Ruby heredoc have two syntax, the one starts with << and other with <<-. The one starts with << needs END marker should be at starting of a line.

def something 
  @abc = <<ABC
    some 
    multiline
    text
ABC
end

but <<- the one with dash, won’t need marker at the start of line, so we can easily intend END marker.

def something 
  @abc = <<-ABC
    some 
    multiline
    text
  ABC
end

while working on erb rendering, something caught my eye, a wierd syntax (at least for me) with heredoc on ruby doc.

ERB.new(<<-'END_PRODUCT'.gsub(/^\s+/, ""), 0, "", "@product").result b
  <%= PRODUCT[:name] %>
  <%= PRODUCT[:desc] %>
END_PRODUCT

Then only I came to know that heredoc can be easily passed to functions like

some_method(<<-START)
  some
  multiline
  text
START

and if you have two heredoc to pass, then

some_method(<<-START, <<-END)
  some
  multiline
  text
START
  another
  multiline
  text
END

Single quoted

just like single quoted string, which won’t evalute the interpolated expression

a = 10
@abc = <<-'ABC'
  some 
  multiline
  text
  #{a}
ABC

will result in

'  some 
  multiline
  text
  #{a}'

Double quoted

As you guessed, this will evalute the interpolated expression and substitute the value.

a = 10
@abc = <<-"ABC"
  some 
  multiline
  text
  #{a}
ABC

will result in

'  some 
  multiline
  text
  10'

Removing unwanted spaces due to indentation

Since we indend the heredoc to make code more readable, this will lead to unnecessary spaces in our text. To avoid this you can use gsub method.

some_method(<<-START.gsub(/^\s+/, ""))
  some
  multiline
  text
START

which will result in

'some 
multiline
text
10'

if you are using Rails/ActiveSupport there is handy method you can use strip_heredoc

some_method(<<-START.strip_heredoc)
  some
  multiline
  text
START