Bringing back Monad Comprehensions (in style)

[George Giorgidze]

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/