Safe Haskell	None
Language	Haskell2010

Graphics.UI.FLTK.LowLevel.FLTKHS

Contents

Motivation
Goals
Obstacles
Installation
- Build With Bundled FLTK
- Compile FLTK Yourself
Demos
Getting Started
Fluid Support
Stack Traces
API Guide
Slow Compilation Issues
Running in the REPL
Core Types
Widgets
Machinery for static dispatch
Association of widgets and functions

Synopsis

Motivation

This library aims to make it easy for users to build native apps that work portably across platforms.

I'm also very interested the user interface renaissance in the programming community, whether the various kinds of functional reactive programming, meta-object protocol UI's, or something like React.js.

The hope is that a low-cost hassle-free way of getting a UI up and running without having to deal with browser, authentication and compilation issues will make it more fun to play around with these great ideas using Haskell.

Why a native toolkit?

Even in this era of web interfaces it is still useful to be able to make native apps. They are usually faster and have fewer security issues.

Why FLTK?

I chose FLTK because it was small enough that one person could bind the whole thing in an initial pass. Larger toolkits like QT, although way slicker, would require many man-years of effort.
FLTK is quite featureful
FLTK is mature and maintained, the project is about 20 years old and I have had good experiences with the community
FLTK comes with a simple but quite useful GUI builder, Fluid which is now able to generate Haskell code. See the `Fluid Support` section for more details.

What about HsQML/WxHaskell/Gtk2Hs?

These are all great projects and produce really nice UI's, but they all fail at least one of criterion listed under the Goals section below.

To my knowledge, as of the second quarter of 2017, no other package in the Haskell ecosystem meets all those constraints.

Goals

The goals of this library are to provide a low level API to the FLTK that:

provides full coverage of the toolkit allowing the user to write GUI's in pure Haskell.
feels like it has polymorphic dispatch, meaning a single function dispatches to the right implementation based on the type of widget it is given
is not monolithic, meaning new widgets can be incorporated the user's application without needing to recompile this library
is easy to install. This library has a minimum of dependencies and FLTK itself compiles cleanly on most architectures. And now there is even a bundled option where Cabal/Stack build FLTK for you behind the scenes.
allows the user to produce statically linked binaries with a minimum of external dependencies
includes a lot of complete working demos so that you can get up and running faster.
comes with GUI builder support to alleviate the tedium of laying out widgets by hand

Obstacles

This section attempts to briefly highlight some possible dealbreakers users might want to know about before proceeding. To be clear, building and deploying portable static application binaries works well on all platforms which is why the library is considered usable. And most of these issues are being aggressively addressed but in the interests of full disclosure ...

Look & Feel

The default look FLTK apps can be charitably described as retro. And FLTK has no support for theming since it was designed for the embedded domain and trades off slickness for speed, portability and binary size.

But that doesn't mean the user is limited to the default! FLTK is quite flexible and it is pretty easy to change the way a widget looks and behaves but there is no "theme file" that applies uniformly to all widgets in the app.

Compile Times

Currently a dense app with ~ 160-180 widgets crammed into the same window takes 9-12 seconds to compile with GHC 7.10.3 on a 32GB quad-core machine. The good news is that this is a known issue.

Installation

There are two ways to install FLTKHS, building with the bundled FLTK GUI library, or compiling and installing FLTK from scratch yourself. The bundled way is by far the easiest on all platforms. It is completely self-contained, you don't need any sudo access to your system.

Build With Bundled FLTK

Linux & *BSD

The steps are:

Make sure to have OpenGL installed if you need it.
Ensure that make, autoconf and autoheader are available on your system
Download & install Stack
Download & install the FLTKHS hello world skeleton
Verify the install by running `fltkhs-hello-world`

Download & Install Stack

Pick the Stack installer that matches your distribution and install according the instructions.

Download & Install the FLTKHS Hello World Skeleton

Downloading Without Git

If git is not installed download the latest version of the fltkhs-hello-world application skeleton from here

Extract and rename the archive:

> tar -zxvf fltkhs-hello-world-master.tar.gz
> mv fltkhs-hello-world-master fltkhs-hello-world

Downloading With Git

If git is available:

> git clone http://github.com/deech/fltkhs-hello-world

Building

Build it with Stack:

> cd fltkhs-hello-world
> stack setup
> stack install --flag fltkhs:bundled
or if you need OpenGL support
> stack install --flag fltkhs:bundled --flag fltkhs:opengl

Verify The Install

Test that the build completed successfully by invoking incredibly unimpressive

> stack exec fltkhs-hello-world

You will be greeted by an incredibly boring little window with a button that says "Hello world", if you click it, it will change to "Goodbye world."

Mac (Yosemite, El Capitan, Sierra)

Mac versions older than El Capitan and Yosemite are not supported.

The general steps are:

Brew Install Stack
Download & install the FLTKHS hello world skeleton
Verify the install by running `fltkhs-hello-world`

Brew Install Stack

This should be as simple as:

> brew install haskell-stack

Brew Install Autoconf

> brew install autoconf

Download & Install the FLTKHS Hello World Skeleton

Downloading Without Git

If git is not installed download the latest version of the fltkhs-hello-world application skeleton from here

Extract the archive:

> cd /Users/<username>/Downloads/
> tar -zxvf fltkhs-hello-world-master.tar.gz
> mv fltkhs-hello-world-master fltkhs-hello-world

Downloading With Git

If git is available:

> git clone http://github.com/deech/fltkhs-hello-world

Building

Build it with Stack:

> cd fltkhs-hello-world
> stack setup
> stack install --flag fltkhs:bundled
or if you need OpenGL support
> stack install --flag fltkhs:bundled --flag fltkhs:opengl

Verify The Install

Test that the build completed successfully by invoking incredibly unimpressive

> stack exec fltkhs-hello-world

You will be greeted by an incredibly boring little window with a button that says "Hello world", if you click it, it will change to "Goodbye world."

Windows(7,8,10)(64-bit)

This install guide has been tested on a Windows 7, 8 and 10.

Install Stack

Downloading and following the default instructions for the standard Windows installer should be enough. If the install succeeded stack should on the PATH. To test run 'cmd.exe' and do:

> stack --version

Now downloading and setup the latest GHC via stack:

> stack setup

From this point on we can live in the MSYS2 shell that comes with Stack. It is a far superior environment to the command prompt. To open the MSYS2 shell do:

> stack exec mintty

Install Necessary Utilities via Pacman

In the MSYS2 shell prompt update and upgrade the MSYS2 installation:

> pacman -Syy
> pacman -Syu

... install packages for download and extracting packages:

> pacman -S wget
> pacman -S tar
> pacman -S unzip
> pacman -S zip
> pacman -S man

... and building C/C++ programs:

> pacman -S autoconf
> pacman -S make
> pacman -S automake

Download And Install The FLTKHS Hello World Skeleton

The fltkhs-hello-world skeleton is a simple Hello World GUI which provides the base structure for FLTKHS applications. Please see the Demos section of this document for examples of apps that show off more complex uses of the API.

> wget --no-check-certificate https://github.com/deech/fltkhs-hello-world/archive/master.zip
> unzip master.zip
> mv fltkhs-hello-world-master fltkhs-hello-world
> cd fltkhs-hello-world

And install with:

> stack install --flag fltkhs:bundled
or if you need OpenGL support
> stack install --flag fltkhs:bundled --flag fltkhs:opengl

To test the installation:

> stack exec fltkhs-hello-world

You will be greeted by an incredibly boring little window with a button that says "Hello world", if you click it, it will change to "Goodbye world."

Packaging A Windows Executable

While the 'fltkhs-hello-world' application is mostly stand-alone the MSYS2 environment bundled with stack seems to require 3 runtime DLLs. The DLLs are bundled with stack so it's easy to zip them up with the executable and deploy. The required DLLs are: 'libstdc++-6.dll', 'libgcc_s_seh-1.dll' and 'libwinpthread-1.dll'.

First create the directory that will contain the executable and DLLs:

> mkdir /tmp/fltkhs-hello-world

Copy the executable over to that directory:

> cp `which fltkhs-hello-world` /tmp/fltkhs-hello-world

Copy over the DLLs. They are usually located in '../<ghc-version>/mingw/bin' but to make the process slightly less fragile we specify the directory relative to whatever ghc is currently in stack 's context:

> cp `dirname $(which ghc)`../mingw/bin/libstdc++-6.dll /tmp/fltkhs-hello-world
> cp `dirname $(which ghc)`../mingw/bin/libgcc_s_seh-1.dll /tmp/fltkhs-hello-world
> cp `dirname $(which ghc)`../mingw/bin/libwinpthread-1.dll /tmp/fltkhs-hello-world

Zip up archive:

> cd /tmp
> zip fltkhs-hello-world.zip fltkhs-hello-world/*

And that's it! Any Windows 10 user should now be able to extract 'fltkhs-hello-world.zip' and run 'fltkhs-hello-world.exe'.

Compile FLTK Yourself

Linux & *BSD

The steps are:

Make sure to have OpenGL installed if you need it
Download & install Stack
Download & install FLTK 1.3.4-1
Download & install the FLTKHS hello world skeleton
Verify the install by running `fltkhs-hello-world`

Download & Install Stack

Pick the Stack installer that matches your distribution and install according the instructions.

Download & Install FLTK-1.3.4-1

Please make sure to only download version FLTK 1.3.4-1. It should build and install smoothly with the standard:

> ./configure --enable-shared --enable-localjpeg --enable-localzlib --enable-localpng
or if you need OpenGL support
> ./configure --enable-gl --enable-shared --enable-localjpeg --enable-localzlib --enable-localpng
> make
> sudo make install

If you didn't install FLTK from source you can use the 'fltk-config' tool to ensure that version 1.3.4-1 is installed:

> fltk-config --version
1.3.4-1

Additionally the FLTK headers should be in the include path and, along with the standard FLTK libraries, fltk_images, and fltk_gl. You will also need the make, autoconf, and autoheader tools to build the Haskell bindings.

The reason we install from source is that some package managers seem to be behind on versions (as of this writing Ubuntu 14.04 is still on 1.3.2) and others put the headers and libraries in a non-standard locations which will cause the Haskell bindings to throw compilation errors.

Download & Install the FLTKHS Hello World Skeleton

Downloading Without Git

If git is not installed download the latest version of the fltkhs-hello-world application skeleton from here

Extract and enter the archive:

> tar -zxvf fltkhs-hello-world-master.tar.gz
> mv fltkhs-hello-world-master fltkhs-hello-world

Downloading With Git

If git is available:

> git clone http://github.com/deech/fltkhs-hello-world

Building

Build it with Stack:

> cd fltkhs-hello-world
> stack setup
> stack install
or if you need OpenGL support
> stack install --flag fltkhs:opengl

Note: If the install step produces a flood of `undefined reference` errors please ensure that you have the right version of FLTK (1.3.4-1) installed and that the headers are in the expected locations. It seems as though some package managers put the libraries and headers in non-standard places so it is best to build from source.

Verify The Install

Test that the build completed successfully by invoking incredibly unimpressive

> stack exec fltkhs-hello-world

You will be greeted by an incredibly boring little window with a button that says "Hello world", if you click it, it will change to "Goodbye world."

Mac (Yosemite & El Capitan)

Unfortunately Mac version older than El Capitan and Yosemite are not supported.

The general steps are:

Brew Install Stack
Download & install the FLTKHS hello world skeleton
Verify the install by running `fltkhs-hello-world`

Brew Install Stack

This should be as simple as:

> brew install haskell-stack

Brew Install Autoconf

> brew install autoconf

Compile & Install FLTK from Source.

The brew package for the current stable release of FLTK is broken. Fortunately installing from source is pretty quick and painless.

> wget http://fltk.org/pub/fltk/1.3.4/fltk-1.3.4-1-source.tar.gz
> tar -zxf fltk-1.3.4-1-source.tar.gz
> cd fltk-1.3.4-1
> ./configure --enable-shared --enable-localjpeg --enable-localzlib --enable-localpng
or if you need OpenGL support
> ./configure --enable-gl --enable-shared --enable-localjpeg --enable-localzlib --enable-localpng
> make
> sudo make install
> fltk-config --version
1.3.4-1

Download & Install the FLTKHS Hello World Skeleton

Downloading Without Git

If git is not installed download the latest version of the fltkhs-hello-world application skeleton from here

Extract the archive:

> cd /Users/<username>/Downloads/
> tar -zxvf fltkhs-hello-world-master.tar.gz
> mv fltkhs-hello-world-master fltkhs-hello-world

Downloading With Git

If git is available:

> git clone http://github.com/deech/fltkhs-hello-world

Building

Build it with Stack:

> cd fltkhs-hello-world
> stack setup
> stack install
or if you need OpenGL support
> stack install --flag fltkhs:opengl

Verify The Install

Test that the build completed successfully by invoking incredibly unimpressive

> stack exec fltkhs-hello-world

You will be greeted by an incredibly boring little window with a button that says "Hello world", if you click it, it will change to "Goodbye world."

Windows(7,8,10)(64-bit)

This install guide has been tested on a Windows 7, 8 and 10.

Install Stack

Downloading and following the default instructions for the standard Windows installer should be enough. If the install succeeded stack should on the PATH. To test run 'cmd.exe' and do:

> stack --version

Now downloading and setup the latest GHC via stack:

> stack setup

From this point on we can live in the MSYS2 shell that comes with Stack. It is a far superior environment to the command prompt. To open the MSYS2 shell do:

> stack exec mintty

Install Necessary Utilities via Pacman

In the MSYS2 shell prompt update and upgrade the MSYS2 installation:

> pacman -Syy
> pacmay -Syu

... install packages for download and extracting packages:

> pacman -S wget
> pacman -S tar
> pacman -S unzip
> pacman -S zip
> pacman -S man

... and building C/C++ programs:

> pacman -S autoconf
> pacman -S make
> pacman -S automake

Download and Install FLTK

Download the latest stable build of FLTK:

> wget --no-check-certificate http://fltk.org/pub/fltk/1.3.4/fltk-1.3.4-1-source.tar.gz

Untar the FLTK archive and enter the directory:

> tar -zxf fltk-1.3.4-1-source.tar.gz
> cd fltk-1.3.4-1

Configure, make and install:

> ./configure --enable-shared --enable-localjpeg --enable-localzlib --enable-localpng
or if you need OpenGL support
> ./configure --enable-gl --enable-shared --enable-localjpeg --enable-localzlib --enable-localpng
> make
> make install

You can test your installation by running:

> fltk-config
1.3.4-1

Download And Install The FLTKHS Hello World Skeleton

The fltkhs-hello-world skeleton is a simple Hello World GUI which provides the base structure for FLTKHS applications. Please see the Demos section of this document for examples of apps that show off more complex uses of the API.

> wget --no-check-certificate https://github.com/deech/fltkhs-hello-world/archive/master.zip
> unzip master.zip
> mv fltkhs-hello-world-master fltkhs-hello-world
> cd fltkhs-hello-world

And install with:

> stack install
or if you need OpenGL support
> stack install --flag fltkhs:opengl

To test the installation:

> stack exec fltkhs-hello-world

You will be greeted by an incredibly boring little window with a button that says "Hello world", if you click it, it will change to "Goodbye world."

Packaging A Windows Executable

While the 'fltkhs-hello-world' application is mostly stand-alone the MSYS2 environment bundled with stack seems to require 3 runtime DLLs. The DLLs are bundled with stack so it's easy to zip them up with the executable and deploy. The required DLLs are: 'libstdc++-6.dll', 'libgcc_s_seh-1.dll' and 'libwinpthread-1.dll'.

First create the directory that will contain the executable and DLLs:

> mkdir /tmp/fltkhs-hello-world

Copy the executable over to that directory:

> cp `which fltkhs-hello-world` /tmp/fltkhs-hello-world

Copy over the DLLs. They are usually located in '../<ghc-version>/mingw/bin' but to make the process slightly less fragile we specify the directory relative to whatever ghc is currently in stack 's context:

> cp `dirname $(which ghc)`../mingw/bin/libstdc++-6.dll /tmp/fltkhs-hello-world
> cp `dirname $(which ghc)`../mingw/bin/libgcc_s_seh-1.dll /tmp/fltkhs-hello-world
> cp `dirname $(which ghc)`../mingw/bin/libwinpthread-1.dll /tmp/fltkhs-hello-world

Zip up archive:

> cd /tmp
> zip fltkhs-hello-world.zip fltkhs-hello-world/*

And that's it! Any Windows 10 user should now be able to extract 'fltkhs-hello-world.zip' and run 'fltkhs-hello-world.exe'.

Demos

FLTKHS has a almost 25 end-to-end demo applications to help the user get started. They are split into two sets, those written manually and those that show how to use FLUID.

The READMEs in the repos have installation instructions but they assume that the user has successfully installed FLTK and the 'fltkhs-hello-world' app (see platform specific instructions above).

Getting Started

By this point, I assume that you have successfully installed hello world (see above) or one of the demo packages.

Quick Start

The quickest way to get started is to look at the source for the FLTKHS project skeleton. Though it is a simple app it shows the basics of widget creation and callbacks.

Other demo packages show more complicated usage of the API.

Since the API is a low-level binding, code using it takes on the imperative style of the underlying toolkit. Fortunately it should look pretty familiar to those who have used object-oriented GUI toolkits before.

Fluid Support

This package also comes with a utility (fltkhs-fluidtohs) that takes a user interface generated using the Fluid GUI builder that ships with FLTK and generates Haskell code.

Now the user can drag and drop widgets into place instead of having to calculate coordinates and sizes by hand. Additionally arbitrary Haskell code can be inserted into Fluid interfaces allowing user to do most of callback wiring directly from Fluid.

The quickest way to get started is to download the Fluid/Haskell project template. The Setup.hs that comes with the skeleton is configured to use the 'fltkhs-fluidtohs' utility to automatically convert any Fluid in src directory into a Haskell module of the same name during the preprocess step. This means using Fluid in a FLTKHS project is as simple as creating a Fluid interface and running 'stack build --flag fltkhs:bundled' or 'stack install --flag fltkhs:bundled'.

Additionally the fltkhs-fluid-demos package comes with a number of demos that show off how Fluid integrates with FLTKS.

Stack Traces

In a traditional callback-heavy API such as FLTKHS null pointers happen which is why FLTKHS supports partial stack traces. All FLTK functions throw an error along with a stack trace when given a null Ref.

For pre-7.10 GHCs stack traces will only be shown if the 'xc' flag is used when compiling FLTKHS.

If compiled with GHC > 7.10 a partial stack trace is transparently available to the user. The recently minted 'CallStack' implicit parameter is used to get a trace of the function that made the offending call along with a file name and line number. So, for example, in the following code:

buttonCb :: Ref Button -> IO ()
buttonCb b' = do
  FL.deleteWidget b'
  l' <- getLabel b'
  ...

main :: IO ()
main = do
 window <- windowNew ...
 begin window
 b' <- buttonNew ...
 setCallback b' buttonCb
 ...

a button is placed inside a window in the main method but the first time it is clicked the callback will delete it and then try the extract the label from the null Ref. The resulting stack trace will look something like:

Ref does not exist. ?loc, called at src/Graphics/UI/FLTK/LowLevel/Fl_Types.chs:395:58 in fltkh_Cx8029B5VOwKjdT0OwMERC:Graphics.UI.FLTK.LowLevel.Fl_Types
  toRefPtr, called at src/Graphics/UI/FLTK/LowLevel/Fl_Types.chs:403:22 in fltkh_Cx8029B5VOwKjdT0OwMERC:Graphics.UI.FLTK.LowLevel.Fl_Types
  withRef, called at src/Graphics/UI/FLTK/LowLevel/Hierarchy.hs:1652:166 in fltkh_Cx8029B5VOwKjdT0OwMERC:Graphics.UI.FLTK.LowLevel.Hierarchy
  getLabel, called at src/Main.hs:11:10 in main:Main

It says that the null pointer was originally detected in the library function toRefPtr function which was called by the library function withRef, which was called by getLabel on line 11 of 'src/Main.hs'. Notice, however, that the trace stops there. It does not tell you getLabel was invoked from buttonCb. For a more detailed trace the CallStack implicit parameter needs to be passed to each function in the chain like:

buttonCb :: (?loc :: CallStack) => Ref Button ...
 ...
main :: IO ()
 ...

API Guide

Guide to the Haddock Docs

Convenient access to the underlying C++ is achieved using typeclasses and type-level programming to emulate OO classes and multiple dispatch. This approach makes Haddock very unhappy and the generated documentation is frequently unhelpful. For instance, newcomers to this library are urged not to look at Graphics.UI.FLTK.LowLevel.Dispatch or Graphics.UI.FLTK.LowLevel.Hierarchy. The purpose of this guide is to point users in a more useful direction.

The documentation provided with this API is not yet self-contained and is meant to be used in tandem with the C++ documentation. The rest of this document is about how the Haskell functions and datatypes map to the C++ ones.

Widget Construction

Each widget has its own module all of which are listed below under the Widgets heading . Most modules include a function named `<widgetName>New` that returns a reference to that widget. Although the user does not have to deal with raw pointers directly it might help to understand that this reference is a pointer to a void pointer to a C++ object.

For instance, windowNew creates a Ref Window which is a pointer to a C++ object of type `Fl_Window`, the FLTK class that knows how to draw, display and handle window events.

This value of type Ref Window is then passed along to various functions which, transparent to the user, extract the pointer and pass it to the appropriate `Fl_Window` instance method.

Widget Methods

The Haskell functions that bind to the instance methods of an FLTK class are listed under the Functions heading in that widget's module. It's worth remembering that these type signatures associated with the functions listed under the Functions heading are not the real ones but are artifically generated because they are much more helpful to users. For instance, the actual type of activate exposes all the type level arithmetic required so it can be used by subclasses of Widget but is pretty unhelpful as a reference compared to the artificial type under Functions heading of Graphics.UI.FLTK.LowLevel.Widget.

Unfortunately to see this more helpful type signature the poor reader has to navigate to the corresponding widget's module, find the Functions header and scroll down to the desired function. Haddock, unfortunately, does not support anchors that link to a named point in the page. I'm very open to ideas on how to make this easier.

Carrying on the previous example from the Widget Creation section, the methods on a Ref Window widget are documented in Graphics.UI.FLTK.LowLevel.Window under Functions. Each function takes the Ref Window reference as its first argument followed by whatever else it needs and delegates it appropriately.

As this is a low-level binding the Haskell functions are kept as close as possible in name and argument list to the underlying C++. This allows users familiar with the FLTK API to use this library with less learning overhead and it lets newcomers to FLTK take advantage of the already extensive C++ documentation.

Functions are named to make it as easy as possible to find the corresponding C++ function, however there are some naming conventions to keep in mind:

setters and getters are prefixed with set and get respectively. In C++ both have the same name; the setter takes an argument while the getter does not. Since Haskell does not support overloading this convention is used.
in many cases C++ uses overloading to provide default values to arguments. Since Haskell does not support overloading, these arguments are Maybe types eg. the hotspot function in Graphics.UI.FLTK.LowLevel.Window. In other cases where the common use case leaves the default arguments unspecified the binding provides two functions, a longer less-convenient-to-type one that takes the default argument and a short one that does not eg. drawBox and drawBoxWithBoxtype, also in Graphics.UI.FLTK.LowLevel.Window.
error codes are Either types
function arguments which are pointers to be filled are not exposed to the API user. For instance, a common C++ idiom is to return a string by taking a pointer to some initialized but empty chunk of memory and filling it up. The corresponding Haskell function just returns a Text.

It is hoped that until the documentation becomes more self-sustaining the user can use these heuristics (and the type signatures) along with the official FLTK documentation to "guess" what the binding functions do.

Widget Hierarchy

Every widget module in the API has a Hierarchy heading that shows all its parents.

The design of the API makes all the parent functions transparently available to that widget. This is also the reason why the actual type signatures are so complicated requiring the manual generation of artificial type signatures.

For instance, the Functions section under Graphics.UI.FLTK.LowLevel.Window shows that a Ref Window can be passed to getModal to check if the window is modal, but it can also be passed to children in Graphics.UI.FLTK.LowLevel.Group which counts up the number of widgets inside the Window and getX in Graphics.UI.FLTK.LowLevel.Widget which returns the X coordinate of the Window's top-left hand corner.

The hierarchy corresponds almost exactly to the underlying C++ class hierarchy so, again, the user is able to take advantage of the C++ documentation to use the binding API.

Overriding C++ methods

The binding API allows a limited but powerful form of "inheritance" allowing users to override certain key FLTK methods with Haskell functions. All GUI elements that derive from the C++ base class Fl_Widget and the Haskell analog Widget now allow Haskell functions to be passed at widget construction time that give Haskell complete control on drawing, handling, resizing and other key functions. This means that the Haskell user has complete control of look-and-feel and the event loop. The table demos are an example of drawing in Haskell. An example of taking over the event loop is an FLTKHS proof-of-concept that overrides the FLTKHS event loop with the Reflex FRP allowing native functional reactive programming. The sky is the limit!

When providing custom methods the object constructor is no longer `<widgetName>New` but `<widgetName>Custom`, which, in addition to the parameters taken by `<widgetName>New` also takes records of Haskell functions which are then passed to the C++ side.

Much like a callback, the Haskell functions are passed as function pointers to the C++ side and called whenever the event loop deems appropriate. Unlike callbacks, they can be set only on object instantiation.

An example of this is Graphics.UI.FLTK.LowLevel.Widget which, since it is a base class for most widgets and doesn't have much functionality of its own, only allows custom construction using widgetCustom. This constructor takes a CustomWidgetFuncs datatype which is a record of functions which tells a Graphics.UI.FLTK.LowLevel.Widget how to handle events and draw, resize and display itself.

Again Graphics.UI.FLTK.LowLevel.Window can be used a motivating example. Its custom constructor windowCustom, in fact, takes two records. A CustomWidgetFuncs which allows the user to override methods in its Graphics.UI.FLTK.LowLevel.Widget parent class and also a CustomWindowFuncs record which allows the user to override flush, a method on the Window class which tells the window how to force a redraw. For example, the demo src/Examples/doublebuffer.hs (which corresponds to the executable 'ftlkhs-doublebuffer') tells both windows how to draw themselves in a Haskell function that uses low-level FLTK drawing routines by overriding the draw function of their Graphics.UI.FLTK.LowLevel.Widget parent.

Every widget that supports customizing also provides a default function record that can be passed to the constructor. For example, Graphics.UI.FLTK.LowLevel.Widget provides defaultCustomWidgetFuncs and Graphics.UI.FLTK.LowLevel.Window has defaultCustomWindowFuncs. In the demo mentioned above the singleWindowCustom function is given defaultCustomWidgetFuncs but with an overridden drawCustom.

Another case where customization comes up a lot is when using Graphics.UI.FLTK.LowLevel.Table which is a low-level table widget that needs to be told, for example, how to draw its cells. The demo src/Examples/table-simple.hs (corresponding to the executable 'fltkhs-table-simple') shows this in action.

Hopefully the demos just mentioned and others included with this library show that even though customizing is limited it is possible to do a lot.

Additionally for most customizable functions a widget provides a corresponding function that calls the default C++ implementation as the function name suffixed with "Super". For instance, the user can provide a custom implementation of handle on Graphics.UI.FLTK.LowLevel.Window via the constructor and every call to handle invokes that implementation, but the user can also call handleSuper to get at the default C++ implementation. This comes in handy when the custom function is just setting up variables or logging and wants underlying implementation to take over at some point.

Warning: since calls to the default implementation of the function, handleSuper, for example, are available to the widget they are automatically available to all subclasses of the widget. I acknowledge this is a confusing aspect of the library's design but hopes the benefit of being able to get at the default implementation out-weighs the trap the user might fall into. If this becomes a pervasive problem I'm open to removing this functionality.

Slow Compilation Issues

As described above, the API emulates multiple dispatch using type-level programming, closed type families and typeclasses. While this is makes for a nice API it has also slowed down compilation of executables much more than expected.

To clarify the time taken to compile the library itself has not changed, but applications that use the library to create executables are taking a lot longer to compile. To further emphasize, there does not appear to be any runtime performance issues. This is only a compile time problem.

To preserve the user's sanity and mine a flag fastCompile has been introduced to the skeleton projects, the fltkhs-demos and to the fltkhs-fluid-demos. This flag which tells the compiler to skip some steps when compiling executables, dramatically decreases compile time but also bloats the resulting executable size and probably makes runtime performance much slower. In this package and fltkhs-fluid-demos it is enabled by default, since the executables are just demos that are not meant to show off performance. To disable this flag, tell Stack to ignore it during the build step:

> stack build --flag fltkhs:bundled --flag fltkhs-demos:-fastCompile

In the fltkhs and the fltkhs-fluid project skeletons this flag is disabled by default to provide the best runtime performance. To enable the flag for a smoother development workflow, tell Stack to enable it during the configure step:

> stack build --flag fltkhs:bundled --flag fltkhs-hello-world:fastCompile

File Layout

Root
  - c-src                 -- The C bindings
  - c-examples            -- demos written using the C bindings (not installed)
  - fltk-<version>.tar.gz -- The bundled FLTK library
  - src
    - TestPrograms        -- Haskell test programs
    - Fluid               -- The Fluid file to Haskell conversion utility
    - Graphics
      - UI
        - FLTK
          - LowLevel      -- Haskell bindings
  - scripts               -- various helper scripts (probably not interesting to anyone but myself)

Running in the REPL

Running GUIs in GHCi is fully supported. Using the hello world skeleton as an example the following steps will run it in the REPL:

> git clone http://github.com/deech/fltkhs-hello-world
> cd fltkhs-hello-world
> stack build --flag fltkhs:bundled
> stack ghci --flag fltkhs:bundled fltkhs-hello-world:exe:fltkhs-hello-world
[1 of 1] Compiling Main ...
Ok, modules loaded: Main ...
Loaded GHCi configuration ...
Prelude Main> replMain

Unfortunately since FLTKHS is hybrid Haskell/C++ there are limitations compared to running a plain 'ol Haskell library on the REPL:

The 'stack build ...' is an essential first step before running 'stack ghci ...'. The reason is it uses '-fobject-code' to link in all the C++ libraries which must be built first.
The use of replMain instead of just ':main' as you might expect. This is because
1. it allows closing the GUI to correctly return control to the REPL prompt and
2. typing 'Ctrl-C' also correctly hands control back to the REPL.

With just ':main' (1) works but (2) results in a "ghosted" UI where the GUI window is still visible but unable to accept any keyboard/mouse input. The reason for the ghosted GUI is that ':main' delegates to the FLTK C++ event loop which is unable to listen for user interrupts on the Haskell side and so has no of knowing that it should destroy itself.replMain emulates the event loop on the Haskell side allowing it to stop, clean up and return control when it catches a UserInterrupt. Thus the replMain is slower than the optimized C++ event loop but hopefully that's not too big an impediment for REPL work.