Re: We need to add role annotations for 7.8

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On Mon, Mar 24, 2014 at 10:28 AM, Brandon Allbery <[email protected]>wrote:

> On Mon, Mar 24, 2014 at 10:14 AM, Mark Lentczner <[email protected]
> > wrote:
>
>> On Fri, Mar 14, 2014 at 2:36 AM, Johan Tibell <[email protected]>wrote:
>>
>>> I'm quite worried about this change though. I thought the default rolefor data type was nominal, as that's the safe default. *If
>>> the default is representational, every package author will need to be aware
>>> of this feature and update their packages.* That's quite a high cost.
>>>
>>
>> On Fri, Mar 14, 2014 at 6:00 AM, Brandon Allbery <[email protected]>
>>  wrote:
>>
>> *Nominal default breaks everything that uses newtype deriving and doesn't
>>> have role annotations, doesn't it?* Representational default means
>>> things behave in 7.8 as they did in earlier GHCs.
>>>
>>
>> Am I reading these pair of statements correctly? It seems to imply to me
>> that *every* parameterized type that uses a type constraint on a
>> parameter *must* be reviewed and possibly annotated to work correctly,
>> one way or the other!
>>
>
> No; if the default is representational, everything works as it did in
> earlier versions, potential bugs/unsafety and all. If the default is
> immediately switched to nominal, *then* every affected type must be
> reviewed immediately. My counter-proposal was to have 7.8 default to
> representational to give library maintainers a release cycle to add the
> necessary annotations, then switch the default to nominal in 7.10 to get
> the additional safety.
>

By the way, since I suspected (and was subsequently proved out) that
appropriate warnings are difficult, perhaps there should be some mechanism
to specify to ghc 7.8 whether a compile should default to representational
or nominal so that authors have a way to test their code / look for
problems.

-- 
brandon s allbery kf8nh                               sine nomine associates
[email protected]                                  [email protected]
unix, openafs, kerberos, infrastructure, xmonad        http://sinenomine.net

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<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote">On M=
on, Mar 24, 2014 at 10:28 AM, Brandon Allbery <span dir=3D"ltr">&lt;<a href=
=3D"mailto:[email protected]" target=3D"_blank">[email protected]</a>&g=
t;</span> wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><div dir=3D"ltr"><div class=3D"gmail_extra">=
<div class=3D"gmail_quote"><div class=3D"">On Mon, Mar 24, 2014 at 10:14 AM=
, Mark Lentczner <span dir=3D"ltr">&lt;<a href=3D"mailto:mark.lentczner@gma=
il.com" target=3D"_blank">[email protected]</a>&gt;</span> wrote:<br=
>

<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex"><div dir=3D"ltr"><div class=3D"gmail_extra">=
On Fri, Mar 14, 2014 at 2:36 AM, Johan Tibell <span dir=3D"ltr">&lt;<a href=
=3D"mailto:[email protected]" target=3D"_blank">[email protected]=
</a>&gt;</span> wrote:<br>

</div><div class=3D"gmail_extra"><div class=3D"gmail_quote"><div><blockquot=
e class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left-width=
:1px;border-left-color:rgb(204,204,204);border-left-style:solid;padding-lef=
t:1ex">



<div dir=3D"ltr"><div>I&#39;m quite worried about this change though. I tho=
ught the default <span>role</span> for data type was nominal, as that&#39;s=
 the safe default. <b>If the default is representational, every package aut=
hor will <span>need</span> to be aware of this feature and update their pac=
kages.</b> That&#39;s quite a high cost.</div>





</div><div></div></blockquote></div></div><br>On Fri, Mar 14, 2014 at 6:00 =
AM, Brandon Allbery=C2=A0<span dir=3D"ltr">&lt;<a href=3D"mailto:allbery.b@=
gmail.com" target=3D"_blank">[email protected]</a>&gt;</span>=C2=A0wrote:=
<div>
<br><blockquote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;bor=
der-left-width:1px;border-left-color:rgb(204,204,204);border-left-style:sol=
id;padding-left:1ex">

<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote"><div=
><b>Nominal default breaks everything that uses newtype deriving and doesn&=
#39;t have role annotations, doesn&#39;t it?</b> Representational default m=
eans things behave in 7.8 as they did in earlier GHCs.</div>



</div></div></div></blockquote><div><br></div></div><div>Am I reading these=
 pair of statements correctly? It seems to imply to me that <i>every</i> pa=
rameterized type that uses a type constraint on a parameter <i>must</i>=C2=
=A0be reviewed and possibly annotated to work correctly, one way or the oth=
er!</div>

</div></div></blockquote><div><br></div></div><div>No; if the default is re=
presentational, everything works as it did in earlier versions, potential b=
ugs/unsafety and all. If the default is immediately switched to nominal, *t=
hen* every affected type must be reviewed immediately. My counter-proposal =
was to have 7.8 default to representational to give library maintainers a r=
elease cycle to add the necessary annotations, then switch the default to n=
ominal in 7.10 to get the additional safety.</div>
</div></div></div></blockquote><div><br></div><div>By the way, since I susp=
ected (and was subsequently proved out) that appropriate warnings are diffi=
cult, perhaps there should be some mechanism to specify to ghc 7.8 whether =
a compile should default to representational or nominal so that authors hav=
e a way to test their code / look for problems.=C2=A0</div>
</div><div><br></div>-- <br><div dir=3D"ltr"><div>brandon s allbery kf8nh =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 sine nomine associates</div><div><a href=3D=
"mailto:[email protected]" target=3D"_blank">[email protected]</a> =C2=
=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0<a href=3D"mailto:ballbery@sinenom=
ine.net" target=3D"_blank">[email protected]</a></div>
<div>unix, openafs, kerberos, infrastructure, xmonad =C2=A0 =C2=A0 =C2=A0 =
=C2=A0<a href=3D"http://sinenomine.net" target=3D"_blank">http://sinenomine=
.net</a></div></div>
</div></div>

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