From: Peter Vandenabeele <peter@...>
Date: 2011-12-16T17:33:21+09:00
Subject: [ruby-core:41695] Re: Documentation of the language itself (syntax, meanings, etc)

--002354471080ae023c04b4317370
Content-Type: text/plain; charset=UTF-8

On Fri, Dec 16, 2011 at 6:17 AM, Yong Li <gilbertly@gmail.com> wrote:

> >
> > Still, I don't like using exceptions as a control flow mechanism for
> entirely un-exceptional situations.
> >
> I strongly agree with this, even though I have myself met a similar
> situation in the past:
>
> def action
>  check_input
>
>  return if handle_special_input1_and_return  # This is valid input
> and return logic, so exception is inappropriate
>
>  return if handle_special_input2_and_return
>
>  handle_common_cases
> end
>
> I did not want to inline "handle_special_inputx_and_return" because it
> would make the action method too long and blur the main logic.
> However, extracting them to private methods would make the code more
> verbose as I cannot return "action" from within
> "handle_special_input_and_return", and this forces me to return a
> value from "handle_special_input_and_return" and check them in
> "action".
>
> I was also trying hard to get around this, but finally settled on this
> for the following reasons:
> * The only way to break out multiple levels of code (nested methods or
> loops or both) is to use a "go-to" like operation, which allows you to
> jump to anywhere in you code. Exceptions can be used for this purpose.
> But again, this is very bad.
> * go-to operations are rarely good to use
> * the purpose and condition of return is clear and explicit, which
> reduces confusions for other people
>
> Maybe I could re-design my code to avoid such situations in the first
> place, but that would be a completely different story.
>
> I would be very glad to hear a more succinct solution and throw my
> silly argument under the bus :-D
>


Certainly a problem I was also bitten by this more than I wanted ...

The problem:

(meta-code)

def show
  user = User.find(params[:id])
  if user.secret?
    flash[:warning] = "sorry, this is secret"
    redirect_to :action => "index"
    return
  end
  if user.vip?
    redirect_to :action => "vip_lounge", :id => params[:id]
    # forgot return
  end
  if user.long_distance?
    redirect_to :action => "long_distance", id => params[:id]
    # line above will BOOM for vip, long_distance user
    return
  end
end

and there you have it ... I forgot 1 return ... A user that is vip? and
intercontinental? will cause an exception (and even if I tested each
separate use case, I may have forgotten to test this _combination_ ...).

Also, duplicating the "return" after each redirect_to or render
seems not DRY.

At one point, I was grepping my code for each "redirect_to",
"render" to see if I had written a return after it ...

A solution ?

Would this be a good use case for Catch / Throw ?

Then the Web Framework  call to User#show could be
wrapped in a catch block like

class WebFramework
...
  catch :render_or_redirect do
    User.show
  end

end

and then we defined a function

def redirect_to_with_throw(*arguments)
  redirect_to(*arguments)
  throw :render_or_redirect
end

which can be used in the controller as:

def show
  user = User.find(params[:id])
  if user.secret?
    flash[:warning] = "sorry, this is secret"
    redirect_to_with_throw :action => "index"
  end
...

That might be a nice extension to the web framework,
to catch a :render_or_redirect throw. If I understand
correctly, the catch in the framework would not
hinder the existing code or developers not using
the feature (that is, never throwing :render_or_redirect).

HTH,

Peter

--002354471080ae023c04b4317370
Content-Type: text/html; charset=UTF-8
Content-Transfer-Encoding: quoted-printable

<div class=3D"gmail_quote">On Fri, Dec 16, 2011 at 6:17 AM, Yong Li <span d=
ir=3D"ltr">&lt;<a href=3D"mailto:gilbertly@gmail.com">gilbertly@gmail.com</=
a>&gt;</span> wrote:<br><blockquote class=3D"gmail_quote" style=3D"margin:0=
 0 0 .8ex;border-left:1px #ccc solid;padding-left:1ex">
<div class=3D"im">&gt;<br>
&gt; Still, I don&#39;t like using exceptions as a control flow mechanism f=
or entirely un-exceptional situations.<br>
&gt;<br>
</div>I strongly agree with this, even though I have myself met a similar<b=
r>
situation in the past:<br>
<br>
def action<br>
 =C2=A0check_input<br>
<br>
 =C2=A0return if handle_special_input1_and_return =C2=A0# This is valid inp=
ut<br>
and return logic, so exception is inappropriate<br>
<br>
 =C2=A0return if handle_special_input2_and_return<br>
<br>
 =C2=A0handle_common_cases<br>
end<br>
<br>
I did not want to inline &quot;handle_special_inputx_and_return&quot; becau=
se it<br>
would make the action method too long and blur the main logic.<br>
However, extracting them to private methods would make the code more<br>
verbose as I cannot return &quot;action&quot; from within<br>
&quot;handle_special_input_and_return&quot;, and this forces me to return a=
<br>
value from &quot;handle_special_input_and_return&quot; and check them in<br=
>
&quot;action&quot;.<br>
<br>
I was also trying hard to get around this, but finally settled on this<br>
for the following reasons:<br>
* The only way to break out multiple levels of code (nested methods or<br>
loops or both) is to use a &quot;go-to&quot; like operation, which allows y=
ou to<br>
jump to anywhere in you code. Exceptions can be used for this purpose.<br>
But again, this is very bad.<br>
* go-to operations are rarely good to use<br>
* the purpose and condition of return is clear and explicit, which<br>
reduces confusions for other people<br>
<br>
Maybe I could re-design my code to avoid such situations in the first<br>
place, but that would be a completely different story.<br>
<br>
I would be very glad to hear a more succinct solution and throw my<br>
silly argument under the bus :-D<br></blockquote><div><br></div><div><br></=
div><div>Certainly a problem I was also bitten by this more than I wanted .=
..</div><div><br></div><div>The problem:</div><div><br></div><div>(meta-cod=
e)</div>
<div><br></div><div>def show</div><div>=C2=A0 user =3D User.find(params[:id=
])</div><div>=C2=A0 if user.secret?</div><div>=C2=A0 =C2=A0 flash[:warning]=
 =3D &quot;sorry, this is secret&quot;</div><div>=C2=A0 =C2=A0 redirect_to =
:action =3D&gt; &quot;index&quot;</div>
<div>=C2=A0 =C2=A0 return</div><div>=C2=A0 end</div><div>=C2=A0 if user.vip=
?</div><div>=C2=A0 =C2=A0 redirect_to :action =3D&gt; &quot;vip_lounge&quot=
;, :id =3D&gt; params[:id]</div><div>=C2=A0 =C2=A0 # forgot return</div><di=
v>=C2=A0 end</div><div>=C2=A0 if user.long_distance?</div>
<div>=C2=A0 =C2=A0 redirect_to :action =3D&gt; &quot;long_distance&quot;, i=
d =3D&gt; params[:id]</div><div>=C2=A0 =C2=A0 # line above will BOOM for vi=
p, long_distance user</div><div>=C2=A0 =C2=A0 return</div><div>=C2=A0 end</=
div><div>end</div><div><br></div><div>
and there you have it ... I forgot 1 return ... A user that is vip? and</di=
v><div>intercontinental? will cause an exception (and even if I tested each=
</div><div>separate use case, I may have forgotten=C2=A0to test this _combi=
nation_ ...).</div>
<div><br></div><div>Also, duplicating the &quot;return&quot; after each red=
irect_to or render</div><div>seems not DRY.</div><div><br></div><div>At one=
 point, I was grepping my code for each &quot;redirect_to&quot;,</div><div>
&quot;render&quot;=C2=A0to see if I had written a return after it ...=C2=A0=
</div><div><br></div><div>A solution ?</div><div><br></div><div>Would this =
be a good use case for Catch / Throw ?</div><div><br></div><div>Then the We=
b Framework =C2=A0call to User#show could be</div>
<div>wrapped in a catch block like</div><div><br></div><div>class WebFramew=
ork</div><div>...</div><div>=C2=A0 catch :render_or_redirect do</div><div>=
=C2=A0 =C2=A0 User.show</div><div>=C2=A0 end</div><div><br></div><div>end</=
div><div><br></div>
<div>and then we defined a function</div><div><br></div><div>def redirect_t=
o_with_throw(*arguments)</div><div>=C2=A0 redirect_to(*arguments)</div><div=
>=C2=A0 throw :render_or_redirect</div><div>end</div><div><br></div><div>wh=
ich can be used in the controller as:</div>
<div><br></div><div><div>def show</div><div>=C2=A0 user =3D User.find(param=
s[:id])</div><div>=C2=A0 if user.secret?</div><div>=C2=A0 =C2=A0 flash[:war=
ning] =3D &quot;sorry, this is secret&quot;</div><div>=C2=A0 =C2=A0 redirec=
t_to_with_throw :action =3D&gt; &quot;index&quot;</div>
<div>=C2=A0 end</div></div><div>...</div><div><br></div><div>That might be =
a nice extension to the web framework,</div><div>to catch a :render_or_redi=
rect=C2=A0throw. If I understand</div><div>correctly, the catch in the fram=
ework would not</div>
<div>hinder the existing code or developers not using</div><div>the feature=
 (that is, never throwing :render_or_redirect).</div><div><br></div><div>HT=
H,</div><div><br></div><div>Peter</div></div>

--002354471080ae023c04b4317370--