How do you define relationships between data types?

Well, why is it any different from other fields? From one of your examples [1], I'd expect you to have a list of questions in the Quiz data type, and if necessary, a quiz field in the Question data type. This might be a bit tricky but certainly achievable [2].

Something like the following:

data Quiz = Quiz {


  description :: String,


  subject     :: String,


  questions   :: [Question]


} deriving (Show, Read)






data Question = Question {

  title   :: String,

  choiceA :: String,

  choiceB :: String,

  choiceC :: String,

  quiz    :: Quiz

} deriving (Show, Read)

[1] http://github.com/chriseidhof/persist/blob/master/examples/Model.phs
[2] http://www.haskell.org/haskellwiki/Tying_the_Knot 

On 28 September 2010 16:13, Chris Eidhof <chris@eidhof.nl> wrote:
Hey Jonathan,

I've done some work on this. The hard part is defining relationships between datatypes: how do you model this in Haskell? I've some code on github: http://github.com/chriseidhof/persist, you might be interested in that.

-chris

On 25 sep 2010, at 21:31, Jonathan Geddes wrote:

> Cafe,
>
> HaskellDB takes a database schema and produces Haskell data structures
> (plus some other query-related stuff for its EDSL query language).
>
> What I'm looking for is the inverse of this functionality. I want to
> create tables based on a Haskell data structure with a few simple
> rules. These rules include: if a field is not of the form `Maybe a'
> then it can't be nullable in the database. If a field is not a
> primitive (in the database) then it is actually stored in another
> table and a reference id is stored in the table. Tables are produced
> recursively, unless they already exist, etc.
>
> The HaskellDB approach is great for interfacing with existing tables,
> but in my case I already have data structures and now I would like a
> quick way to create tables to persist them.
>
> Does such a thing exist? If not, would you find it useful? I may take
> this up as a side project if it does not already exist and others
> would find it useful.
>
> Thanks,
>
> --Jonathan
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe@haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe

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Ozgur Akgun