HotDrink is a declarative, constraint-based UI system that models the flow of data across the elements in the UI[1]. Trivially, it can repurposed for layout; although, most UI designers balk at the loss of control. The basic idea comes from a sizing dialog:

    height: [   ]

    width : [   ]

     area : [   ]

If the user enters a height & width, the form should update area [2]. If the user then updates area, then height is the free constraint (last recently updated). The whole system scales to work with incredibly large interfaces [3]. The experience section in [4] describes Adobe's experience porting chunks of Photoshop to using this system. (I did the figures for [4]; I'm quite proud of them.)

[1] https://code.google.com/p/hotdrink/

[2] http://dl.acm.org/citation.cfm?id=2371413

[3] http://dl.acm.org/citation.cfm?id=2047892&CFID=690849385&CFT...; http://dl.acm.org/citation.cfm?id=1449927&CFID=690849385&CFT...; http://dl.acm.org/citation.cfm?id=1621630&CFID=690849385&CFT...

[4] https://parasol.tamu.edu/~jarvi/papers/gpce08.pdf