Add getters and setters concept and exercise (#687)

* Start on concept

* More progress

* Update config and concept text

* Fix config

* Fix config name

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* Fix formatting again

* Changes based on feedback

* Update config

concepts/getters-setters/.meta/config.json

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+{
+  "blurb": "Crystal has macros for defining getters and setters. These are used to access and modify the instance variables of a class.",
+  "authors": ["meatball133"],
+  "contributors": ["ryanplusplus"]
+}

concepts/getters-setters/.meta/template.md

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+{% $getters-setters %}

concepts/getters-setters/about.md

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+# Getters and setters
+
+Getters and setters are methods that allow you to read and write the value of an object's property. 
+Crystal has macros, which makes it easy to define getters and setters for a property.
+Macros are a way to generate code at compile time, which will be covered later in the macro concept.
+
+In Ruby these methods are defined using the `attr_reader`, `attr_writer` and `attr_accessor` methods and are very similar to Crystals implementation on the surface.
+Crystal has defined these as `getter`, `setter` and `property` macros.
+
+## Getters
+
+Getter is a macro that defines a method that returns the value of an instance variable.
+This means that you no longer have to write `@` in front of the instance variable when you want to access it (in methods, excluding initialize); instead, you can call the method that the getter macro has defined.
+
+The getter macro can accept multiple instance variables by separating them with commas.
+
+```crystal
+# This:
+class Person
+  @nane : String
+  @age : Int32
+
+  def name
+    @name
+  end
+
+  def age
+    @age
+  end
+end
+
+# Is the same as this:
+class Person
+  @name : String
+  @age : Int32
+
+  getter name, :age
+end
+```
+
+As you can see, using getter reduces the amount of code you write and makes it easier to read.
+Also, Ruby and Crystal differ in that Crystal accepts both the variable name and symbol as arguments for the getter macro.
+Symbols will be covered later in the symbol concept.
+
+## Setters
+
+Setter is a macro that defines a method that sets the value of an instance variable.
+This macro isn't that often found and is commonly the `property` macro used instead.
+The methods that will be created will look like `name_of_method=`; the `=` is what makes it so the property can be set.
+
+This method definition is a bit special since the argument the method receives is after the `=` sign.
+Several other special methods in Crystal use this syntax, like the `+` method.
+
+As with the getter macro, when you want to update the value of an instance variable after defining a setter, you can call the method that the setter macro has defined.
+
+```crystal
+# This:
+class Person
+  @name : String
+  @age : Int32
+
+  def name=(name : String)
+    @name = name
+  end
+
+  def age=(age : Int32)
+    @age = age
+  end
+end
+
+# Is the same as this:
+class Person
+  @name : String
+  @age : Int32
+
+  setter name, :age
+end
+```
+
+## Property
+
+Property is a macro that defines both a getter and a setter for an instance variable.
+
+```crystal
+class Person
+  @name : String
+  @age : Int32
+
+  property name, :age
+end
+```
+
+[getters_and_macros]: https://crystal-lang.org/reference/syntax_and_semantics/methods_and_instance_variables.html#getters-and-setters
+[getter]: https://crystal-lang.org/api/Object.html#getter%28%2Anames%2C%26block%29-macro
+[setter]: https://crystal-lang.org/api/Object.html#setter%28%2Anames%29-macro
+[property]: https://crystal-lang.org/api/Object.html#property%28%2Anames%2C%26block%29-macro

concepts/getters-setters/introduction.md

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+# Getters and setters
+
+Getters and setters are methods that allow you to read and write the value of an object's property. 
+Crystal has macros, which makes it easy to define getters and setters for a property.
+Macros are a way to generate code at compile time, which will be covered later in the macro concept.
+
+In Ruby these methods are defined using the `attr_reader`, `attr_writer` and `attr_accessor` methods and are very similar to Crystals implementation on the surface.
+Crystal has defined these as `getter`, `setter` and `property` macros.
+
+## Getters
+
+Getter is a macro that defines a method that returns the value of an instance variable.
+This means that you no longer have to write `@` in front of the instance variable when you want to access it (in methods, excluding initialize); instead, you can call the method that the getter macro has defined.
+
+The getter macro can accept multiple instance variables by separating them with commas.
+
+```crystal
+# This:
+class Person
+  @nane : String
+  @age : Int32
+
+  def name
+    @name
+  end
+
+  def age
+    @age
+  end
+end
+
+# Is the same as this:
+class Person
+  @name : String
+  @age : Int32
+
+  getter name, :age
+end
+```
+
+As you can see, using getter reduces the amount of code you write and makes it easier to read.
+Also, Ruby and Crystal differ in that Crystal accepts both the variable name and symbol as arguments for the getter macro.
+Symbols will be covered later in the symbol concept.
+
+## Setters
+
+Setter is a macro that defines a method that sets the value of an instance variable.
+This macro isn't that often found and is commonly the `property` macro used instead.
+The methods that will be created will look like `name_of_method=`; the `=` is what makes it so the property can be set.
+
+This method definition is a bit special since the argument the method receives is after the `=` sign.
+Several other special methods in Crystal use this syntax, like the `+` method.
+
+As with the getter macro, when you want to update the value of an instance variable after defining a setter, you can call the method that the setter macro has defined.
+
+```crystal
+# This:
+class Person
+  @name : String
+  @age : Int32
+
+  def name=(name : String)
+    @name = name
+  end
+
+  def age=(age : Int32)
+    @age = age
+  end
+end
+
+# Is the same as this:
+class Person
+  @name : String
+  @age : Int32
+
+  setter name, :age
+end
+```
+
+## Property
+
+Property is a macro that defines both a getter and a setter for an instance variable.
+
+```crystal
+class Person
+  @name : String
+  @age : Int32
+
+  property name, :age
+end
+```
+
+[getter]: https://crystal-lang.org/api/Object.html#getter%28%2Anames%2C%26block%29-macro
+[setter]: https://crystal-lang.org/api/Object.html#setter%28%2Anames%29-macro
+[property]: https://crystal-lang.org/api/Object.html#property%28%2Anames%2C%26block%29-macro

concepts/getters-setters/links.json

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+[
+  {
+    "url": "https://crystal-lang.org/reference/syntax_and_semantics/methods_and_instance_variables.html#getters-and-setters",
+    "description": "Crystal docs: getters and setters"
+  }
+]

config.json

@@ -105,6 +105,18 @@
         ],
         "status": "beta"
       },
+      {
+        "slug": "weighing-machine",
+        "name": "Weighing Machine",
+        "uuid": "317930f4-4c97-4962-a04e-76e6d60b7fba",
+        "concepts": [
+          "getters-setters"
+        ],
+        "prerequisites": [
+          "strings"
+        ],
+        "status": "beta"
+      },
       {
         "slug": "high-school-sweetheart",
         "name": "High School Sweetheart",
@@ -1524,6 +1536,11 @@
       "slug": "strings",
       "name": "Strings"
     },
+    {
+      "uuid": "b5f40f9a-d3bf-4e82-9918-a04649f08655",
+      "slug": "getters-setters",
+      "name": "Getters and Setters"
+    },
     {
       "uuid": "4dd28cdc-f197-490b-bbfb-5cd33bbdb6af",
       "slug": "binary-octal-hexadecimal",

exercises/concept/weighing-machine/.docs/hints.md

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+# Hints
+
+## General
+
+- To create the getter and setter methods you should use the `getter`, `setter` and `property` macros.
+
+## 1. Create an initial state for the weighing machine
+
+- To initialize the weighing machine you should use the `initialize` method.
+- The method should take two arguments, `precision` and `metric`, which should be an `Int32` and `Bool`, respectively.
+  These should be used to set the instance variables `@precision` and `@metric`.
+- The instance variable `@weight` should be set to `0.0`.
+
+## 2. Allow the weighing machine to have a precision
+
+- The `getter` macro allows you to define a method that returns the value of an instance variable.
+
+## 3. Allow the weight to be set on the weighing machine
+
+- The `property` macro allows you to define a method that gets and sets the value of an instance variable.
+
+## 4. Allow the machine to be switch between metric and imperial units
+
+- The `setter` macro allows you to define a method that sets the value of an instance variable.

exercises/concept/weighing-machine/.docs/instructions.md

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+# Instructions
+
+In this exercise you'll be modeling a weighing machine.
+
+## 1. Create an initial state for the weighing machine
+
+When initialized, the weighing machine should refer to its factory settings which is different for where the machine is sold.
+Thereby the machine should be able to be initialized with a precision and if it is metric or imperial.
+
+Implement the `WeighingMachine#initialize` method which takes two arguments, `precision` which is an `Int32` and `metric` which is a `Bool`.
+The `metric` argument when `true` means that the machine should use the metric system, otherwise it should use the imperial system.
+The initialize method set should also set the instance variable `@weight` to `0.0`.
+
+```crystal
+precision = 3
+metric = true
+vm = WeighingMachine.new(precision, metric)
+```
+
+## 2. Allow the weighing machine to have a precision
+
+To cater to different demands, we allow each weighing machine to be customized with a precision (the number of digits after the decimal separator).
+
+Implement the `WeighingMachine#precision` getter method to return the precision of the weighing machine.
+
+```crystal
+precision = 3
+metric = true
+vm = WeighingMachine.new(precision, metric)
+vm.precision
+# => 3
+```
+
+## 3. Allow the weight to be set on the weighing machine
+
+Implement the `WeighingMachine#weight` property to allow the weight to be get _and_ set:
+
+```crystal
+precision = 3
+metric = true
+wm = WeighingMachine.new(precision, metric)
+wm.weight = 60.5
+wm.weight
+# => 60.5
+
+wm.weigh
+# => 60.5
+```
+
+## 4. Allow the machine to be switch between metric and imperial units
+
+Implement the `WeighingMachine#metric=` property to allow the unit to be set.
+It should accept a boolean value.
+
+```crystal
+precision = 3
+metric = true
+wm = WeighingMachine.new(precision, metric)
+vm.weight = 60.5
+wm.metric = false
+
+vm.weigh
+# => 133.377
+```