Field names starting Upper [Was: [haskell-cafe/ghc-proposals] Allow reserved identifiers as fields ...

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I said (
https://mail.haskell.org/pipermail/haskell-cafe/2024-December/136986.html)

On Wed, 25 Dec 2024 at 16:49, Anthony Clayden <[email protected]>
wrote:

> ...
>
> [**] I'm particularly keen on field names starting upper case, because
> they're injective/part of the structure, like data constructors.
>
>
>
And previously with Hugs (
https://mail.haskell.org/pipermail/hugs-users/2020-May/000927.html)


> * I've taught Trex to support field labels starting upper-case, because,
>
well for me they're more like constructors than variables.
>
>

So can I combine that work? Yes, providing I use Upper-case labels only
within `{ ... }`. These can combine lower with Upper field names within the
same data type; can pun on the lower.

>
>     data Upup a = MkUp{ down :: a, Up :: Int }   deriving (Eq, Show)
>
>     anUp  = MkUp{ down = 'a', Up = 5 }      -- build
>     recUp = anUp{ Up = 7 }              -- field update by Up label,
record syntax
>
>     succUp (MkUp {down, Up = u} ) = MkUp{ down = succ down, Up = succ u }
>

You can't use a field selection function: 'ERROR - "Up" is not a data
constructor'.
(It looks like Hugs does generate such a function; this error is a syntax
rejection in trying to use it.)

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<div dir=3D"ltr"><div>I said (<a href=3D"https://mail.haskell.org/pipermail=
/haskell-cafe/2024-December/136986.html">https://mail.haskell.org/pipermail=
/haskell-cafe/2024-December/136986.html</a>)</div><br><div class=3D"gmail_q=
uote gmail_quote_container"><div dir=3D"ltr" class=3D"gmail_attr">On Wed, 2=
5 Dec 2024 at 16:49, Anthony Clayden &lt;<a href=3D"mailto:anthony.d.clayde=
[email protected]">[email protected]</a>&gt; wrote:<br></div><blockquot=
e class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px s=
olid rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr"><div>...</div><div=
><br></div><div><div>[**] I&#39;m particularly keen on field names starting=
 upper case, because they&#39;re injective/part of the structure, like data=
 constructors.</div><div><br></div><div><br></div></div></div></blockquote>=
<div><br></div><div>And previously with Hugs (<a href=3D"https://mail.haske=
ll.org/pipermail/hugs-users/2020-May/000927.html">https://mail.haskell.org/=
pipermail/hugs-users/2020-May/000927.html</a>)</div><div><br></div><blockqu=
ote class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px=
 solid rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr"><div><div><br cl=
ass=3D"gmail-Apple-interchange-newline"><span style=3D"color:rgb(0,0,0);whi=
te-space:pre-wrap">* I&#39;ve taught Trex to support field labels starting =
upper-case, because,</span></div></div></div></blockquote><div><blockquote =
class=3D"gmail_quote" style=3D"margin:0px 0px 0px 0.8ex;border-left:1px sol=
id rgb(204,204,204);padding-left:1ex"><div dir=3D"ltr"><div><div><span styl=
e=3D"color:rgb(0,0,0);white-space:pre-wrap">  well for me they&#39;re more =
like constructors than variables.</span><br class=3D"gmail-Apple-interchang=
e-newline"><br></div></div></div></blockquote><div><br></div></div><div><br=
></div><div>So can I combine that work? Yes, providing I use Upper-case lab=
els only within `{ ... }`. These can combine lower with Upper field names w=
ithin the same data type; can pun on the lower.</div><div><br></div><div>&g=
t;</div><div>&gt;=C2=A0 =C2=A0 =C2=A0data Upup a =3D MkUp{ down :: a, Up ::=
 Int } =C2=A0 deriving (Eq, Show)</div><div>&gt;</div><div>&gt;=C2=A0 =C2=
=A0 =C2=A0anUp =C2=A0=3D MkUp{ down =3D &#39;a&#39;, Up =3D 5 }=C2=A0 =C2=
=A0 =C2=A0 -- build</div>&gt;=C2=A0 =C2=A0 =C2=A0recUp =3D anUp{ Up =3D 7 }=
 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0 =C2=A0-- field update by Up labe=
l, record syntax</div><div class=3D"gmail_quote gmail_quote_container">&gt;=
</div><div class=3D"gmail_quote gmail_quote_container">&gt;=C2=A0=C2=A0 =C2=
=A0 succUp (MkUp {down, Up =3D u} ) =3D MkUp{ down =3D succ down, Up =3D su=
cc u }<br><div>&gt;</div><div><br></div><div>You can&#39;t use a field sele=
ction function: &#39;ERROR - &quot;Up&quot; is not a data constructor&#39;.=
</div><div>(It looks like Hugs does generate such a function; this error is=
 a syntax rejection in trying to use it.)</div><div><br></div><div><br></di=
v></div></div>

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