m (→Composition of TVs)
m (fix cabal install directions)
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Latest revision as of 15:39, 14 August 2013
 1 Abstract
TV is a library for composing tangible values ("TVs"), i.e., values that carry along external interfaces. In particular, TVs can be composed to create new TVs, and they can be directly executed with a friendly GUI, a process that reads and writes character streams, or many other kinds interfaces. Values and interfaces are combined for direct use, and separable for composition. This combination makes for software that is ready to use and ready to reuse.
TV can be thought of as a simple functional formulation of the Model-View-Controller pattern. (My thanks to an anonymous ICFP referee for pointing out this connection.) The value part of a TV is the model, and the "interface" part, or "output" as it is called below, is the viewer. Outputs are built up compositionally from other outputs and from inputs (the controllers), as described below.
Besides this wiki page, here are more ways to learn about TV:
- Visit the Hackage page for library documentation.
- Install with cabal install TV.
As of version 0.2, I have moved the GUI functionality out of TV and into a small new package GuiTV. I moved it out to eliminate the dependency of core TV on Phooey and hence on wxHaskell, as the latter can be difficult to install. The GUI examples below require GuiTV.
GuiTV (better named "wxTV") is bit-rotten. There is also a very similar package to generate Gtk-based GUIs.
I'd love to hear your comments at the Talk:TV page.
 2 First Example
Here is a tangible reverse function:
reverseT :: CTV (String -> String) reverseT = tv (oTitle "reverse" defaultOut) reverse
We'll see later that "
runUI reverseT runIO reverseT *Examples> runIO reverseT reverse: Hello, reversible world. .dlrow elbisrever ,olleH *Examples>
 3 OutputsWhat I've been calling an "interface" is a value of type
total :: Show a => COutput a total = oTitle "total" showOut
 4 Inputs and function-valued outputs
Just as an output is a way to deliver (or consume) a value, an "input" is a way to obtain (or produce) a value. For example, here are two inputs, each specifying an initial value and a value range, and each given a title.
apples, bananas :: CInput Int apples = iTitle "apples" defaultIn bananas = iTitle "bananas" defaultIn
shoppingO :: COutput (Int -> Int -> Int) shoppingO = oTitle "shopping list" $ oLambda apples (oLambda bananas total)
And a TV:
shopping :: CTV (Int -> Int -> Int) shopping = tv shoppingO (+)
 5 A variation
Here is an uncurried variation:
shoppingPr :: CTV ((Int,Int) -> Int) shoppingPr = tv ( oTitle "shopping list -- uncurried" $ oLambda (iPair apples bananas) total ) (uncurry (+))
shoppingPr = uncurryA $$ shopping
 6 The general story
TVs, outputs, and inputs are not restricted to GUIs and IO. In general, they are parameterized by the mechanics of "transmitting values", i.e., delivering ("sinking") output and gathering ("sourcing") input.
data Input src a data Output src snk a type TV src snk a
The "sources" will be applicative functors (AFs), and the "sinks" will be contravariant functors.In the examples above, we've used two different mechanisms, namely Phooey's
runUI :: TV UI IU a -> IO () runUI = runTV runIO :: TV IO OI a -> IO () runIO = runTV
 7 Common Ins and OutsThe examples
type CInput a = forall src. (CommonIns src) => Input src a type COutput a = forall src snk. (CommonIns src, CommonOuts snk) => Output src snk a type CTV a = forall src snk. (CommonIns src, CommonOuts snk) => TV src snk a
 8 Sorting examplesHere's a sorting TV (see <div class="inline-code">
sortT :: (Read a, Show a, Ord a) => CTV ([a] -> [a]) sortT = tv (oTitle "sort" $ interactLineRS ) sort
- runUI (sortT :: CTV ([String] -> [String]))
 9 Composition of TVs
So far, we done a little composition of interfaces and combined them with values to construct TVs. Now let's look at composition of TVs.First, wrap up the
wordsT :: CTV (String -> [String]) wordsT = tv ( oTitle "function: words" $ oLambda (iTitle "sentence in" defaultIn) (oTitle "words out" defaultOut)) words
unwordsT :: CTV ([String] -> String) unwordsT = tv ( oTitle "function: unwords" $ oLambda (iTitle "words in" defaultIn) (oTitle "sentence out" defaultOut)) unwords
sortWordsT :: CTV (String -> String) sortWordsT = wordsT ->| sortT ->| unwordsT
The operator "<div class="inline-code">
runUI sortWordsT runIO sortWordsT sentence in: The night Max wore his wolf suit sentence out: Max The his night suit wolf wore
 10 Transmission-specific interfaces
While some interfaces can be implemented for different means of transmission, others are more specialized.
 10.1 GUIs
Here are inputs for our shopping example above that specifically work with Phooey's UI applicative functor.
applesU, bananasU :: Input UI Int applesU = iTitle "apples" (islider 3 (0,10)) bananasU = iTitle "bananas" (islider 7 (0,10)) shoppingUO :: Output UI (Int -> Int -> Int) shoppingUO = oTitle "shopping list" $ oLambda applesU (oLambda bananasU total)
We can then make curried and uncurried TVs:
 10.2 IOWe can use
interactOut :: Output IO OI (String -> String) interactOut = oLambda contentsIn stringOut
Assuming we have a file "test.txt" containing some lines of text, we can use it to test string transformations.
testO :: Output IO OI (String -> String) testO = oLambda (fileIn "test.txt") defaultOut
First, let's define higher-order functions that apply another function to the lines or on the words of a string.
onLines, onWords :: ([String] -> [String]) -> (String -> String) onLines f = unlines . f . lines onWords f = unwords . f . words
Next, specializations that operate on each line or word:
perLine,perWord :: (String -> String) -> (String -> String) perLine f = onLines (map f) perWord f = onWords (map f)
string functionf runIO (tv test0 f) id To see a World in a Grain of Sand And a Heaven in a Wild Flower, Hold Infinity in the palm of your hand And Eternity in an hour. - William Blake reverse ekalB mailliW - .ruoh na ni ytinretE dnA dnah ruoy fo mlap eht ni ytinifnI dloH ,rewolF dliW a ni nevaeH a dnA dnaS fo niarG a ni dlroW a ees oT onLines reverse - William Blake And Eternity in an hour. Hold Infinity in the palm of your hand And a Heaven in a Wild Flower, To see a World in a Grain of Sand perLine reverse dnaS fo niarG a ni dlroW a ees oT ,rewolF dliW a ni nevaeH a dnA dnah ruoy fo mlap eht ni ytinifnI dloH .ruoh na ni ytinretE dnA ekalB mailliW - perLine (perWord reverse) oT ees a dlroW ni a niarG fo dnaS dnA a nevaeH ni a dliW ,rewolF dloH ytinifnI ni eht mlap fo ruoy dnah dnA ytinretE ni na .ruoh - mailliW ekalB