From: Piotr Szotkowski <chastell@...>
Date: 2011-05-31T05:55:39+09:00
Subject: [ruby-core:36596] Re: [Ruby 1.9 - Feature #4801][Open] Shorthand Hash Syntax for Strings


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Cezary:

> I though exactly the same thing, until I realized
> that having keys of different types in a Hash
> isn't really part of the general Hash concept.

Why? [citation needed]

> Consider the following:

> { nil =3D> 0, :foo =3D> 1, 'foo' =3D> 2 }

> Conceptually, people expect Hash keys to be of the same type,
> except maybe for "hacks" like that nil above that can simplify code.

Well, they either do or don=E2=80=99t, then. :)

> If someone out there in the world actually demands that such a Hash
> is valid and that :foo and 'foo' are different keys, you could always
> wrap Hash to support that for that single, specialized case.

Hm, IMHO =E2=80=98any object can be a key, just as any object can be
a value=E2=80=99 is the general case, and =E2=80=98I want my Strings and Sy=
mbols
to be treated the same when they=E2=80=99re similar, oh, and maybe with
the nil handled separately for convenience=E2=80=99 is the specialised case.

> In Ruby "foo" + 123 raises a TypeError. Adding a string
> key to a symbol-keyed Hash doesn't even show a warning.

I don=E2=80=99t see why it should =E2=80=93 as long as it still
responds to #hash and #eql?, it=E2=80=99s a valid Hash key.

Hashes in Ruby serve a lot of purposes (they even maintain insertion
order); if you want to limit their functionality, feel free to subclass.

> I consider hashes with different key types different types of hashes,
> that shouldn't even be allowed to merge together without conversion.

There=E2=80=99s nothing preventing you from subclassing Hash to
create StringKeyHash, SymbolKeyHash or even MonoKeyHash
that would limit the keys=E2=80=99 class to the first one defined.

How would you treat subclasses? Let=E2=80=99s say I have a Hash with
keys being instances of People, Employees and Volunteers (with
Employees ans Volunteers being subclasses of People). Should
they all be allowed as keys in a single MonoKeyHash or not?

What about String-only keys, but with different
keys having their own different singleton methods?

(For discussion=E2=80=99s sake: what about if a couple of the Strings
had redefined #hash and #eql? methods, on an instance level?)

> I think the meaning of symbols and hashes are too similar for such
> different types to be allowed as keys in the same Hash instance.

But that would introduce a huge exception in the current
very simple model. Ruby is complicated enough; IMHO we
should strive to make it less complicated, not more.

=E2=80=94 Piotr Szotkowski
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// sometimes I believe compiler ignores all my comments




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