From: Rocky Bernstein <rockyb@...>
Date: 2011-06-06T07:04:53+09:00
Subject: [ruby-core:36772] Re: [Ruby 1.9 - Feature #4824] Provide method Kernel#executed?

--0015174ff252fec85204a4fe2c74
Content-Type: text/plain; charset=ISO-8859-1

On Sun, Jun 5, 2011 at 5:33 PM, Clifford Heath <clifford.heath@gmail.com>wrote:

> On 05/06/2011, at 11:29 PM, Benoit Daloze wrote:
>
>> Clifford Heath wrote:
>>
>>> That is, the problem is that it's not obvious to a newcomer that __FILE__
>>> means
>>> the current source-code file, or that $0 means the name of the script
>>> being executed. Especially the latter...
>>>
>>
>> $PROGRAM_NAME is an alias for $0.
>> But "if __FILE__ == $PROGRAM_NAME" is quite long.
>>
>
> Length is not a problem, if the text includes the meaning.
>
> Things that are used often should be succinct, and the reader's
> knowledge should be assumed. Things that are used only a
> few times in a program do not need to be succinct.
>
> To use an API call requires that the user knows (or looks up)
> the meaning. This kind of semantic hiding is completely
> unnecessary and *counter-productive*.
>

In my opinion, the reason for adding  executed? or something like this is
because __FILE__ == $PROGRAM_NAME does not reliably test whether a program
has been invoked directly from Ruby or is required/loaded.

The sources of variability (and thus unreliability) stem from *both* sides
of the "==".  First, __FILE__ can be a relative file name. Consider this
code in two files /tmp/foolme.rb and /tmp/foolme/foolme.rb:

# File /tmp/foolme.rb
puts "++1 #{__FILE__}"
Dir.chdir('/tmp/foolme') { require './foolme' }

# File /tmp/foolme/foolme.rb
puts "++2 #{__FILE__}"

If you run this on MRI 1.8 or Rubinius 1.8 (e.g. 1.2.3) you will get this
output:

++1 ./foolme.rb
++2 ./foolme.rb

On MRI 1.9 the output is different.

On the other side of the ==, you have the problem that $0 or $PROGRAM_NAME
can be set.

Thus, if it is useful to be able to tell if a file has been invoked
initially as main script, then it would be nice have a reliable and way to
do so.

I think it useful to distinguish the two in order to more reliably support
the style of programming I use.  I generally put little demo code, at the
bottom of each class or module. You can think of this demo code as something
like unit test code; often it is modified and becomes unit test code. But
demo code and test code have a slightly different focus, since demoing and
testing are different.



> Clifford Heath.
>
>
>

--0015174ff252fec85204a4fe2c74
Content-Type: text/html; charset=ISO-8859-1
Content-Transfer-Encoding: quoted-printable

On Sun, Jun 5, 2011 at 5:33 PM, Clifford Heath <span dir=3D"ltr">&lt;<a hre=
f=3D"mailto:clifford.heath@gmail.com">clifford.heath@gmail.com</a>&gt;</spa=
n> wrote:<br><div class=3D"gmail_quote"><blockquote class=3D"gmail_quote" s=
tyle=3D"margin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;">
<div class=3D"im">On 05/06/2011, at 11:29 PM, Benoit Daloze wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex">
Clifford Heath wrote:<br>
<blockquote class=3D"gmail_quote" style=3D"margin:0 0 0 .8ex;border-left:1p=
x #ccc solid;padding-left:1ex">
That is, the problem is that it&#39;s not obvious to a newcomer that __FILE=
__ means<br>
the current source-code file, or that $0 means the name of the script<br>
being executed. Especially the latter...<br>
</blockquote>
<br>
$PROGRAM_NAME is an alias for $0.<br>
But &quot;if __FILE__ =3D=3D $PROGRAM_NAME&quot; is quite long.<br>
</blockquote>
<br></div>
Length is not a problem, if the text includes the meaning.<br>
<br>
Things that are used often should be succinct, and the reader&#39;s<br>
knowledge should be assumed. Things that are used only a<br>
few times in a program do not need to be succinct.<br>
<br>
To use an API call requires that the user knows (or looks up)<br>
the meaning. This kind of semantic hiding is completely<br>
unnecessary and *counter-productive*.<br></blockquote><div><br></div><div>I=
n my opinion, the reason for adding =A0executed? or something like this is =
because __FILE__ =3D=3D $PROGRAM_NAME=A0does not reliably test whether a pr=
ogram has been invoked directly from Ruby or is required/loaded.</div>
<div><br></div><div>The sources of=A0variability (and thus unreliability) s=
tem from *both* sides of the &quot;=3D=3D&quot;. =A0First, __FILE__ can be =
a relative file name. Consider this code in two files /tmp/foolme.rb and /t=
mp/foolme/foolme.rb:=A0</div>
<div><br></div><div># File /tmp/foolme.rb</div><div><div>puts &quot;++1 #{_=
_FILE__}&quot;=A0</div><div>Dir.chdir(&#39;/tmp/foolme&#39;) { require &#39=
;./foolme&#39; }</div></div><div><br></div><div># File /tmp/foolme/foolme.r=
b</div>
<div><div>puts &quot;++2 #{__FILE__}&quot;</div></div><div><br></div><div>I=
f you run this on MRI 1.8 or Rubinius 1.8 (e.g. 1.2.3) you will get this ou=
tput:</div><div><br></div><div><div>++1 ./foolme.rb</div><div>++2 ./foolme.=
rb</div>
</div><div><br></div><div>On MRI 1.9 the output is different.=A0</div><div>=
<br></div><div>On the other side of the =3D=3D, you have the problem that $=
0 or $PROGRAM_NAME can be set.</div><div><br></div><div>Thus, if it is usef=
ul to be able to tell if a file has been invoked initially as main script, =
then it would be nice have a reliable and way to do so.=A0</div>
<div><br></div><div>I think it useful to distinguish the two=A0in order to =
more reliably support the style of programming I use. =A0I generally put li=
ttle demo code, at the bottom of each class or module. You can think of thi=
s demo code as something like unit test code; often it is modified and beco=
mes unit test code. But demo code and test code have a slightly different f=
ocus, since demoing and testing are different.=A0</div>
<div><br></div><div><br></div><blockquote class=3D"gmail_quote" style=3D"ma=
rgin:0 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex;"><font color=
=3D"#888888">
<br>
Clifford Heath.<br>
<br>
<br>
</font></blockquote></div><br>

--0015174ff252fec85204a4fe2c74--