FW: Two Proposals

[Simon Peyton-Jones]

[adding ghc-users]

It's not easy, Phil.  Do you have any ideas?

For the 'then' case the name of the function serves as the verb.  One might say

	then take 4
or
	then takeWhile by salary > 40

For grouping one might like to say the same  thing, such as
	then groupBy by salary
but the typing rule is quite different, so we really need a different keyword.  We chose the compound keyword "then group" to avoid needing a whole new keyword ("group" is treated specially only in tthis context). So you write
	then group by salary using groupBy

Using this order of the pieces for the sorting case is harder. What would one say?  "then process"?  Like this?
	then process by salary > 40 using takeWhile
Not very nice.

One could use a new keyword for grouping "theng" say, thus:
	theng groupBy by salary
But that is hardly beautiful either.

So the current story is not great, but it's the best I could think of. Improvements welcome.

Simon

|  -----Original Message-----
|  From: Philip Wadler [mailto:wadler at inf.ed.ac.uk]
|  Sent: 04 October 2011 18:15
|  To: Simon Peyton-Jones; George Giorgidze
|  Subject: Re: FW: Two Proposals
|  
|  George,
|  
|  Nice proposal.  I like the idea of symmetry, but don't at all like the
|  idea that f comes before e for 'then' but f comes after e for 'then
|  group'.  Can you rethink it and come up with something even more
|  symmetric?
|  
|  Yours,  -- P
|  
|  
|  On Tue, Oct 4, 2011 at 9:23 AM, Simon Peyton-Jones
|  <simonpj at microsoft.com> wrote:
|  > FYI
|  >
|  > -----Original Message-----
|  > From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-
|  users-bounces at haskell.org] On Behalf Of George Giorgidze
|  > Sent: 30 September 2011 18:28
|  > To: glasgow-haskell-users at haskell.org
|  > Subject: Two Proposals
|  >
|  > GHC Users,
|  >
|  > I would like to make to the following two proposals:
|  >  * Eliminate the default grouping close from SQL-like comprehensions
|  >  * Introduce a GHC extension for list literal overloading
|  >
|  > OK, let me start with the first proposal.
|  >
|  > Currently, the SQL-like comprehension notation (both in its list comprehension
|  and monad comprehension variants) features the following five clauses:
|  >
|  > then f
|  > then f by e
|  > then group by e
|  > then group using f
|  > then group by e using f
|  >
|  > The first two clauses are used for specifying transformations of type [a] -> [a]
|  (or Monad m => m a-> m a for monad comprehensions). The following three
|  clauses are used for specifying transformations of type [a] -> [[a]] (or Monad m,
|  Functor f => m a -> m (f a) for monad comprehensions). See [1] for further
|  details.
|  >
|  > Note that the third clause does not mention which function is used for grouping.
|  In this case GHC.Exts.groupWith function is used as a default for list
|  comprehensions and the mgroupWith function from the MonadGroup class is used
|  as a default for monad comprehensions.
|  >
|  > I would like to suggest to remove the third clause for the following reasons:
|  > * Currently the syntax is asymmetrical. Note that there is the default case for
|  the 'then group' clause and not for the 'then' clause.
|  > * In the current notation it is not clear which grouping function is used in the
|  default case
|  > * For many monads including lists it is not clear which function should be
|  selected as a default (e.g., the groupWith function also does sorting and it is not
|  clear to me why this should be the default)
|  > * Gets rid of the MonadGroup class. Currently the sole purpose of this class is to
|  introduce a default grouping function for monad comprehensions.
|  > * Explicit mention of the grouping function would make  monad/list
|  comprehensions much easier to read by making it immediately apparent which
|  function is used for grouping.
|  >
|  > My second proposal is to introduce the OverloadedLists extension that overloads
|  list literals. See Section 5.2 in [1] for details.
|  >
|  > Basically the idea is to treat list literals like:
|  >
|  > [1,2,3]
|  >
|  > as
|  >
|  > fromList [1,2,3]
|  >
|  > where
|  >
|  > class IsList l where
|  >  type Item l
|  >  fromList :: [Item l] -> l
|  >
|  > In the following I give useful instances of the IsList class.
|  >
|  > instance IsList [a] where
|  >  type Item [a] = a
|  >  fromList = id
|  >
|  > instance (Ord a) => IsList (Set a) where
|  >  type Item (Set a) = a
|  >  fromList = Set.fromList
|  >
|  > instance (Ord k) => IsList (Map k v) where
|  >  type Item (Map k v) = (k,v)
|  >  fromList = Map.fromList
|  >
|  > instance IsList (IntMap v) where
|  >  type Item (IntMap v) = (Int,v)
|  >  fromList = IntMap.fromList
|  >
|  > instance IsList Text where
|  >  type Item Text = Char
|  >  fromList = Text.pack
|  >
|  > As you can see the extension would allow list literals to be used for sets, maps
|  and integer maps. In addition the suggested OverloadedLists extension would
|  subsume OverloadedStrings extension (see the instance for Text, for example).
|  Having said that, for now, I am not suggesting to remove the OverloadedStrings
|  extension as it appears to be widely used.
|  >
|  > This extension could also be used for giving data-parallel array literals instead of
|  the special syntax used currently.
|  >
|  > Unless there is a vocal opposition to the aforementioned two proposals, I would
|  like to implement them in GHC. Both changes appear to be straightforward to
|  implement.
|  >
|  > Thanks in advance for your feedback.
|  >
|  > Cheers, George
|  >
|  > [1] http://www-db.informatik.uni-tuebingen.de/files/giorgidze/haskell2011.pdf
|  > _______________________________________________
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|  > http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
|  >
|  >
|  >
|  
|  
|  
|  --
|  .\ Philip Wadler, Professor of Theoretical Computer Science
|  ./\ School of Informatics, University of Edinburgh
|  /  \ http://homepages.inf.ed.ac.uk/wadler/
|  
|  The University of Edinburgh is a charitable body, registered in
|  Scotland, with registration number SC005336.
|