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ruby-on-railsrubyargumentsblock

Parenthesis for arguments before blocks

I notice that I need to put parenthesis around arguments I pass to a Ruby method before the block. Wat is the reason for that? Boiling down to the smallest example to illustrate what I mean:

def test a
   yield 100
end

test 50 { |x| x*x + 2 } # Gives an error in irb!

test(50) { |x| x*x + 2 } # Gives no error!

I didn't understand why Ruby complains about the first case. The error is along the lines of:

SyntaxError: compile error
(irb):18: syntax error, unexpected '{', expecting $end
Solution

  • This is related to the fact that {...} sometimes delimits a block and sometimes a Hash literal. Disambiguating those two uses has some odd side-effects.

    If you use do...end, it works without the parentheses:

    test 50 do |x| x*x + 2 end