From: dblack@...
Date: 2007-09-05T19:49:57+09:00
Subject: Re: assigning to hash keys when there is a default value?

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Hi --

On Wed, 5 Sep 2007, Robert Klemme wrote:

> 2007/9/5, Pe=F1a, Botp <botp@delmonte-phil.com>:
>> From: Robert Klemme [mailto:shortcutter@googlemail.com]
>> # 2007/9/4, Yossef Mendelssohn <ymendel@pobox.com>:
>> # > If '"x+=3D1" will always be "x=3Dx+1"', what's the problem in
>> # having x ||=3D
>> # > 1 always be x =3D x || 1?
>> # That an assignment of x=3Dx is useless (basically a nop in the standar=
d
>> # case of x being a variable).
>>
>> consider,
>>
>> irb(main):074:0* h=3DHash.new
>> =3D> {}
>> irb(main):075:0> h[5] =3D h[5]
>> =3D> nil
>> irb(main):076:0> h
>> =3D> {5=3D>nil}
>> irb(main):062:0* h=3DHash.new(1)
>> =3D> {}
>> irb(main):063:0> h[5] =3D h[5]
>> =3D> 1
>> irb(main):064:0> h
>> =3D> {5=3D>1}
>>
>> thus h[k] =3D h[k] could mean
>>     h[k] =3D h.default   or
>>     h[k] =3D nil
>>
>> definitely not useless and not noop.
>
> I said "noop in the standard case of a variable".  We can certainly
> debate about usefulness or uselessness.  I concede that it's not free
> of side effects in the case of a Hash, but the only noticeable side
> effect is the change of the key set.  Assigning with the Hash's
> default is not visible through Hash#[] although it is with #fetch
> which seems to be rarely used from what I see.  So I still say that
> usefulness of self assignment is very limited.

I'd be happy for unnecessary assignments to be optimized away, but I
like the conceptual clarity of the x =3D x || y interpretation for x ||=3D
y.


David

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