Simon Marlow
On 06/10/2010 00:26, Max Bolingbroke wrote:
On 5 October 2010 15:41, George Giorgidze
wrote: 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.
They do: see the comments by MichaelAdams at http://haskell.org/haskellwiki/Simonpj/Talk:ListComp. Last I checked, the code there was slightly buggy but correct in spirit.
What *doesn't* generalise is the zip comprehensions extension:
[(x, y) | x<- xs | y<- ys] == zip xs ys
I wonder how much that extension is actually used, e.g. in all of Hackage? Could we deprecate it?
Cheers, Simon
I personally, do not have strong feelings about the zip comprehension notation. Unfortunately, I do not now how widely is this feature used by the Haskell community. I have just checked that in GHC 6.12 -XTransformListComp does not imply -XParallelListComp. The zip comprehension notation is supported as a separate language extension. As a start, one could implement -XMonadComp that supports standard list comprehensions (i.e., generators, filters and let binding) and subsequently extended it with 'order by' and 'group by'. This should only require GHC extension and does not need addition of new type classes/subclasses to the standard library. Monad and MonadPlus (for filtering) should be enough. Afterwards, if -XParallelListComp remains as a supported extension of GHC, one can identify a suitable (sub) type class for that and ensure that -XParallelListComp plays nicely with -XMonadComp. Cheers, George