Bringing back Monad Comprehensions (in style)

[Don Stewart]

You can also buy the tshirt for this ticket :-)

    http://haskell.spreadshirt.com/bring-back-monad-comprehensions-A6499530

simonpj:
> Good idea.  I've made a new Trac ticket and responded there.  I suggest that others do the same, so the conversation is captured in the ticket.
> http://hackage.haskell.org/trac/ghc/ticket/4370
> You can add yourself to the cc list of the ticket to stay in the loop.
> 
> Of course, do use the mailing list too for clarifying discussion, and then dump the conclusion in the ticket.
> 
> Simon
> 
> | -----Original Message-----
> | From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-
> | users-bounces at haskell.org] On Behalf Of George Giorgidze
> | Sent: 05 October 2010 15:42
> | To: glasgow-haskell-users at haskell.org
> | Subject: Bringing back Monad Comprehensions (in style)
> | 
> | Bringing back Monad Comprehensions (in style).
> | 
> | Dear GHC users,
> | 
> | My colleagues and I are working on Haskell embedded DSL for data-intensive
> | and
> | data-parallel applications [1]. The idea is to provide the Haskell list
> | prelude combinators to manipulate database-resident data. The combinators are
> | not executed in Haskell runtime, instead they are compiled down to SQL,
> | executed on relational database systems and the results are marshalled back
> | to
> | Haskell for further in-heap processing or generation of new database-able
> | embedded programs.
> | 
> | Although programming with the standard list processing combinators is
> | feasible, the embedded programs are much more concisely formulated using the
> | list comprehension notation, especially, when extended with 'order by' and
> | 'group by' constructs [2].
> | 
> | Unfortunately, in Haskell, the list comprehension notation is only available
> | for processing lists.
> | 
> | In order to support the list comprehension notation, we have built a
> | quasiquter that desugars the list comprehension notation, but, instead of
> | generating code using the Haskell list prelude combinators the quasiquter
> | generates code that uses list processing combinators from our embedded
> | language.
> | 
> | Although the quasiquoting approach worked for us, it has a number of
> | drawbacks:
> | 
> |   * Introduces extra syntactic noise
> | 
> |   * Error messages are hard to understand as they refer to generated code
> | 
> |   * Needs to be re-implemented for every list-based embedded language
> | 
> | One way to address the aforementioned drawbacks is to define our queries as a
> | monad (similar to list monad) and use the monad comprehension notation. The
> | do
> | notation can be used but it is less suited for query languages.
> | 
> | Unfortunately monad comprehensions were removed from Haskell, prior to
> | Haskell
> | 98. However, I think that the notation is extremely useful not only for
> | lists,
> | but for other list like data structures, list-based query languages (see
> | above), maybe even for wider range of EDSLs and monads. I think the feature
> | deserves to be supported at least as a GHC language extension.
> | 
> | Thus, I would like to propose to design and implement the monad comprehension
> | notation as a GHC language extension. I am willing to invest some time and
> | contribute to this effort.
> | 
> | One can also look at how recently introduced 'order by' and 'group by'
> | constructs generalise to monad comprehensions. If that works, one could
> | implement even more "stylish" monad comprehension notation.
> | 
> | Feedback from GHC users and developers would be very much appreciated.
> | 
> |   * Do you think that this is a good idea?
> | 
> |   * Would you use monad comprehensions (if available) for your
> |     library/EDSL/application?
> | 
> |   * Do you think that it would be hard to integrate this extension into
> |     current GHC codebase?
> | 
> |   * Have you already thought about how to generalise 'order by' and 'group
> |     by' to monad comprehensions?
> | 
> |   * Have you already thought about how to address the original objections to
> |     the monad comprehension notation?
> | 
> | Cheers, George
> | 
> | [1] http://www-db.informatik.uni-
> | tuebingen.de/files/weijers/IFL2010complete.pdf
> | 
> | [2] http://research.microsoft.com/en-us/um/people/simonpj/papers/list-comp/
> | 
> | 
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