Difference between revisions of "Phooey"
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== Abstract == 
== Abstract == 

−  '''Phooey''' is an [[arrow]]based functional UI library for [[Haskell]]. 

+  '''Phooey''' is a functional UI library for [[Haskell]]. Or it's two of them, as it provides a <hask>Monad</hask> interface ''and'' an <hask>Applicative</hask> interface. The simplicity of Phooey's implementation is due to its use of [[Reactive]] for applicative, datadriven computation. (Before version 2.0, Phooey used the [[DataDriven]] library.) 

−  * [http://darcs.haskell.org/packages/phooey/doc/html Read the Haddock docs] (with source code, additional examples, and Comment/Talk links) 

+  Besides this wiki page, here are more ways to find out about Phooey: 

−  * Get the code repository: '''<tt>darcs get http://darcs.haskell.org/packages/phooey</tt>''' 

+  * Read [http://hackage.haskell.org/package/phooey2.0 the Haddock docs] (with source code, additional examples, and Comment/Talk links). 

−  * Or grab a [http://darcs.haskell.org/packages/phooey/dist distribution tarball]. 

+  * Get the code repository: '''<tt>darcs get http://conal.net/repos/phooey</tt>'''. 

−  * See the [http://darcs.haskell.org/packages/phooey/README README]. 

+  
+  The package can be installed from [[Hackage]], using [[cabalinstall]]: 

+  cabal install phooey 

+  
+  Phooey is also used in [[GuiTV]], a library for composable interfaces and "tangible values". 

+  
+  Since Reactive is currently broken (as of February 2010), Phooey is also broken. 

== Introduction == 
== Introduction == 

Line 12:  Line 18:  
GUIs are usually programmed in an unnatural style, in that implementation dependencies are inverted, relative to logical dependencies. This reversal results directly from the ''push'' (datadriven) orientation of most GUI libraries. While outputs depend on inputs from a user and semantic point of view, the ''push'' style imposes an implementation dependence of inputs on outputs. 
GUIs are usually programmed in an unnatural style, in that implementation dependencies are inverted, relative to logical dependencies. This reversal results directly from the ''push'' (datadriven) orientation of most GUI libraries. While outputs depend on inputs from a user and semantic point of view, the ''push'' style imposes an implementation dependence of inputs on outputs. 

−  A second drawback of the push style is that it is ''imperative'' rather than declarative. A GUI program describes actions to update a model 
+  A second drawback of the push style is that it is ''imperative'' rather than declarative. A GUI program describes actions to update a model and view in reaction to user input. In contrast to the ''howtoupdate'' style of an imperative program, a functional GUI program would express ''whatitis'' of a model in terms of the inputs and of the view in terms of the model. 
−  The questions of 
+  The questions of pushvspull and imperativevsdeclarative are related. While an imperative GUI program could certainly be written to pull (poll) values from input to model and model to view, thus eliminating the dependency inversion, I don't know how a declarative program could be written in the inverteddependency style. ([[Talk:PhooeyDo you?]]). 
−  A important reason for using push rather than pull in a GUI implementation is that push is typically much more efficient. A simple pull implementation would either waste time recomputing an unchanging model and view (pegging your CPU for no benefit), or deal with the complexity of avoiding that recomputation. The push style computes only when inputs change. ( 
+  A important reason for using push rather than pull in a GUI implementation is that push is typically much more efficient. A simple pull implementation would either waste time recomputing an unchanging model and view (pegging your CPU for no benefit), or deal with the complexity of avoiding that recomputation. The push style computes only when inputs change. (Continuous change, i.e. animation, negates this advantage of push.) 
−  Phooey ("'''Ph'''unctional '''oo'''s'''e'''r '''y'''nterfaces") adopts the declarative style, in which outputs are expressed in terms of inputs. Under the hood, however, the implementation is pushbased (datadriven). Phooey 
+  Phooey ("'''Ph'''unctional '''oo'''s'''e'''r '''y'''nterfaces") adopts the declarative style, in which outputs are expressed in terms of inputs. Under the hood, however, the implementation is pushbased (datadriven). Phooey uses the [[Reactive]] library to perform the dependency inversion invisibly, so that programmers may express GUIs simply and declaratively while still getting an efficient implementation. 
+  
+  Phooey came out of [http://conal.net/Pajama Pajama] and [http://conal.net/papers/Eros Eros]. Pajama is a reimplementation of the [http://conal.net/Pan Pan] language and compiler for function synthesis of interactive, continuous, infinite images. Pan and Pajama use a monadic style for specifying GUIs and are able to do so because they use the implementation trick of [http://conal.net/papers/jfpsaig Compiling Embedded Languages], in which one manipulates expressions rather than values. (This trick is mostly transparent, but the illusion shows through in places.) 

+  
+  == One example, two interfaces == 

As an example, below is a simple shopping list GUI. The <hask>total</hask> displayed at the bottom of the window always shows the sum of the values of the <hask>apples</hask> and <hask>bananas</hask> input sliders. When a user changes the inputs, the output updates accordingly. 
As an example, below is a simple shopping list GUI. The <hask>total</hask> displayed at the bottom of the window always shows the sum of the values of the <hask>apples</hask> and <hask>bananas</hask> input sliders. When a user changes the inputs, the output updates accordingly. 

+  : [[Image:ui1.png]] 

+  
+  Phooey presents two styles of functional GUI interfaces, structured as a [[monad]] and as an [[applicative functor]]. (I have removed the original [[arrow]] interface.) Below you can see the code for the shopping list example in each of these styles. 

+  
+  The examples below are all found under [http://darcs.haskell.org/packages/phooey/src/Examples <code>src/Examples/</code>] in the phooey distribution, in the modules [http://darcs.haskell.org/packages/phooey/src/Examples/Monad.hs <code>Monad.hs</code>], and [http://darcs.haskell.org/packages/phooey/src/Examples/Applicative.hs <code>Applicative.hs</code>]. In each case, the example is run by loading the corresponding example module into ghci and typing <hask>runUI ui1</hask>. 

+  
+  === Monad === 

−  : http://conal.net/phooey/images/ui1.png 

+  Here is a definition for the GUI shown above, formulated in terms of Phooey's monadic interface. See the [http://darcs.haskell.org/packages/phooey/doc/html/GraphicsUIPhooeyMonad.html monad interface] and its [http://darcs.haskell.org/packages/phooey/doc/html/src.Graphics.UI.Phooey.Monad.hs.html source code]. 

−  Phooey is structured as an arrow, and this example uses arrow notation. The code: 

<haskell> 
<haskell> 

−  ui1 :: UI 
+  ui1 :: UI () 
ui1 = title "Shopping List" $ 
ui1 = title "Shopping List" $ 

−  proc () > do 

+  do a < title "apples" $ islider (0,10) 3 

−  +  b < title "bananas" $ islider (0,10) 7 

−  +  title "total" $ showDisplay (liftA2 (+) a b) 

−  title "total" showDisplay < a+b 

</haskell> 
</haskell> 

−  There are more examples [http://darcs.haskell.org/packages/phooey/doc/html/GraphicsUIPhooey.html in the Haddock docs]. 

+  The relevant library declarations: 

+  <haskell> 

+   Input widget type (with initial value) 

+  type IWidget a = a > UI (Source a) 

+   Output widget type 

+  type OWidget a = Source a > UI () 

+  
+  islider :: (Int,Int) > IWidget Int 

+  showDisplay :: Show a => OWidget a 

+  title :: String > UI a > UI a 

+  </haskell> 

−  == Motivation == 

+  The <hask>Source</hask> type is a (datadriven) source of [[Reactivetimevarying values]]. (<hask>Source</hask> is a synonym for <hask>Reactive</hask>.) By using <hask>Source Int</hask> instead of <hask>Int</hask> for the type of <hask>a</hask> and <hask>b</hask> above, we do not have to rebuild the GUI every time an input value changes. 

−  Phooey came out of [http://conal.net/Pajama Pajama] and [http://conal.net/papers/Eros]. Pan is a reimplementation of the [http://conal.net/Pan Pan] language and compiler for function synthesis of interactive, continuous, infinite images. Pan and Pajama use a monadic style for specifying GUIs and are able to do so because they use the implementation trick of [http://conal.net/papers/jfpsaig Compiling Embedded Languages], in which one manipulates expressions rather than values. (This trick is mostly transparent, but the illusion shows through in places.) 

+  The down side of using source types is seen in the <hask>showDisplay</hask> line above, which requires lifting. We could partially hide the lifting behind overloadings of <hask>Num</hask> and other classes (as in [http://conal.net/Fran Fran], [http://conal.net/Pan Pan], and other systems). Some methods, however, do not not have sufficiently flexible types (e.g., <hask>(==)</hask>), and the illusion becomes awkward. The <hask>Arrow</hask> and <hask>Applicative</hask> interfaces hide the source types. 

−  Switching from expressions to values raises a challenge for a declarative [[monad]]ic approach. For instance, consider this (problematic) monadic UI. 

+  Before we move on to other interface styles, let's look at some refactorings. First pull out the slider minus initial value: 

<haskell> 
<haskell> 

−  factOopsUI :: UIM () 

+  sl0 :: IWidget Int 

−  +  sl0 = islider (0,10) 

−  title "factorial" (showDisplay (fact n)) 

</haskell> 
</haskell> 

−  If <hask>n :: Int</hask>, then the second half of the GUI depends on the dynamic runtime values flowing out of the first half, and so must be constructed for each new value of <hask>n</hask>. The problem is a failure to separate a GUI into its static and dynamic parts. A solution with the monadic framework would be to give <hask>n</hask> a different type, such as <hask>Source Int</hask>, which captures the static nature of the input GUI while giving access to a ''source'' of dynamic values. Phooey provides this monadic solution in its [http://darcs.haskell.org/packages/phooey/doc/html/GraphicsUIPhooeyMonadUI.html MonadUI] module. The awkwardness is that we can no longer use such simple formulations as <hask>factOopsUI</hask> above, since expressions like <hask>product [1..n]</hask> won't typecheck. (We can play overloading tricks as in [http://conal.net/Fran Fran], Pan, and Pajama, but they don't always work.) Instead, we could use <hask>return</hask>, <hask>liftM</hask>, <hask>liftM2</hask>, etc. 

+  
+  Then the titled widgets: 

<haskell> 
<haskell> 

−  factUI :: UIM () 

+  apples, bananas :: UI (Source Int) 

−  factUI = do n < title "n" (islider 3 (return (0,20))) 

+  apples = title "apples" $ sl0 3 

−  title "factorial" (showDisplay (liftM fact n)) 

+  bananas = title "bananas" $ sl0 7 

+  
+  total :: Num a => OWidget a 

+  total = title "total" . showDisplay 

</haskell> 
</haskell> 

−  Rather than introducing the complexity of the <hask>Source</hask> type and the need for explicit lifting, we can solve the problem by replacing the [[Monad]] abstraction with one that separates static and dynamic aspects. Getting that separation is the point of the [[Arrow]] abstraction, and thus Phooey's primary interface is formulated as an arrow rather than a monad. Phooey's UI arrow is implemented on top of its UI monad using a simple, reusable pattern. See the [http://darcs.haskell.org/packages/phooey/doc/html/GraphicsUIPhooeyArrowUI.html ArrowUI module doc] and its [http://darcs.haskell.org/packages/phooey/doc/html/src/Graphics/UI/Phooey/ArrowUI.hs.html source code]. 

+  And use them: 

+  <haskell> 

+  ui1x :: UI () 

+  ui1x = title "Shopping List" $ 

+  do a < apples 

+  b < bananas 

+  total (liftA2 (+) a b) 

+  </haskell> 

−  Phooey is also used in [[TV]], a library for composable interfaces and "tangible values". 

+  We can go pointfree by using <hask>liftM2</hask> and <hask>(>>=)</hask>: 

+  <haskell> 

+   Sum UIs 

+  infixl 6 .+. 

+  
+  (.+.) :: Num a => UIS a > UIS a > UIS a 

+  (.+.) = liftA2 (liftA2 (+)) 

+  
+  fruit :: UI (Source Int) 

+  fruit = apples .+. bananas 

+  
+  ui1y :: UI () 

+  ui1y = title "Shopping List" $ fruit >>= total 

+  </haskell> 

+  
+  === Applicative Functor === 

+  
+  [[Applicative functor]]s (AFs) provide still another approach to separating static and dynamic information. Here is our example, showing just the changes relative to the [[#Monadmonadic]] version. (See the 

+  [http://darcs.haskell.org/packages/phooey/doc/html/GraphicsUIPhooeyApplicative.html Applicative interface doc] and its [http://darcs.haskell.org/packages/phooey/doc/html/src.Graphics.UI.Phooey.Applicative.hs.html source code].) 

+  <haskell> 

+  ui1 :: UI (IO ()) 

+  ui1 = title "Shopping List" $ fruit <**> total 

+  
+  fruit :: UI Int 

+  fruit = liftA2 (+) apples bananas 

+  
+  total :: Num a => OWidget a 

+  total = title "total" showDisplay 

+  </haskell> 

+  I chose reversed AF application <hask>(<**>)</hask> rather than <hask>(<*>)</hask> so the fruit (argument) would be displayed above the total (function). 

+  
+  The UIbuilding functions again have the same types as before, relative to these new definitions: 

+  <haskell> 

+  type IWidget a = a > UI a 

+  type OWidget a = UI (a > IO ()) 

+  </haskell> 

+  
+  Notes: 

+  * Output widgets are functionvalued UI. 

+  * <hask>fruit</hask> has a simpler definition, requiring only one lifting instead of two. 

+  * <hask>total</hask> is subtly different, because output widgets are now ''functionvalued''. 

+  * <hask>ui1</hask> uses the [http://www.haskell.org/ghc/docs/latest/html/libraries/base/ControlApplicative.html#v%3A%3C%2A%2A%3E reverse application operator] <hask>(<**>)</hask>. This reversal causes the function to appear after (below) the argument. 

+  * <hask>ui1</hask> is an IOvalued UI. 

+  
+  The applicative UI interface (<hask>Graphics.UI.Phooey.Applicative</hask>) is implemented as a very simple layer on top of the monadic interface, using type composition (from [[TypeCompose]]): 

+  <haskell> 

+  type UI = M.UI :. Source 

+  </haskell> 

+  Thanks to properties of <hask>O</hask>, this definition suffices to make <hask>UI</hask> an AF. 

+  
+  == Layout == 

+  
+  By default, UI layout follows the order of the specification, with earlierspecified components above laterspecified ones. This layout may be overridden by explicit layout functions. For instance, the following definitions form variations of <hask>ui1</hask> laid out from bottom to top and from left to right. 

+  
+  GUIs & code: 

+  
+  : [[Image:UiB1.png]] 

+  : [[Image:UiL1.png]] 

+  
+  <haskell> 

+  uiB1 = fromBottom ui1 

+  uiL1 = fromLeft ui1 

+  </haskell> 

+  
+  
+  We can also lay out a subassembly, as in <hask>ui3</hask> below 

+  
+  : [[Image:Ui3.png]] 

+  
+  <haskell> 

+  ui3 = fromBottom $ 

+  title "Shopping List" $ 

+  fromRight fruit >>= total 

+  </haskell> 

+  
+  == Event Examples == 

+  
+  The shopping examples above demonstrate the simple case of outputs (<hask>total</hask>) as functions of varying inputs (<hask>apples</hask> and <hask>bananas</hask>). Events were hidden inside the implementation of [[Reactive#Data.Reactivereactive values]]. 

+  
+  This section shows two classic functional GUI examples involving a visible notion of [[Reactive#Data.Reactiveevents]]. 

+  
+  === Counter === 

+  
+  Here is simple counter, which increments or decrements when the "up" or "down" button is pressed. The example is from "[http://www.citeulike.org/user/conal/article/1617415 Structuring Graphical Paradigms in TkGofer]" 

+  
+  : [[Image:PhooeyUpDown.png]] 

+  
+  The first piece in making this counter is a button, having a specified value and a label. The button GUI's value is an ''event'' rather than a source: 

+  <haskell> 

+  smallButton :: a > String > UI (Event a) 

+  </haskell> 

+  To make the up/down counter, we'll want two such buttons, labeled "up" and "down". But with what values? The buttons won't know what the counter value is, but they will know how to change the value, so the events will be functionvalued. The two events resulting from the two buttons are then merged into a single functionvalued event via <hask>mappend</hask>. (If you're curious about events at this point, take a detour and [[Reactive#Data.Reactiveread about them]].) 

+  
+  The pair of buttons and combined event could be written as follows: 

+  <haskell> 

+  upDown :: Num a => UIE (a > a) 

+  upDown = do up < smallButton (+ 1) "up" 

+  down < smallButton (subtract 1) "down" 

+  return (up `mappend` down) 

+  </haskell> 

+  If you've been hanging around with monad hipsters, you'll know that we can write this definition more simply: 

+  <haskell> 

+  upDown = liftM2 mappend (smallButton (+ 1) "up") 

+  (smallButton (subtract 1) "down") 

+  </haskell> 

+  Personally, I'm on an <hask>Applicative</hask> kick lately, so I prefer <hask>liftA2</hask> in place of <hask>liftM2</hask>. 

+  
+  Still more sleekly, let's hide the <hask>liftM2</hask> (or <hask>liftA2</hask>) by using the <hask>Monoid (UI o) </hask> instance, which holds whenever <hask>Monoid o</hask>. 

+  <haskell> 

+  upDown = smallButton (+ 1) "up" `mappend` 

+  smallButton (subtract 1) "down" 

+  </haskell> 

+  
+  To finish the counter, use the <hask>accumR</hask> function, which makes a source from an initial value and an functionvalued event. The source begins as the initial value and grows by applying the functions generated by the event. 

+  <haskell> 

+  accumR :: a > UI (Event (a > a)) > UI (Source a) 

+  
+  counter :: UI () 

+  counter = title "Counter" $ fromLeft $ 

+  do e < upDown 

+  showDisplay (0 `accumR` e) 

+  </haskell> 

+  
+  === Calculator === 

+  
+  The second event example is a calculator, as taken from "[http://citeseer.ist.psu.edu/vullinghs95lightweight.html Lightweight GUIs for Functional Programming]". 

+  
+  : [[Image:Calc.png]] 

+  
+  The basic structure of this example is just like the previous one. Each key has a functionvalued event, and the keys are combined (visually and semantically) using <hask>mappend</hask>. 

+  
+  First a single key. For variety, we'll postpone interpreting the key's event as a function. 

+  <haskell> 

+  key :: Char > UIE Char 

+  key c = button' c [c] 

+  </haskell> 

+  
+  We'll combine keys with the help of a friend of <hask>concatMap</hask>: 

+  <haskell> 

+  mconcatMap :: Monoid b => (a > b) > [a] > b 

+  mconcatMap f = mconcat . map f 

+  </haskell> 

+  
+  With this helper, it's especially easy to turn several keys into a row and several rows into a keyboard. 

+  <haskell> 

+  row :: [Char] > UIE Char 

+  row = fromLeft . mconcatMap key 

+  
+  rows :: [[Char]] > UIE Char 

+  rows = fromTop . mconcatMap row 

+  
+  calcKeys :: UIE Char 

+  calcKeys = rows [ "123+" 

+  , "456" 

+  , "789*" 

+  , "C0=/" ] 

+  </haskell> 

+  
+  Next, let's turn <hask>calcKeys</hask>'s charactervalued event into a functionvalued event. While the state of the [[Phooey#Countercounter]] was a single number, the calculator state is a little more complicated. It consists of a number being formed and a continuation. 

+  <haskell> 

+  type CState = (Int, Int > Int) 

+  
+  startCS :: CState 

+  startCS = (0, id) 

+  </haskell> 

+  
+  Keyboard characters have interpretations as state transitions. 

+  <haskell> 

+  cmd :: Char > CState > CState 

+  cmd 'C' _ = startCS 

+  cmd '=' (d,k) = (k d, const (k d)) 

+  cmd c (d,k)  isDigit c = (10*d + ord c  ord '0', k) 

+   otherwise = (0, op c (k d)) 

+  
+  op :: Char > Int > Int > Int 

+  op c = fromJust (lookup c ops) 

+  where 

+  ops :: [(Char, Binop Int)] 

+  ops = [('+',(+)), ('',()), ('*',(*)), ('/',div)] 

+  </haskell> 

+  
+  To compute the (reactive) value, from a keygenerating event, accumulate transitions, starting with the initial state, and extract the value. 

+  <haskell> 

+  compCalc :: Event Char > Source Int 

+  compCalc key = fmap fst (startCS `accumR` fmap cmd key) 

+  </haskell> 

+  Show the result: 

+  <haskell> 

+  showCalc :: Event Char > UI () 

+  showCalc = title "result" . showDisplay . compCalc 

+  </haskell> 

+  
+  The whole calculator then snaps together: 

+  <haskell> 

+  calc :: UI () 

+  calc = title "Calculator" $ calcKeys >>= showCalc 

+  </haskell> 

== Portability == 
== Portability == 

−  Phooey is built on wxHaskell. Quoting from the [http://wxhaskell.sourceforge.net wxHaskell home page] 
+  Phooey is built on [[wxHaskell]]. Quoting from the [http://wxhaskell.sourceforge.net wxHaskell home page], 
<blockquote> 
<blockquote> 

wxHaskell is therefore built on top of [http://www.wxwidgets.org wxWidgets]  a comprehensive C++ library that is portable across all major GUI platforms; including GTK, Windows, X11, and MacOS X. 
wxHaskell is therefore built on top of [http://www.wxwidgets.org wxWidgets]  a comprehensive C++ library that is portable across all major GUI platforms; including GTK, Windows, X11, and MacOS X. 

</blockquote> 
</blockquote> 

−  So I expect that Phooey runs on all of these platforms. That said, I have only tried Phooey on Windows. Please give it a try and leave a message on the Talk page 
+  So I expect that Phooey runs on all of these platforms. That said, I have only tried Phooey on Windows. Please give it a try and leave a message on the [[Talk:Phooeytalk page]]. 
+  
+  == Known problems == 

+  * Recursive examples don't work (consumes memory) in the Arrow or Applicative interface. 

+  
+  == Plans == 

+  * Use Javascript and HTML in place wxHaskell, and hook it up with [[Yhc/Javascript]]. 

−  [[Category:User 
+  [[Category:User interfaces]] 
[[Category:Arrow]] 
[[Category:Arrow]] 

[[Category:Monad]] 
[[Category:Monad]] 

[[Category:Libraries]] 
[[Category:Libraries]] 

[[Category:Packages]] 
[[Category:Packages]] 

+  [[Category:wxHaskell]] 
Latest revision as of 14:34, 3 April 2012
Contents
Abstract
Phooey is a functional UI library for Haskell. Or it's two of them, as it provides a Monad
interface and an Applicative
interface. The simplicity of Phooey's implementation is due to its use of Reactive for applicative, datadriven computation. (Before version 2.0, Phooey used the DataDriven library.)
Besides this wiki page, here are more ways to find out about Phooey:
 Read the Haddock docs (with source code, additional examples, and Comment/Talk links).
 Get the code repository: darcs get http://conal.net/repos/phooey.
The package can be installed from Hackage, using cabalinstall:
cabal install phooey
Phooey is also used in GuiTV, a library for composable interfaces and "tangible values".
Since Reactive is currently broken (as of February 2010), Phooey is also broken.
Introduction
GUIs are usually programmed in an unnatural style, in that implementation dependencies are inverted, relative to logical dependencies. This reversal results directly from the push (datadriven) orientation of most GUI libraries. While outputs depend on inputs from a user and semantic point of view, the push style imposes an implementation dependence of inputs on outputs.
A second drawback of the push style is that it is imperative rather than declarative. A GUI program describes actions to update a model and view in reaction to user input. In contrast to the howtoupdate style of an imperative program, a functional GUI program would express whatitis of a model in terms of the inputs and of the view in terms of the model.
The questions of pushvspull and imperativevsdeclarative are related. While an imperative GUI program could certainly be written to pull (poll) values from input to model and model to view, thus eliminating the dependency inversion, I don't know how a declarative program could be written in the inverteddependency style. (Do you?).
A important reason for using push rather than pull in a GUI implementation is that push is typically much more efficient. A simple pull implementation would either waste time recomputing an unchanging model and view (pegging your CPU for no benefit), or deal with the complexity of avoiding that recomputation. The push style computes only when inputs change. (Continuous change, i.e. animation, negates this advantage of push.)
Phooey ("Phunctional ooser ynterfaces") adopts the declarative style, in which outputs are expressed in terms of inputs. Under the hood, however, the implementation is pushbased (datadriven). Phooey uses the Reactive library to perform the dependency inversion invisibly, so that programmers may express GUIs simply and declaratively while still getting an efficient implementation.
Phooey came out of Pajama and Eros. Pajama is a reimplementation of the Pan language and compiler for function synthesis of interactive, continuous, infinite images. Pan and Pajama use a monadic style for specifying GUIs and are able to do so because they use the implementation trick of Compiling Embedded Languages, in which one manipulates expressions rather than values. (This trick is mostly transparent, but the illusion shows through in places.)
One example, two interfaces
As an example, below is a simple shopping list GUI. The total
displayed at the bottom of the window always shows the sum of the values of the apples
and bananas
input sliders. When a user changes the inputs, the output updates accordingly.
Phooey presents two styles of functional GUI interfaces, structured as a monad and as an applicative functor. (I have removed the original arrow interface.) Below you can see the code for the shopping list example in each of these styles.
The examples below are all found under src/Examples/
in the phooey distribution, in the modules Monad.hs
, and Applicative.hs
. In each case, the example is run by loading the corresponding example module into ghci and typing runUI ui1
.
Monad
Here is a definition for the GUI shown above, formulated in terms of Phooey's monadic interface. See the monad interface and its source code.
ui1 :: UI ()
ui1 = title "Shopping List" $
do a < title "apples" $ islider (0,10) 3
b < title "bananas" $ islider (0,10) 7
title "total" $ showDisplay (liftA2 (+) a b)
The relevant library declarations:
 Input widget type (with initial value)
type IWidget a = a > UI (Source a)
 Output widget type
type OWidget a = Source a > UI ()
islider :: (Int,Int) > IWidget Int
showDisplay :: Show a => OWidget a
title :: String > UI a > UI a
The Source
type is a (datadriven) source of timevarying values. (Source
is a synonym for Reactive
.) By using Source Int
instead of Int
for the type of a
and b
above, we do not have to rebuild the GUI every time an input value changes.
The down side of using source types is seen in the showDisplay
line above, which requires lifting. We could partially hide the lifting behind overloadings of Num
and other classes (as in Fran, Pan, and other systems). Some methods, however, do not not have sufficiently flexible types (e.g., (==)
), and the illusion becomes awkward. The Arrow
and Applicative
interfaces hide the source types.
Before we move on to other interface styles, let's look at some refactorings. First pull out the slider minus initial value:
sl0 :: IWidget Int
sl0 = islider (0,10)
Then the titled widgets:
apples, bananas :: UI (Source Int)
apples = title "apples" $ sl0 3
bananas = title "bananas" $ sl0 7
total :: Num a => OWidget a
total = title "total" . showDisplay
And use them:
ui1x :: UI ()
ui1x = title "Shopping List" $
do a < apples
b < bananas
total (liftA2 (+) a b)
We can go pointfree by using liftM2
and (>>=)
:
 Sum UIs
infixl 6 .+.
(.+.) :: Num a => UIS a > UIS a > UIS a
(.+.) = liftA2 (liftA2 (+))
fruit :: UI (Source Int)
fruit = apples .+. bananas
ui1y :: UI ()
ui1y = title "Shopping List" $ fruit >>= total
Applicative Functor
Applicative functors (AFs) provide still another approach to separating static and dynamic information. Here is our example, showing just the changes relative to the monadic version. (See the Applicative interface doc and its source code.)
ui1 :: UI (IO ())
ui1 = title "Shopping List" $ fruit <**> total
fruit :: UI Int
fruit = liftA2 (+) apples bananas
total :: Num a => OWidget a
total = title "total" showDisplay
I chose reversed AF application (<**>)
rather than (<*>)
so the fruit (argument) would be displayed above the total (function).
The UIbuilding functions again have the same types as before, relative to these new definitions:
type IWidget a = a > UI a
type OWidget a = UI (a > IO ())
Notes:
 Output widgets are functionvalued UI.

fruit
has a simpler definition, requiring only one lifting instead of two. 
total
is subtly different, because output widgets are now functionvalued. 
ui1
uses the reverse application operator(<**>)
. This reversal causes the function to appear after (below) the argument. 
ui1
is an IOvalued UI.
The applicative UI interface (Graphics.UI.Phooey.Applicative
) is implemented as a very simple layer on top of the monadic interface, using type composition (from TypeCompose):
type UI = M.UI :. Source
Thanks to properties of O
, this definition suffices to make UI
an AF.
Layout
By default, UI layout follows the order of the specification, with earlierspecified components above laterspecified ones. This layout may be overridden by explicit layout functions. For instance, the following definitions form variations of ui1
laid out from bottom to top and from left to right.
GUIs & code:
uiB1 = fromBottom ui1
uiL1 = fromLeft ui1
We can also lay out a subassembly, as in ui3
below
ui3 = fromBottom $
title "Shopping List" $
fromRight fruit >>= total
Event Examples
The shopping examples above demonstrate the simple case of outputs (total
) as functions of varying inputs (apples
and bananas
). Events were hidden inside the implementation of reactive values.
This section shows two classic functional GUI examples involving a visible notion of events.
Counter
Here is simple counter, which increments or decrements when the "up" or "down" button is pressed. The example is from "Structuring Graphical Paradigms in TkGofer"
The first piece in making this counter is a button, having a specified value and a label. The button GUI's value is an event rather than a source:
smallButton :: a > String > UI (Event a)
To make the up/down counter, we'll want two such buttons, labeled "up" and "down". But with what values? The buttons won't know what the counter value is, but they will know how to change the value, so the events will be functionvalued. The two events resulting from the two buttons are then merged into a single functionvalued event via mappend
. (If you're curious about events at this point, take a detour and read about them.)
The pair of buttons and combined event could be written as follows:
upDown :: Num a => UIE (a > a)
upDown = do up < smallButton (+ 1) "up"
down < smallButton (subtract 1) "down"
return (up `mappend` down)
If you've been hanging around with monad hipsters, you'll know that we can write this definition more simply:
upDown = liftM2 mappend (smallButton (+ 1) "up")
(smallButton (subtract 1) "down")
Personally, I'm on an Applicative
kick lately, so I prefer liftA2
in place of liftM2
.
Still more sleekly, let's hide the liftM2
(or liftA2
) by using the Monoid (UI o)
instance, which holds whenever Monoid o
.
upDown = smallButton (+ 1) "up" `mappend`
smallButton (subtract 1) "down"
To finish the counter, use the accumR
function, which makes a source from an initial value and an functionvalued event. The source begins as the initial value and grows by applying the functions generated by the event.
accumR :: a > UI (Event (a > a)) > UI (Source a)
counter :: UI ()
counter = title "Counter" $ fromLeft $
do e < upDown
showDisplay (0 `accumR` e)
Calculator
The second event example is a calculator, as taken from "Lightweight GUIs for Functional Programming".
The basic structure of this example is just like the previous one. Each key has a functionvalued event, and the keys are combined (visually and semantically) using mappend
.
First a single key. For variety, we'll postpone interpreting the key's event as a function.
key :: Char > UIE Char
key c = button' c [c]
We'll combine keys with the help of a friend of concatMap
:
mconcatMap :: Monoid b => (a > b) > [a] > b
mconcatMap f = mconcat . map f
With this helper, it's especially easy to turn several keys into a row and several rows into a keyboard.
row :: [Char] > UIE Char
row = fromLeft . mconcatMap key
rows :: [[Char]] > UIE Char
rows = fromTop . mconcatMap row
calcKeys :: UIE Char
calcKeys = rows [ "123+"
, "456"
, "789*"
, "C0=/" ]
Next, let's turn calcKeys
's charactervalued event into a functionvalued event. While the state of the counter was a single number, the calculator state is a little more complicated. It consists of a number being formed and a continuation.
type CState = (Int, Int > Int)
startCS :: CState
startCS = (0, id)
Keyboard characters have interpretations as state transitions.
cmd :: Char > CState > CState
cmd 'C' _ = startCS
cmd '=' (d,k) = (k d, const (k d))
cmd c (d,k)  isDigit c = (10*d + ord c  ord '0', k)
 otherwise = (0, op c (k d))
op :: Char > Int > Int > Int
op c = fromJust (lookup c ops)
where
ops :: [(Char, Binop Int)]
ops = [('+',(+)), ('',()), ('*',(*)), ('/',div)]
To compute the (reactive) value, from a keygenerating event, accumulate transitions, starting with the initial state, and extract the value.
compCalc :: Event Char > Source Int
compCalc key = fmap fst (startCS `accumR` fmap cmd key)
Show the result:
showCalc :: Event Char > UI ()
showCalc = title "result" . showDisplay . compCalc
The whole calculator then snaps together:
calc :: UI ()
calc = title "Calculator" $ calcKeys >>= showCalc
Portability
Phooey is built on wxHaskell. Quoting from the wxHaskell home page,
wxHaskell is therefore built on top of wxWidgets  a comprehensive C++ library that is portable across all major GUI platforms; including GTK, Windows, X11, and MacOS X.
So I expect that Phooey runs on all of these platforms. That said, I have only tried Phooey on Windows. Please give it a try and leave a message on the talk page.
Known problems
 Recursive examples don't work (consumes memory) in the Arrow or Applicative interface.
Plans
 Use Javascript and HTML in place wxHaskell, and hook it up with Yhc/Javascript.