From: Robert Klemme <shortcutter@...>
Date: 2010-10-07T18:09:13+09:00
Subject: Re: "map" a deeply nested structure: Object#deep_map

On Thu, Oct 7, 2010 at 10:22 AM, Guido De Rosa <guidoderosa@gmail.com> wrote:
> Robert Klemme wrote:
>
>> But according to that logic *all* collections returned should be
>> Arrays.  So out = {} would become out = [].
>
> Enumerable#map returns an Array on any object except wen the object is a
> Hash: in such case it returns another Hash. And so should do #deep_map.

11:04:25 ~$ allruby -e 'p({1=>2}.map {|a,b| [b,a]}.class)'
CYGWIN_NT-5.1 padrklemme2 1.7.7(0.230/5/3) 2010-08-31 09:58 i686 Cygwin
========================================
ruby 1.8.7 (2008-08-11 patchlevel 72) [i386-cygwin]
Array
========================================
ruby 1.9.1p429 (2010-07-02 revision 28523) [i386-cygwin]
Array
========================================
jruby 1.4.0 (ruby 1.8.7 patchlevel 174) (2009-11-02 69fbfa3) (Java
HotSpot(TM) Client VM 1.6.0_21) [x86-java]
Array

I see you noticed already.

>> The difference is that in a Hash or Array values are usually uniform
>> while with a recursive structure it is much more likely that they are
>> not.  So in the case of Enumerable#map you typically know what objects
>> you map while in the recursive case you rather need checks.
>>
>> I still wonder about the usability of this.
>
> Of course it depends by the usage scenario, and I actually found such a
> need in one of my production projects. I may tell you longer detail if
> you wish...

Yes, please.

> Of course there's no need that the values belong to the same class or to
> derived class, they just need to be uniform in the duck-typing sense.
>
> Consider something like:
>
>    my_deep_structure.deep_map{|v| v.to_s}

What do you need that for?  If it is for printing or logging then you
can use my_deep_structure.to_s or my_deep_structure.inspect.

> any of this value may belong to a *different* class but still in
> Your::Application::NameSpace::
>
> This doesn't appear so uncommon or unrealistic.
>
> You may have designed your application so that any of your classes
> respond to to_s or some other method which "exports" data.
>
> The result of deep_map will not contain any reference to your
> application logic and may easily "inter-operate" with others: I fond
> this more convenient that overwriting to_json, to_yaml, marshal_dump and
> so on...

You really make me curious about your use case.

Cheers

robert

-- 
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