Feature #3131: add Kernel#Hash() method like Kernel#Array()

• Status: Closed
• Assignee: matz (Yukihiro Matsumoto)
• Target version: 2.0.0

[sunaku (Suraj Kurapati)]

=begin
Hello,

There is an imbalance of power in the Ruby core API (when it comes
to arrays and hashes) because it is easier to convert nil values
into empty arrays, thanks to Kernel#Array(), than it is to convert
nil values into empty hashes, due to lack of Kernel#Hash().

To correct this asymmetry and restore a balance of power, please
add a Kernel#Hash() method for converting nil, Array, and Hash
values into hashes:

 module Kernel
   def Hash(value)
     if value.respond_to? :to_hash
       value.to_hash
     elsif value.respond_to? :to_ary
       Hash[*value.to_ary]
     elsif value.nil?
       {}
     else
       raise ArgumentError, "invalid value for Hash: #{value}"
     end
   end
 end

For example, here is how I would use the above API:

 #-------------------------------------------------------------------------
 # CASE 1: to_hash
 #-------------------------------------------------------------------------
 
 real_hash = {:real => true}
 Hash(real_hash) # => {:real=>true}

 fake_hash = Object.new
 def fake_hash.to_hash
   {:fake => true}
 end
 Hash(fake_hash) # => {:fake=>true}

 #-------------------------------------------------------------------------
 # CASE 2: to_ary
 #-------------------------------------------------------------------------
 
 real_array = [:real, true]
 Hash(real_array) # => {:real=>true}

 fake_array = Object.new
 def fake_array.to_ary
   [:fake, true]
 end
 Hash(fake_array) # => {:fake=>true}

 #-------------------------------------------------------------------------
 # CASE 3: nil
 #-------------------------------------------------------------------------

 Hash(nil) # => {}

 #-------------------------------------------------------------------------
 # CASE 4: unsupported arguments
 #-------------------------------------------------------------------------
 
 >> Hash(true)
 ArgumentError: invalid value for Hash: true
         from (irb):74:in `Hash'
         from (irb):80
         from /usr/bin/irb:12:in `<main>'

 >> Hash(false)
 ArgumentError: invalid value for Hash: false
         from (irb):74:in `Hash'
         from (irb):81
         from /usr/bin/irb:12:in `<main>'

 >> Hash(123)
 ArgumentError: invalid value for Hash: 123
         from (irb):74:in `Hash'
         from (irb):82
         from /usr/bin/irb:12:in `<main>'

Thanks for your consideration.
=end

[sunaku (Suraj Kurapati)]

=begin
I forgot to show that case 2 supports empty arrays:

Hash([])
=> {}

Just like Kernel#Array() supports empty hashes:

Array({})
=> []

Thanks for your consideration. And sorry for the noise.
=end

[matz (Yukihiro Matsumoto)]

=begin
Hi,

In message "Re: [ruby-core:29462] [Feature #3131] add Kernel#Hash() method like Kernel#Array()"
on Mon, 12 Apr 2010 08:32:52 +0900, Suraj Kurapati [e-mail] writes:

|There is an imbalance of power in the Ruby core API (when it comes
|to arrays and hashes) because it is easier to convert nil values
|into empty arrays, thanks to Kernel#Array(), than it is to convert
|nil values into empty hashes, due to lack of Kernel#Hash().

Having Hash() might be a good idea. But since conversion from
arrays only meaningful for specific case (array of 2-element arrays),
I am not sure whether Hash() should support conversion from Array in
general or not.

						matz.

=end

[sunaku (Suraj Kurapati)]

=begin
Hi,

matz wrote:

since conversion from arrays only meaningful for specific case
(array of 2-element arrays), I am not sure whether Hash() should
support conversion from Array in general or not.

Hash::[] does not support conversion from odd-length Array (because
it is uncertain what Ruby must do with the 2n+1'th element) and will
raise an error in such case. So, in my view, Kernel#Hash() need not
support conversion from odd-length Array either.

If you wish to avoid even-length Array conversion in Kernel#Hash()
because odd-length Array conversion is not supported, then I request
that Kernel#Hash() must still support conversion from empty Array
because Kernel#Array() already supports conversion from empty Hash:

Array({})
=> []

In summary, I request that Kernel#Hash() supports conversion from:

• nil
• Hash
• empty Array

It would be nice if Kernel#Hash() also supports conversion from:

• even-length Array

Thanks for your consideration.
=end

[sunaku (Suraj Kurapati)]

=begin
Hi,

Please allow me to clarify.

Suraj Kurapati wrote:

Hash::[] does not support conversion from odd-length Array (because
it is uncertain what Ruby must do with the 2n+1'th element) and will
raise an error in such case.

I was referring to this particular behavior:

Hash[1,2,3,4,5,6]
=> {1=>2, 3=>4, 5=>6}

Hash[1,2,3,4,5,6,7]
ArgumentError: odd number of arguments for Hash
from (irb):3:in []' from (irb):3 from /usr/bin/irb:12:in '

I forgot that Hash::[] can convert Array of 2-element Array into Hash:

Hash[1,2,3,4,[5,6],[7]] # non 2-element Array ignored
=> {1=>2, 3=>4, [5, 6]=>[7]}

Hash[[1,2,3,4,[5,6],[7]]] # implicit pair value is nil
=> {5=>6, 7=>nil}

Hash[[1,2,3,4,[5,6],[7,8],[7]]] # previous pair is overwritten
=> {5=>6, 7=>nil}

Thanks for your consideration.
=end

[sunaku (Suraj Kurapati)]

=begin
Hi Matz,

To avoid delaying this request forever due to unnecessary features,
I have narrowed the requirements for Kernel#Hash() to the following:

Hash() must convert (1) nil, (2) Hash, and (3) empty Array into Hash.

 module Kernel
   def Hash(value)
     if value.respond_to? :to_hash
       value.to_hash
     elsif value.nil? or Array(value).empty?
       {}
     else
       raise ArgumentError, "invalid value for Hash: #{value}"
     end
   end
 end

Thanks for your consideration.
=end

[matz (Yukihiro Matsumoto)]

=begin
Hi,

In message "Re: [ruby-core:29644] [Feature #3131] add Kernel#Hash() method like Kernel#Array()"
on Tue, 20 Apr 2010 13:36:00 +0900, Suraj Kurapati [e-mail] writes:

|To avoid delaying this request forever due to unnecessary features,
|I have narrowed the requirements for Kernel#Hash() to the following:
|
|Hash() must convert (1) nil, (2) Hash, and (3) empty Array into Hash.

Sounds reasonable. But the new feature window for 1.9.2 was closed.
So, I will consider (positively) adding this feature after 1.9.2 release.

						matz.

=end

[shyouhei (Shyouhei Urabe)]

• Status changed from Open to Assigned
• Assignee set to matz (Yukihiro Matsumoto)

=begin

=end

[sunaku (Suraj Kurapati)]

=begin
Hi,

Matz wrote:

I will consider (positively) adding this feature after 1.9.2
release.

Thanks Matz! In the mean time, I have created a gem with this
functionality so that people can start using it, if they wish:

http://rubygems.org/gems/kernel_hash

The source code (and unit tests) is available here:

http://github.com/sunaku/kernel_hash

Cheers.
=end

[sunaku (Suraj Kurapati)]

=begin
Hi,

Please set the "Done %" for this issue to 80%.

The remaining 20% is for (possibly) rewriting
my proposed Kernel#Hash() Ruby code in C.

Thanks for your consideration.
=end

[runpaint (Run Paint Run Run)]

• File kernel-hash.patch kernel-hash.patch added

=begin
I believe the attached patch reflects the consensus between matz and Suraj. The only material difference is that a TypeError is raised instead of an ArgumentError for consistency with Integer(), Float(), etc.
=end

[sunaku (Suraj Kurapati)]

=begin
Integer() and Float() in Ruby 1.9.2 raise TypeError and ArgumentError:

$ irb

ruby 1.9.2p180 (2011-02-18 revision 30909) [x86_64-linux]

Float(nil)
TypeError: can't convert nil into Float
from (irb):6:in Float' from (irb):6 from /usr/bin/irb:12:in '
Float('meow')
ArgumentError: invalid value for Float(): "meow"
from (irb):8:in Float' from (irb):8 from /usr/bin/irb:12:in '

I don't know which error type we should raise in Kernel#Hash().
=end

[matz (Yukihiro Matsumoto)]

=begin
Hi,

In message "Re: [ruby-core:35857] [Ruby 1.9 - Feature #3131] add Kernel#Hash() method like Kernel#Array()"
on Sat, 23 Apr 2011 08:52:12 +0900, Suraj Kurapati [e-mail] writes:

|Integer() and Float() in Ruby 1.9.2 raise TypeError and ArgumentError:

I think they should raise TypeError, if incompatibly here is not an
issue, I will fix.

						matz.

=end

[sunaku (Suraj Kurapati)]

Any chance of this getting into Ruby 1.9.3? Thanks.

[yeban (Anurag Priyam)]

On Mon, Sep 26, 2011 at 1:52 AM, Suraj Kurapati [e-mail] wrote:

Issue #3131 has been updated by Suraj Kurapati.

Any chance of this getting into Ruby 1.9.3?  Thanks.

Doesn't look like it. See ruby-core 39693.

--
Anurag Priyam

[sunaku (Suraj Kurapati)]

Thanks for the pointer. I'll just wait for Ruby 1.9.4 then.

[matz (Yukihiro Matsumoto)]

I accept this patch for trunk, except that Hash(nil) should return an empty hash, not nil.

Matz.

[ko1 (Koichi Sasada)]

Who should work on it?

[naruse (Yui NARUSE)]

• Status changed from Assigned to Closed

This has already done, only missed to close.