Hash[] vs to_h, are they the same, and if so which is better?

Question

These appear to be identical. Is there a preference, performance differences, etc?

Hash[[[:a, 'foo'], [:b, 'bar']]]
#=> {:a=>"foo", :b=>"bar"}
[[:a, 'foo'], [:b, 'bar']].to_h
#=> {:a=>"foo", :b=>"bar"}

Answer 1

They are not the same. Both can take an array of arrays:

Hash[[[1, 2], [3, 4]]] #=> {1=>2, 3=>4}
[[1, 2], [3, 4]].to_h #=> {1=>2, 3=>4}

but in addition to that, Hash.[] has more quirky syntax. It can accept elements directly:

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

If you try something in between, you will get unexpected results:

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

And although giving it odd numbers of elements usually returns an error, which will help you in debugging:

Hash[1, 2, 3]
#=> ArgumentError: odd number of arguments for Hash

with certain kind of elements, it will silently return an empty hash (but with a warning), which will be hard to debug:

Hash[[1, 2]]
#=> {}
#=> warning: wrong element type Fixnum at 0 (expected array)

For this reason, Hash.[] is tricky. Since Array#to_h has been introduced, there is no reason to use Hash.[]. Array#to_h is better. The Ruby developers will not remove Hash.[] just for compatibility.