User:JRV

I'm developing a tutorial on this page (slowly). I'm keeping it here so I can check out format, figures, etc. before putting the whole think in the Tutorial area.

Very rough draft:

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Why?

Why would you want to do this? Those with Xcode/Interface Builder/ Cocoa experience will not need an explanation. On the chance that someone else is browsing here, I'll list a few things.

• Cocoa is not a language, it is a vast library of Objective-C objects.

• Cocoa covers tasks such as:
  • windowing, accessibility,
  • pretty easy printing, with pdf automatically available,
  • working with tables, trees, fonts, colors, images, and
  • on and on...

• Objective-C is built on C, and is basically syntactic sugar for C.

• Interface Builder provides quick and easy construction of a complex user

interface with

• • input fields, menus, toolbars,
  • display of tables, outlines, trees,
  • text fields (with built in editing), and
  • more.

In Xcode together with Interface Builder, one can define most user interfaces with simple drag and drop from a library, then connect them with your code by dragging and drop in the IB screen.

You automatically get full integration with the Mac system. You get the Mac Aqua look.

Enough said. What Cocoa doesn't provide are the tools for mathematical modeling of an application domain that Haskell provides. Parsing comes immediately to mind, as does information visualization (see [[<Delhttp://themonadreader.wordpress.com The Monad Reader, 14]).

Use of a Haskell model fits nicely with the Apple Model–View–Controller design pattern.

In the interest of full disclosure—I'm not an old time Mac person. I've had my Mac for two years. I only decided to learn Objective-C and Cocoa after I explored doing an application using Python and Qt, and someone in the Python community said “Objective-C is easy. Why don't you write a real Mac application”. Neither am I a Haskell expert.

Overview of the app

Here are screen shots of the simple application we will develop. The one on the leftshows the application widow (together with the menu automatically generated by Xcode), ready to enter a number. The one on the right shows the same scene after entering a number in the text box, and pressing “return”.

The entire user interface was built in a short time using the graphical Interface Builder, by dragging text the entry box, and the labels from a library of “controls” onto the window. Notice that the labels are dynamic, in the line “Fibonacci Number for n = … is: … ”.

I won't discuss Interface Builder any further in this tutorial.

The Haskell module

For this test I used the same code as used in [[Calling_Haskell_from_C|Calling Haskell from C], with some slight modifications. Here is the Haskell code, in a file called FibTest.hs.

{-# LANGUAGE ForeignFunctionInterface #-}

module FibTest where

import Foreign.C.Types

fibonacci :: Int -> Int
fibonacci n = fibs !! n
    where fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

fibonacci_hs :: CInt -> CInt
fibonacci_hs = fromIntegral . fibonacci . fromIntegral

foreign export ccall fibonacci_hs :: CInt -> CInt

We compile this with ghc, viz:

   $ ghc -c -O FibTest.hs

This produces the following files that we will import into Xcode:

• FibTest.o
• FibTest_stub.h
• FibTest_stub.o

and the following files that we won't need:

• FibTest_stub.c
• FibTest.hi

Import into Xcode project

First we start an Xcode project in the usual way. I started this as a plain Cocoa application. I called my application CocoaHaskellFib.

Several steps can be done in any order:

Copy Files

FibTest.o, FibTest_stub.h, and FibTest_stub.o into the folder where you have saved your CocoaHaskellFib project.

Then add them to the project by Project ➝ Add To Project .

Create application class

Next, create a Cocoa Objective-C class using the Xcode menu, File→New. I named mine CocoaFib. Xcode will create both an interface file (.h) and an implementation file (.m).

Here is my interface file:

<pre-c>

1. import <Cocoa/Cocoa.h>

@interface CocoaFib : NSObject {

   IBOutlet NSTextField *integerInput;
   IBOutlet NSTextField *fibOutput;
   IBOutlet NSTextField *forNis;

}

-(IBAction)generate:(id)sender;

@end </pre-c>

The three outlets are for the user input, fibonacci number output, and the label.

And here is my implementation code:

<pre-c>

1. import "CocoaFib.h"
2. include "HsFFI.h"
3. include "FibTest_stub.h"

@implementation CocoaFib

-(IBAction)generate:(id)sender{

   int i = [intValue integerInput];
   NSLog(@" in generate, integer input is %d\n",i );
   unsigned int j = fibonacci_hs(i);
   [setIntValue:i forNis];
   [setIntValue:j fibOutput];

}

-(void)dealloc{

   hs_exit();
   [dealloc super];

}

@end </pre-c>

Note the include lines for 1) hsFFI.h, and FibTest_stub.h.

Note also the hs_exit call in dealloc.

Modify main.m

When creating a Cocoa project, Xcode automatically generates a file main.m. Normally, one never touches this file. However, when using a Haskell module, one needs to initialize the Haskell run-time by calling hs_init. Normally, I would do this in an init call of CocoaFib.m, rather than in main.m, but since hs_init needs an argc and argv, and since main.m already has them hanging around, I took the easy way out.

Here is main.m

<pre-c>

1. import <Cocoa/Cocoa.h>
2. include "FibTest_stub.h"

int main(int argc, char *argv[]) {

   hs_init(&argc, &argv);
   return NSApplicationMain(argc,  (const char **) argv);

}

</pre-c>

The only changes to the main.m provided by Xcode are the addition of include "FibTest_stub.h" and the call to hs_init.

Compile and run

This is the easy part. Just kidding. It should be easy, if it were easy to make a library or executable containing FibTest.o, FibTest_stub.o and all their dependencies. Unfortunately I've been unable to do that, in spite of spending most of my spare time for a week trying, and posting questions on Haskell-cafe.

Anyway, here is how I did it. If you have a better way, here is a good place to edit this tutorial!

Build and go, the first time

First, I added libffi.a to my project for good measure.

Then select build and go from the menu or the toolbar. This will result something like 26 failures, all due to undefined symbols.

What to do?

What I did was go to the …/usr/lib/ghc-6.10.4 directory of my installation, and run:

find . -name "lib*.a" | xargs nm > ~/develop/haskellLibInfo/libInfo

This results in a file with a list of all the available symbols from all the libraries in the Haskell installation.

Entries look something like this:

./array-0.2.0.0/libHSarray-0.2.0.0.a(Base__1.o):
         U _arrayzm0zi2zi0zi0_DataziArrayziBase_STUArray_con_info
000000b0 D _arrayzm0zi2zi0zi0_DataziArrayziBase_zdWSTUArray_closure
0000009c T _arrayzm0zi2zi0zi0_DataziArrayziBase_zdWSTUArray_info
00000090 t _arrayzm0zi2zi0zi0_DataziArrayziBase_zdWSTUArray_info_dsp
00000078 t _s7RK_info
00000070 t _s7RK_info_dsp
…

Entries with a flag T (for text) are code. U of course indicates undefined, and is no help to us. D indicates data, and is used (if I understood correctly) for defined global constants.

OK. So what do you do with this?

Your Xcode output will say something like:

  "_newCAF", referenced from:
      _FibTest_a3_info in FibTest.o
  "_base_GHCziBase_plusInt_closure", referenced from:
      _FibTest_a3_info in FibTest.o

  …

Now go look in your libinfo folder and look for T _newCAF. It is in libHSrts.a.

Now go back to Xcode. In a Finder window, locate the file with libHSrts.a in it. Drag libHSrts.a into the Groups and Files pane of the Xcode window. You will get a dialog asking if you want to add it to the project. Say yes, but don't copy the file into the project. Xcode will go and find it when it tries to link.

Now when you redo Build and Go, you will no doubt get even more failures due to undefined symbols.

But don't dispair …

Iterate build and go

After a few interations of looking for the library containing missing symbols you'll get a successful build.

Well, except I had one further problem.

Symlink renamed libgmp.a

I have a bunch of libgmp.a, libgmp.so, and libgmp.dylib on my machine. Don't know where they all came from. Nevertheless, Xcode linking tries to use the ones early in its search path, and they don't work with Haskell.

To get around this, I made a symbolic link in the usr/lib/ghc-6.10.4 directory as follows:

ln -s libgmp.a lib-h-gmp.a

and added lib-h-gmp.a to my project.

That's all there is to it

Well, its a bit tedious, but consider the following:

You can (and probably should) test your Haskell model without any GUI, when it's working as desired, then make a version with a C interface.

Once you import this into your Xcode project, with all the attendant adding of .a files, you shouldn't have to change any of the .a file additions again. Now you can concentrate on the View and Control part of your project.

To do