[Haskell-cafe] Hierachical abstraction

[Sebastian Fischer]

On Jan 28, 2010, at 9:31 PM, Luke Palmer wrote:

> I don't remember the name, but there is a technique where you compose
> the features you want and then take its fixed point to get your AST.

Did you think of "Data types à la carte" by Wouter Swierstra?

     http://www.cs.nott.ac.uk/~wss/Publications/DataTypesALaCarte.pdf

Also see the comments on Phil Wadler's blog:

     http://wadler.blogspot.com/2008/02/data-types-la-carte.html

> You can make a typeclass for each feature that uses it on any data
> structure in which it is present.

I don't fully understand this sentence but it reminds me of "Finally  
Tagless, Partially Evaluated" by Jacques Carette, Oleg Kiselyov and  
Chung-chieh Shan:

     http://www.cs.rutgers.edu/~ccshan/tagless/jfp.pdf

Regarding the question of representing in the type system which  
features are used, phantom types might also be helpful. If I remember  
correctly, they were introduced by Daan Leijen and Erik Meijer to  
restrict a data type that represents syntactically
correct database queries such that only type correct queries can be  
made:

     http://research.microsoft.com/en-us/um/people/emeijer/Papers/HaskellDB.pdf

Sebastian


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Underestimating the novelty of the future is a time-honored tradition.
(D.G.)