LGtk

What is it?

LGtk is a lens-based API for Gtk. LGtk is built on Gtk2Hs.

Most Haskellers would like to use a mature FRP-based API for creating graphical user interfaces. FRP may not be the best tool for special user interfaces, like interfaces which consists of buttons, checkboxes, combo boxes, text entries and menus only. LGtk has a lens-based API which fits better these user interfaces.

Semantics overview

The semantics of LGtk is given by a reference implementation. The reference implementation is given in the following stages.

Lenses

LGtk use simple lenses defined in the data-lens package:

newtype Lens a b = Lens { runLens :: a -> Store b a }

This data type is isomorphic to (a -> b, b -> a -> a), the well-know lens data type. The isomorphism is established by the following operations:

getL :: Lens a b -> a -> b

setL :: Lens a b -> b -> a -> a

lens :: (a -> b) -> (b -> a -> a) -> Lens a b

Lens laws

The three well-known laws for lenses:

• get-set: set k (get k a) a === a
• set-get: get k (set k b a) === b
• set-set: set k b2 (set k b1 a) === set k b2 a

Impure lenses, i.e. lenses which brake lens laws are allowed in certain places. Those places are explicitly marked and explained in this overview.

References

It is convenient to use lenses with the state monad (see this use case) and the reader monad.

Let's call a value with type Lens s a a reference of the state monad State s and the reader monad Reader s.

References are distinguished from lenses, because we would like to make the program state s unaccessible to guarantee linear use of the state for example.

There are several ways to model references in Haskell. LGtk models them as type classes with associated types, but that is just a technical detail.

Types

Instead of giving a concrete implementation in Haskell, suppose that

• s is a fixed type,
• Ref :: * -> * ~ Lens s, references are lenses from s to the type of the referred value,
• R :: * -> * ~ Reader s, the reference reading monad is the reader monad over s,
• S :: * -> * ~ State s, the reference reading and writing monad is the state monad over s.

The three equality constraints are not exposed in the API.

Operations

Exposed operations of Ref, S and R:

• The Monad instance of S and R

• The monad morphism between R and S

liftReadPart :: R a -> S a
liftReadPart = gets . runReader

• Reference read

readRef :: Ref a -> R a
readRef = reader . getL

• Reference write

writeRef :: Ref a -> a -> S ()
writeRef = modify . setL r

• Lens application on a reference

lensMap :: Lens a b -> Ref a -> Ref b
lensMap = (.)

• Reference join

joinRef :: R (Ref a) -> Ref a
joinRef = Lens . join . (runLens .) . runReader

• The unit reference

unitRef :: Ref ()
unitRef = lens (const ()) (const id)

Note that the identity lens is not a reference because that would leak the program state s.

Reference creation

There should be a possibility for new reference creation too.

New reference creation with a given initial value extends the state. For example, if the state is (1, 'c') :: (Int, Char), extending the state with True :: Bool would yield the state ((1, 'c'), True) :: ((Int, Char), Bool).

We could model the type change of the state with an indexed monad, but that would complicate both the API and the implementation too.

Instead of changing the type of the state, we use an extensible state, a data type S with the operations

empty :: S

extend :: a -> State S (Lens S a)

such that the following laws hold:

• extend v >> return () === return ()

• extend v >>= liftReadPart . readRef === return v

The first law sais that extend has no side-effect, i.e. extending the state does not change the values of existing references. The second law sais that extending the state with value v creates a reference with inital value v.

Is there a state with a well-behaved extend function?

The answer is yes, but we should guarantee linear usage of the state. This is great, because linear usage is guaranteed with the restricted API!

How can such an S be implemented? And how can such an S be implemented efficiently? These questions are not relevant for the semantics, but there is an efficient implementation with IORefs.

Reference creation API

Let S be an extensible state as specified before. Let s = S in the definition of references.

• C :: (* -> *) -> * -> * ~ StateT S, the reference creating monad is the state monad transformer over S.

The equality constraint is not exposed in the API. The following functions are exposed:

• New reference creation

newRef :: Monad m => a -> C m (Ref a)
newRef v = mapStateT (return . runIdentity) $ extend v

• Lift reference read and write operations

liftWrite :: Monad m => S a -> C m a
liftWrite = mapStateT (return . runIdentity)

• Derived MonadTrans instance of C to be able to lift operations in the m monad.

Note that C is parameterized by a monad because in this way it is easier to add other effects later.

Running

The last missing function in the API is the running of the C m monad.

• Run the reference creation monad

runC :: Monad m => C m a -> m a
runC x = runStateT x empty

Lens-trees

TODO

Effects

TODO

Examples

Hello World

main = runWidget $ label $ return "Hello World!"

return is neded because labels may be dynamic, see the next examples.

Copy

The following applications presents an entry and a label below of it. When a text is entered in the entry, the label is changed to the entered text.

main = runWidget $ action $ do
   r <- newRef "enter text here"
   return $ vcat
       [ entry r
       , label $ readRef r
       ]

action gives acces to a monad in which new references can be made by newRef. A crutial feature of LGtk is that you cannot change the value of references in this monad (you can read them though).

label gives acces to a monad in which you can read references but no reference creation or write is possible.

Addition

Two entries of integers and a label which shows the sum:

main = runWidget $ action $ do
   r1 <- newRef (12 :: Integer)
   r2 <- newRef 4
   return $ vcat
       [ entryShow r1
       , entryShow r2
       , label $ liftM show $ liftM2 (+) (readRef r1) (readRef r2)
       ]

Complex examples

You can find more complex examples in the source code of LGtk. More examples will be presented here also.

Status

Features of lgtk-0.5

• The API is closed, you can safely use any constructs as far as you obey the documented laws.
• Support for asynchronous events. Using LGtk is a safe way for writing multithreaded Gtk applications.
• The semantics is getting stable but it is not yet documented.

TODO list:

• Add an efficient implementation for LGtk. LGtk has only a reference implementation currently.
• Add support for styles (layout, colors, etc).
• Support more Gtk constructs.

Demo application

You can try the demo application with the following commands:

cabal install gtk2hs-buildtools
cabal install lgtk
lgtkdemo

Changelog

lgtk-0.5.1

• Documentation fixes and cleanup
• Try to support Haskell Platform 2012.4.0.0

lgtk-0.5

• Do not use monadic lenses any more.
• Support for asynchronous events.
• Lazily created tabs.
• Unactive tabs are really unactive (event handlers are detached).
• File references watch the files. When the file changes, the GUI is updated.
• References with inherent identity (makes defining auto-sensitive buttons easier)
• Experimental support for colored buttons.
• More examples in the demo application.
• Lots of inner changes.

External links

• API documentation at HackageDB
• Darcs repo on hub.darcs.net
• Initial announcement

Related Stackoverflow questions:

• What is this special functor structure called?
• Howto abort getChar safely?

Reddit comments:

• 15 April
• 17 April
• 27 May