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This chapter is from the book

2.3 Using Here-Documents

If you want to represent a long string spanning multiple lines, you can certainly use a regular quoted string:

str = "Once upon a midnight dreary,
       While I pondered, weak and weary..."

However, the indentation will be part of the string.

Another way is to use a here-document, a string that is inherently multiline. (This concept and term are borrowed from older languages and contexts.) The syntax is the << symbol, followed by an end marker, then zero or more lines of text, and finally the same end marker on a line by itself:

str = <<EOF
Once upon a midnight dreary,
While I pondered weak and weary,...

Be careful about things such as trailing spaces on the final end marker line. Current versions of Ruby will fail to recognize the end marker in those situations.

Note that here-documents may be “stacked”; for example, here is a method call with three such strings passed to it:

some_method(<<STR1, <<STR2, <<STR3)
first piece
of text...
second piece...
third piece
of text.

By default, a here-document is like a double-quoted string—that is, its contents are subject to interpretation of escape sequences and interpolation of embedded expressions. But if the end marker is single-quoted, the here-document behaves like a single-quoted string:

str = <<'EOF'
This isn't a tab: \t
and this isn't a newline: \n

If a here-document’s end marker is preceded by a hyphen, the end marker may be indented. Only the spaces before the end marker are deleted from the string, not those on previous lines:

str = <<-EOF
  Each of these lines
  starts with a pair
  of blank spaces.

To delete the spaces from the beginning of each line, we need another method. The ActiveSupport gem (included in Rails) defines a strip_heredoc method that works similarly to this one:

class String
  def strip_heredoc
    # Find the margin whitespace on the first line
    margin = self[/\A\s*/]
    # Remove margin-sized whitespace from each line

The amount of whitespace before the start of the first line is detected, and that amount of whitespace is then stripped off of each line. It’s used in this way:

str = <<end.strip_heredoc
  This here-document has a "left margin"
  set by the whitespace on the first line.

  We can do inset quotations here,
  hanging indentions, and so on.

The word end is used naturally enough as an end marker. (This, of course, is a matter of taste. It looks like the reserved word end but is really just an arbitrary marker.) Many text editors use the end marker as a hint for syntax highlighting. As a result, using <<SQL or <<RUBY can make it dramatically easier to read blocks of code inside here-docs in those editors.

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