Permitting trailing commas for record syntax ADT declarations

[Andreas Abel]

I agree with Edward that TupleSections are too useful to trade for more 
convenience to write large tuples on the *expression* level.

(If you habitually write write large tuple expressions instead of using 
records, you should revisit your coding style, seriously!)

However, I see no problem to allow extra commas in *administrative* 
tuples like in import/export lists.  Thus, I see no problem if these two 
kinds of tuples adhere to different syntax rules.

In previous discussions, there was a caveat raised that extra commata in 
lists and tuples mean something else.

   [,a,b,]  is [a,b]
   (,a,b,)  is a tuple section

However, I never felt like having list sections would see abundant 
usage.  Thus, this discrepancy is ok, imho.

Cheers,
Andreas

On 24.09.2014 08:50, Edward Kmett wrote:
> I'm personally of the "it should be a language extension like everything
> else" mindset. Sure, it is a pain, but then so is working with
> EmptyCase, TupleSections, DoRec, EmptyDataDecls, ImplicitParams,
> KindSignatures, MultiWayIf, TypeOperators, UnicodeSyntax, etc. All of
> which just make a few more programs compile in the same sense, and fit
> into the gaps in the existing grammar.
>
> The conflict with TupleSections came up the last time this was proposed.
>
> If we limit it to record-like notions, and import/export lists, then we
> don't have to deal with conflicts with TupleSections and while it is
> inconsistent to have tuples behave differently, than other
> comma-separated lists, I'd really rather retain tuple sections, which I
> use somewhat heavily, than lose them to mindless uniformity over how we
> handle comma-separated lists.
>
> I use TupleSections a fair bit, whereas I've adopted prefixed
> comma-lists in Haskell to avoid the need for an extension like this one,
> so it'd be quite a shift in my programming style to get to where this
> helps me. The one place I'd really want something like this proposal is
> for the first line of my export list, where adopting the prefixed ','
> convention + haddock sections makes for an annoying visual asymmetry.
>
> -Edward
>
> On Tue, Sep 23, 2014 at 4:55 AM, Simon Peyton Jones
> <simonpj at microsoft.com <mailto:simonpj at microsoft.com>> wrote:
>
>     PS I have to admit that GHC already fails to adhere to H-2010 by
>     accepting trailing commas in module import lists, *without* a
>     language extension.  That is naughty, but I suppose it is a foot in
>     the door.   What to others think?
>
>     Incidentally, trailing commas in tuples have quite a different
>     meaning. With TupleSections,  (a,,b,) means \x y -> (a,x,b,y).
>
>     Simon
>
>     | -----Original Message-----
>     | From: Simon Peyton Jones
>     | Sent: 23 September 2014 08:32
>     | To: 'Alexander Berntsen'; Johan Tibell
>     | Cc: ghc-devs at haskell.org <mailto:ghc-devs at haskell.org>
>     | Subject: RE: Permitting trailing commas for record syntax ADT
>     | declarations
>     |
>     | | > have a language extension TrailingCommas (or something) to enable
>     | | > the extension
>     | | For clarification: are you overruling the "do we sneak it in HEAD or
>     | | use pragma(s)"-vote and telling me to do the latter?
>     |
>     | Well, it *is* a language extension, exactly like lots of other
>     language
>     | extensions, isn't it? (E.g. UnicodeSyntax.)  What alternative action,
>     | exactly, are you proposing?   Why do you propose to treat it
>     differently
>     | to other language extensions?  I would be reluctant to simply add it
>     | without any indication; then GHC would accept non-Haskell 2010
>     programs.
>     | And we have not done that with other extensions.
>     |
>     | Simon
>     |
>     | |
>     | | If we can sneak it into HEAD (this is @ you Johan, too), I suggest
>     | | that someone applies my patches to make import and export lists
>     | | support leading commas (presently they only support trailing commas,
>     | | per the report) -- and following this I can just send a bunch of
>     | | "Permit leading/trailing ',' in Foo" patches to Phabricator, and you
>     | | guys can bikeshed over there about which ones you actually want to
>     | | commit. ;-)
>     | |
>     | | If I am to go down the pragma route, I guess I can make a
>     | | RudundantCommas pragma or something like that, that implements
>     | | trailing commas for imports/exports, and leading/trailing commas for
>     | | the suggestions in this thread.
>     | |
>     | | I'm +1 on the GHC HEAD route, but I'm not exactly violently
>     opposed to
>     | | the pragma route either.
>     | | - --
>     | | Alexander
>     | | alexander at plaimi.net <mailto:alexander at plaimi.net>
>     | | https://secure.plaimi.net/~alexander
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-- 
Andreas Abel  <><      Du bist der geliebte Mensch.

Department of Computer Science and Engineering
Chalmers and Gothenburg University, Sweden

andreas.abel at gu.se
http://www2.tcs.ifi.lmu.de/~abel/