# Writing the Lisp Source

In this section we'll write Lisp code that duplicates the features provided by the Objective-C code in Apple's tutorial. In Apple's tutorial, the explanation of the Objective C code begins with the section Bridging the Model and View: The Controller.

The Lisp code in this section of the HOWTO is considerably simpler than the corresponding Objective-C code, in part because we can ignore the conventions that XCode uses for laying out source files. We can just write all our definitions into a single Lisp source file, and load that file into Clozure CL when we are ready to build the application.

## First Things First

Place the following line at the top of your Lisp source file:

    (in-package "CCL")

Clozure CL's Objective-C bridge code is defined in the "CCL" package. Usually, when building an application, you'll create a package for that application and import the definitions you need to use. In order to keep the discussion short in this simple example, we just place all our definitions in the "CCL" package.

## Defining the Converter Class

We begin by defining the Converter class. Recall from Apple's tutorial that this is the Model class that implements the conversion between dollars and other currencies. Here is the Lisp definition that implements the class you created in InterfaceBuilder:

    (defclass converter (ns:ns-object)
      ()
      (:metaclass ns:+ns-object))

This is an ordinary CLOS class definition, with a couple of simple wrinkles. First, the superclass it inherits from is the NS-OBJECT class in the "NS" package. NS-OBJECT is an Objective-C class, the ancestor of all Objective-C objects. This CLOS definition actually creates a new Objective-C class named "Converter".

We tell Clozure CL how to build the right kind of class object by including the :METACLASS option in the definition:

      (:metaclass ns:+ns-object)

The Objective-C bridge knows that when the metaclass is `ns:+ns-object`, it must lay out the class object in memory as an Objective-C class, rather than a normal CLOS STANDARD-CLASS.

Next, we define the method "convertCurrency:atRate:":

    (objc:defmethod (#/convertCurrency:atRate: :float)
        ((self converter) (currency :float) (rate :float))
      (* currency rate))

This is the method that actually does the currency conversion. It's a Lisp method that will be called when the AppKit sends the Objective-C message "convertCurrency:atRate:" It's very simple—really, it just multiples `currency` times `rate`. Most of the text in the definition is Objective-C bridge code that links the definition to the right class with the right argument and return types.

`objc:defmethod` is a version of DEFMETHOD that creates methods that can execute in response to Objective-C message-sends.

The syntax `#/convertCurrency:atRate:` uses the "#/" reader macro to read a symbol with case preserved, so that you can see in your code the same name that Objective-C uses for the method, without worrying about how the name might be converted between Lisp and Objective-C conventions.

The number of arguments to an Objective-C method is the number of colons in the name, plus one. Each colon indicates an argument, and there is always an extra "self" argument that refers to the object that receives the message. These are normal Objective-C conventions, but we perhaps need to emphasize the details, since we are using Lisp code to call the Objective-C methods.

We indicate the return type and the types of arguments in the method definition by surrounding parameters and the method name with parentheses, and appending the type name.

Thus, for example,

    (#/convertCurrency:atRate: :float)

means that the return type of the method is :FLOAT, and

    (self converter)

means that the type of the receiving object is Converter.

You will see these same conventions repeated in the next section.

## Defining the ConverterController Class

The previous section defined the Model class, Converter. All we need now is a definition for the ConverterController class. Recall from your reading of Apple's Tutorial that the CurrencyConverter example uses the Model-View-Controller paradigm. You used InterfaceBuilder to construct the application's views. The Converter class provides the model that represents application data. Now we define the controller class, ConverterController, which connects the View and the Model.

Here's the definition of the ConverterController class:

    (defclass converter-controller (ns:ns-object)
      ((amount-field :foreign-type :id :accessor amount-field)
       (converter :foreign-type :id :accessor converter)
       (dollar-field :foreign-type :id :accessor dollar-field)
       (rate-field :foreign-type :id :accessor rate-field))
      (:metaclass ns:+ns-object))

Once again we use the Objective-C bridge to define an Objective-C class. This time, we provide several instance-variable definitions in the class, and name accessors for each of them explicitly. The `:FOREIGN-TYPE` initargs enable us to specify the type of each field in the foreign (Objective-C) class.

Each field in the definition of the ConverterController class is an outlet that will be used to store a reference to one of the text fields that you created in InterfaceBuilder. For example, `amount-field` will be connected to the "Amount" text field.

Why did we spell the name "amount-field" in Lisp code, and "amountField" when creating the outlet in InterfaceBuilder? The Objective-C bridge automatically converts Lisp-style field names (like "amount-field") to Objective-C-style field names (like "amountField"), when handling class definitions.

The `converter` field at launch time contains a reference to the Converter object, whose class definition is in the previous section.

The final piece of the implementation is a definition of the "convert:" method. This is the method that is called when a user clicks the "Convert" button in the user interface.

    (objc:defmethod (#/convert: :void) ((self converter-controller) sender)
      (declare (ignore sender))
      (let* ((conv (converter self))
             (dollar-field (dollar-field self))
             (rate-field (rate-field self))
             (amount-field (amount-field self))
             (dollars (#/floatValue dollar-field))
             (rate (#/floatValue rate-field))
             (amount (#/convertCurrency:atRate: conv dollars rate)))
        (#/setFloatValue: amount-field amount)
        (#/selectText: rate-field self)))

Just as in the Apple example, this method reads the dollar and rate values, and passes them to the "convertCurrency:atRate:" method of the Converter class. It then sets the text of the amount-field to reflect the result of the conversion. The only significant difference between this implementation and Apple's is that the code is written in Lisp rather than Objective-C.

This completes the definition of the CurrencyConverter's behavior. All that remains is to actually build the Cocoa application. The next section shows how to do that.