Tangible Value

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, or a *nix-like process that reads and writes character streams. Values and interfaces are combined for direct use, and separable for composition.

TV is for creating software that is ready to use and ready to reuse.

Beside this page, here are some ways to explore TV:

• Read the Haddock docs (with source code, additional examples, and Comment/Talk links)
• Get the code repository: darcs get http://darcs.haskell.org/packages/TV
• Or grab a distribution tarball.
• See the README.

Tangible values

As a first example, here is a tangible reverse function:

reverseT :: CTV (String -> String)
reverseT = tv (oTitle "reverse" defaultOut) reverse

The result of runUI reverseT looks like this:

The 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 we use runIO instead of runUI (underlining user input):

*Examples> runIO reverseT
reverse: Hello, reversible world!
!dlrow elbisrever ,olleH
*Examples>

Outputs

What I've been calling an "interface" is a value of type COutput a for a type a (for reverseT, a == 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"  (intIn 3 (0,10))
bananas = iTitle "bananas" (intIn 7 (0,10))

Now for the fun part. Lets combine the apples and 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 (+)

The result of runUI shopping:

And of runIO shopping:

*Examples> runIO shopping
shopping list: apples: 8
bananas: 5
total: 13

A variation

Here is an uncurried variation:

shoppingPr :: CTV ((Int,Int) -> Int)
shoppingPr = tv ( oTitle "shopping list" $ 
                  oLambda (iPair apples bananas) total )
                (uncurry (+))

The runIO presentation is identical to that of shopping. The runUI presentation:

The general story

TVs, outputs and inputs are not restricted to GUIs and IO. In general, theyre parameterized by an arrow.

data Output (~>) a
data Input  (~>) a
type TV     (~>) a

In the examples above, we've used two different arrows, namely Phooeys UI arrow (<http://conal.net/Phooey>) and KIO, defined simply as

type KIO = Kleisli IO

Any other monad may be used in place of IO, and other arrows in place of UI and KIO.

Common Ins and Outs

The examples reverseT and shoppingT above used not only the generic Output and Input operations, but also some operations that apply to arrows belonging to the CommonInsOuts class, which includes UI and KIO. The type constructors CInput, COutput, and CTV are universally quantified over CommonInsOuts arrows.

type Common f a = forall (~>). CommonInsOuts (~>) => f (~>) a

type CInput  a = Common Input a
type COutput a = Common Output a
type CTV     a = Common TV a

Sorting examples

Here's a sorting TV:

sortT :: (Read a, Show a, Ord a) => CTV ([a] -> [a])
sortT = tv (oTitle "sort" $ interactRSOut []) sort

Since sortT is polymorphic in value, you may want to type-annotate its uses, e.g.,

runUI (sortT :: CTV ([String] -> [String]))

Otherwise, a will default to Int.

With runUI:

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 words and unwords functions:

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

Finally, compose wordsT, unwordsT, and sortT

sortWordsT :: CTV (String -> String)
sortWordsT = wordsT ->| sortT ->| unwordsT

With runUI and runIO:

*Examples> runIO sortWordsT
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.

Motivation

Portability

Known Problems

Plans