You have implemented very general idea as a part of your library (Data type a la carte). Maybe it's better to make a separate package? Other developers would benefit from it. I saw some other packages and they implement it too. Anton 2012/7/1 Liam O'Connor

I know it's somewhat evil (:P), but I was unable to implement the subsumption constraints (:<:) in a way that avoided it. It's there because I'm (ab)using the type-class system to do computation (specifically an O(n) search though long chain of coproducts for a specific constructor). E.g, determining Foo :<: Bar :+: Foo :+: Baz. There may be a way to rearrange the type level stuff to make it go away, perhaps with some new type extensions in 7.4 that I haven't studied extensively yet.

This is also a problem in Swierstra's original paper where he introduces this technique. In practice, it doesn't pose a problem, because you're not meant to define your own instances of :<: anyway, and you shouldn't care what order the constructors occur.

Regards, Liam O'Connor

On Sunday, 1 July 2012 at 6:37 PM, Krzysztof Skrzętnicki wrote:

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Hello,

This library looks very nice. Thank you for realeasing it! I realise it took quite an effort to write it.

Looking from examples on Github I noticed that you use -XOverlappingInstances. Why do you need such extension?

Best regards, Krzysztof Skrzętnicki

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Hi all,

wizards is an Haskell library designed for the quick and painless development of interrogative programs, which revolve around a "dialogue" with the user, who is asked a series of questions in a sequence much like an installation wizard.

Everything from interactive system scripts, to installation wizards, to full-blown shells can be implemented with the support of wizards.

It is developed transparently on top of a free monad (see Swierstra's excellent paper on this topic at http://www.cs.ru.nl/~W.Swierstra/Publications/DataTypesALaCarte.pdf ( http://www.cs.ru.nl/%7EW.Swierstra/Publications/DataTypesALaCarte.pdf)http://www.cs.ru.nl/%7EW.Swierstra/Publications/DataTypesALaCarte.pdf%29), which separates out the semantics of the program from the wizards interface. A variety of backends exist, including a full featured backend for Haskeline, a debug-friendly simpler implementation in terms of System.IO primitives, and a completely pure implementation modelled as a function from an input string to output. It is also possible to write your own backends, or extend the existing back-ends with new features.

While both built-in IO backends operate on a console, there is no reason why wizards cannot also be used for making GUI wizard interfaces.

The library is highly extensible - back-ends can be written or extended with the type system helpfully tracking what features are supported by which back-ends.

Installation instructions and some educational examples are at the github page:

https://github.com/liamoc/wizards

Information on how to write backends or extend backends, as well as structured API documentation is available on Hackage:

http://hackage.haskell.org/package/wizards

(Or, you can just run cabal haddock to generate the documentation from

On Sun, Jul 1, 2012 at 10:17 AM, Liam O'Connor wrote: the source).

...

Regards, Liam O'Connor

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