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:
- Read the Haddock docs (with source code, additional examples, and Comment/Talk links)
- Get the code repository: darcs get http://code.haskell.org/~conal/code/TV
- Or grab a distribution tarball.
- See the use of TV in Eros.
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.
I'd love to hear your comments at the Talk:TV page.
Here is a tangible reverse function:
reverseT :: CTV (String -> String) reverseT = tv (oTitle "reverse" defaultOut) reverse
tv function combines an interface and a value. In this example, the interface is the default for string functions, wrapped with the title "reverse".
TV "interfaces" are more than just GUIs. Here are two different renderings of
reverseT. (User input is shown in italics in the
*Examples> runIO reverseT reverse: Hello, reversible world. .dlrow elbisrever ,olleH *Examples>
We'll see later that "
runUI" and "
runIO" are both type-specialized synonyms for a more general function.
What I've been calling an "interface" is a value of type
COutput a for a type
a. For instance, for
String->String. The reason for the
C prefix is explained below. At the heart of TV is a small algebra for constructing these outputs. Weve already seen one output function,
oTitle. Another one is
showOut, which is an output for all
Show types. For instance,
total :: Show a => COutput a total = oTitle "total" showOut
Inputs and function-valued outputs
Just as an output is a way to deliver a value, an "input" is a way to obtain 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
Now for the fun part. Lets combine the
bananas inputs and the
total output to make a function-valued output.
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 (+)
shopping list: apples: 8 bananas: 5 total: 13
Here is an uncurried variation:
shoppingPr :: CTV ((Int,Int) -> Int) shoppingPr = tv ( oTitle "shopping list -- uncurried" $ oLambda (iPair apples bananas) total ) (uncurry (+))
However, there's a much more elegant formulation, using
$$ from DeepArrow:
shoppingPr = uncurryA $$ shopping
shopping list: apples: 8 bananas: 5 total: 13
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 cofunctors.
In the examples above, we've used two different mechanisms, namely Phooey's
UI AF and
IO. The sinks are counterparts
runIO used in examples above are simply type-specialized synonyms for
runUI :: TV UI IU a -> IO () runUI = runTV runIO :: TV IO OI a -> IO () runIO = runTV
Common Ins and Outs
shoppingT above used not only the generic
Input operations, but also some operations that apply to AFs having a few methods for sourcing and sinking a few common types (strings, readables, showables, and booleans). The type constructors
CTV are universally quantified over sources and sinks having the required methods.
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
Here's a sorting TV (see
interactLinesRS), tested with
sortT :: (Read a, Show a, Ord a) => CTV ([a] -> [a]) sortT = tv (oTitle "sort" $ interactLinesRS ) sort
sortT is polymorphic in value, and the type variable
a as defaulted to
Int. You could instead type-annotate its uses, e.g.,
runUI (sortT :: CTV ([String] -> [String]))
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
sentence in: The night Max wore his wolf suit sentence out: Max The his night suit wolf wore
The operator "
->|" is part of a general approach to value composition from DeepArrow.
While some interfaces can be implemented for different means of transmission, others are more specialized.
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:
code runUI rendering
tv shoppingUO (+)
uncurryA $$ tv shoppingUO (+)
Note: We could define other type classes, besides
CommonInsOuts. For instance,
islider could be made a method of a
We can use
IO operations in TV interfaces. The corresponding sink is
OI, defined in TypeCompose. TV provides a few functions in its
IO module, including a close counterpart to the standard
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)
runIO (tv test0 f)
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
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
- 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
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
There are more examples in the TV repository and in the in the GuiTV repository. See also "separating IO from logic -- example".