/structs.md

@@ -122,6 +122,7 @@ when you leave the function.
 {id: struct-from-json-upper-case}
 
 * We cannot have attributes starting with upper case so we have to convert the field names to lowercase:
+* Using [attribute annotation](https://crystal-lang.org/reference/syntax_and_semantics/annotations/index.html)
 
 ![](examples/struct/struct_from_json_upper.cr)
 ![](examples/struct/struct_from_json_upper.out)