Proc throws error when used with do end [duplicate]

Question

This question already has an answer here:

• Code block passed to each works with brackets but not with 'do'-'end' (ruby) 3 answers

This doesn't work:

run Proc.new do |env|
  [200, 
   {
    "Content-Type" => "application/json; charset=UTF-8"
  }, ["{\"name\":\"Rack App\"}"]]
end

But this does:

run Proc.new { |env|
  [200, 
   {
    "Content-Type" => "application/json; charset=UTF-8"
  }, ["{\"name\":\"Rack App\"}"]]
}

Any ideas, why its throwing error when used with do..end?

Error I am getting:

app.ru:1:in new: tried to create Proc object without a block (ArgumentError)

Answer 1

Your first code is interpreted as:

run(Proc.new) do |env|
  ...
end

and the block is is passed to run instead of new. The problem can be solved by doing:

run(Proc.new do |env|
  ...
end)