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Bug #14380

Expected transform_keys! to work just as transform_keys, but it doesn't

Added by taw (Tomasz Wegrzanowski) almost 3 years ago. Updated over 2 years ago.

Status:
Closed
Priority:
Normal
Target version:
-
ruby -v:
ruby 2.5.0p0 (2017-12-25 revision 61468) [x86_64-darwin17]
[ruby-core:85385]

Description

This seriously violates the Principle of Least Surprise to me:

{1 => :a, -1 => :b}.transform_keys{|k| -k} #=> {-1=>:a, 1=>:b}
{1 => :a, -1 => :b}.transform_keys!{|k| -k} # => {1=>:a}

# This fails:
ht=(1..10).map{|k| [k,k]}.to_h; ht.transform_keys(&:succ) # => {2=>1, 3=>2, 4=>3, 5=>4, 6=>5, 7=>6, 8=>7, 9=>8, 10=>9, 11=>10}
ht=(1..10).map{|k| [k,k]}.to_h; ht.transform_keys!(&:succ) # => {11=>1}

# This code with same issue works just because of key ordering:
ht=(1..10).map{|k| [k,k]}.to_h; ht.transform_keys(&:pred) #=> {0=>1, 1=>2, 2=>3, 3=>4, 4=>5, 5=>6, 6=>7, 7=>8, 8=>9, 9=>10}
ht=(1..10).map{|k| [k,k]}.to_h; ht.transform_keys!(&:pred) #=> {0=>1, 1=>2, 2=>3, 3=>4, 4=>5, 5=>6, 6=>7, 7=>8, 8=>9, 9=>10}

Of course in these examples it's very easy to see the problem, but in bigger programs it could be really difficult.

If the implementation instead did equivalent of:

class Hash
  def transform_values!(&block)
    replace transform_values(&block)
  end
end

it would be much less surprising.

Hash#transform_keys / Hash#transform_keys! inherently require that resulting values don't collide, but in these examples it works in surprising ways even though there's no collision between results.

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