Ruby, hashes, and default params

whoa.md

I was surprised by where my params were going with a call to a method with defaulted hash parameters.

So I tested it in IRB, and here are the results.

Create test program

def test(a, h1={}, h2={})
  pp a
  pp h1
  pp h2
end

Just pass first param, leave second to default

> test 1, :foo=>'bar'
1
{:foo=>"bar"}
{}

Same, but create explicit hash-literal

> test 1, {:foo=>'bar'}
1
{:foo=>"bar"}
{}

Pass both hash params, both explicit hashes. Works as expected.

> test 1, {:foo=>'bar'}, {:tee=>'fee'}
1
{:foo=>"bar"}
{:tee=>"fee"}

Same, but 2nd as a single parameter. Ruby will see no more elements, and create a hash for this.

> test 1, {:foo=>'bar'}, :tee=>'fee'
1
{:foo=>"bar"}
{:tee=>"fee"}

Same again, but 1st as a single parameter. Error.

> test 1, :foo=>'bar', {:tee=>'fee'}
SyntaxError: (irb):46: syntax error, unexpected '\n', expecting tASSOC

Now, two params passed in, both implicate single element hashes :-

test 1, :foo=>'bar', :tee=>'fee'
1
{:foo=>"bar", :tee=>"fee"}
{}

Ruby sees the second param, and greedily merges that into the first hash!

Watch out for this!