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From: Corey Huinker <corey.huinker@gmail.com>
Date: Wed, 22 Feb 2017 18:15:01 -0500
Message-ID: 
 <CADkLM=dqO2G9yLtDB9PdJqyySau+EYFS1ypK5SSthya7yLEg2w@mail.gmail.com>
Subject: Re: \if, \elseif, \else, \endif (was Re: PSQL commands:
 \quit_if, \quit_unless)
To: Tom Lane <tgl@sss.pgh.pa.us>
Cc: Fabien COELHO <coelho@cri.ensmp.fr>, Greg Stark <stark@mit.edu>,
 Erik Rijkers <er@xs4all.nl>,
	Robert Haas <robertmhaas@gmail.com>, Daniel Verite <daniel@manitou-mail.org>,
	Jim Nasby <Jim.Nasby@bluetreble.com>,
 PostgreSQL <pgsql-hackers@postgresql.org>,
	pgsql-hackers-owner@postgresql.org
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On Wed, Feb 22, 2017 at 5:59 PM, Tom Lane <tgl@sss.pgh.pa.us> wrote:

> Ah, I see why *that* wants to know about it ... I think.  I suppose you're
> arguing that variable expansion shouldn't be able to insert, say, an \else
> in a non-active branch?  Maybe, but if it can insert an \else in an active
> branch, then why not non-active too?  Seems a bit inconsistent.
>

The major reason was avoiding situations like what Daniel showed: where
value of a variable that is meaningless/undefined in the current
false-block context gets expanded anyway, and thus code inside a false
block has effects outside of that block. Granted, his example was
contrived. I'm open to removing that feature and seeing what breaks in the
test cases.

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<div dir=3D"ltr"><div class=3D"gmail_extra"><div class=3D"gmail_quote">On W=
ed, Feb 22, 2017 at 5:59 PM, Tom Lane <span dir=3D"ltr">&lt;<a href=3D"mail=
to:tgl@sss.pgh.pa.us" target=3D"_blank">tgl@sss.pgh.pa.us</a>&gt;</span> wr=
ote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border=
-left:1px #ccc solid;padding-left:1ex"><div id=3D":2ue" class=3D"a3s aXjCH =
m15a680d90471eae5">Ah, I see why *that* wants to know about it ... I think.=
=C2=A0 I suppose you&#39;re<br>
arguing that variable expansion shouldn&#39;t be able to insert, say, an \e=
lse<br>
in a non-active branch?=C2=A0 Maybe, but if it can insert an \else in an ac=
tive<br>
branch, then why not non-active too?=C2=A0 Seems a bit inconsistent.</div><=
/blockquote></div><br>The major reason was avoiding situations like what Da=
niel showed: where value of a variable that is meaningless/undefined in the=
 current false-block context gets expanded anyway, and thus code inside a f=
alse block has effects outside of that block. Granted, his example was cont=
rived. I&#39;m open to removing that feature and seeing what breaks in the =
test cases.<br></div></div>

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