Fwd: Two Proposals
George Giorgidze
giorgidze at gmail.com
Tue Oct 11 12:05:30 CEST 2011
For some reasons Philip's email was rejected by the mailing list.
I am reposting his message. See below.
Cheers, George
Begin forwarded message:
> From: Philip Wadler <wadler at inf.ed.ac.uk>
> Date: 2011-October-11 11:48:31 GMT+02:00
> To: Simon Peyton-Jones <simonpj at microsoft.com>, George Giorgidze <george.giorgidze at uni-tuebingen.de>
> Subject: Fwd: Two Proposals
>
> FYI. -- P
>
>
> ---------- Forwarded message ----------
> From: <glasgow-haskell-users-owner at haskell.org>
> Date: Tue, Oct 11, 2011 at 9:28 AM
> Subject: Re: Two Proposals
> To: wadler at inf.ed.ac.uk
>
>
> You are not allowed to post to this mailing list, and your message has
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>
> ---------- Forwarded message ----------
> From: Philip Wadler <wadler at inf.ed.ac.uk>
> To: George Giorgidze <giorgidze at gmail.com>
> Date: Tue, 11 Oct 2011 09:28:05 +0100
> Subject: Re: Two Proposals
> George, Thanks very much for this. I like your suggestion, which
> fits the logical structure perfectly; and you've suggested a neat way
> around the ugliness of 'group groupBy'. I also note that if we aren't
> so worried about not introducing new keywords, that 'then group' could
> become 'group'. Yours, -- P
>
> On Mon, Oct 10, 2011 at 11:21 PM, George Giorgidze <giorgidze at gmail.com> wrote:
>> A quick thought that came to me after hoogling [a] -> [[a]].
>> The first four functions in the search result are named after what they
>> return (noun in plural) rather than what they do (verb). I am talking about
>> inits, permutations, subsequence and tails.
>> So I thought the following syntax might work as well if (as it is already
>> common) grouping functions are named after what they return.
>> then f
>> then f by e
>> then group f
>> then group f by e
>> For example the following code fragments read well:
>> then group inits
>> then group permutations
>> then group subsequences
>> then group tails
>> Here we use the special identifier group as a verb.
>> I have not told you about the fifth result of the hoogling, the groupWith
>> function. The following really looks ugly:
>> then group groupWith by e
>> But following the aforementioned naming convention the groupWith function
>> could as well be named as equals. Now this reads well:
>> then group equals by e
>> Cheers, George
>>
>> On 2011-Oct-5, at 09:14 , Simon Peyton-Jones wrote:
>>
>> [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
>> | > _______________________________________________
>> | > Glasgow-haskell-users mailing list
>> | > Glasgow-haskell-users at haskell.org
>> | > 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.
>> |
>>
>>
>>
>
>
>
> --
> .\ 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.
>
>
>
>
>
>
> --
> .\ Philip Wadler, Professor of Theoretical Computer Science
> ./\ School of Informatics, University of Edinburgh
> / \ http://homepages.inf.ed.ac.uk/wadler/
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