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The <<- operator on Ruby, where is it documented?

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执笔经年
执笔经年 2020-12-03 05:31

I recently used the <<- operator to output a multi-line string, like this:

<<-form
  
Name to say hi!
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  • 小鲜肉
    2020-12-03 06:03

    From The Ruby Programming Language:

    Here Documents

    For long string literals, there may be no single character delimiter that can be used without worrying about remembering to escape characters within the literal. Ruby's solution to this problem is to allow you to specify an arbitrary sequence of characters to serve as the delimiter for the string. This kind of literal is borrowed from Unix shell syntax and is historically known as a here document. (Because the document is right here in the source code rather than in an external file.)

    Here documents begin with << or <<-. These are followed immediately (no space is allowed, to prevent ambiguity with the left-shift operator) by an identifier or string that specifies the ending delimiter. The text of the string literal begins on the next line and continues until the text of the delimiter appears on a line by itself. For example:

    document = <<HERE        # This is how we begin a here document
This is a string literal.
It has two lines and abruptly ends...
HERE

    The Ruby interpreter gets the contents of a string literal by reading a line at a time from its input. This does not mean, however, that the << must be the last thing on its own line. In fact, after reading the content of a here document, the Ruby interpreter goes back to the line it was on and continues parsing it. The following Ruby code, for example, creates a string by concatenating two here documents and a regular single-quoted string:

    greeting = <<HERE + <<THERE + "World"
Hello
HERE
There
THERE

    The <<HERE on line 1 causes the interpreter to read lines 2 and 3. And the <<THERE causes the interpreter to read lines 4 and 5. After these lines have been read, the three string literals are concatenated into one.

    The ending delimiter of a here document really must appear on a line by itself: no comment may follow the delimiter. If the here document begins with <<, then the delimiter must start at the beginning of the line. If the literal begins with <<- instead, then the delimiter may have whitespace in front of it. The newline at the beginning of a here document is not part of the literal, but the newline at the end of the document is. Therefore, every here document ends with a line terminator, except for an empty here document, which is the same as "":

    empty = <<END
END

    If you use an unquoted identifier as the terminator, as in the previous examples, then the here document behaves like a double-quoted string for the purposes of interpreting backslash escapes and the # character. If you want to be very, very literal, allowing no escape characters whatsoever, place the delimiter in single quotes. Doing this also allows you to use spaces in your delimiter:

    document = <<'THIS IS THE END, MY ONLY FRIEND, THE END'
    .
    . lots and lots of text goes here
    . with no escaping at all.
    .
THIS IS THE END, MY ONLY FRIEND, THE END

    The single quotes around the delimiter hint that this string literal is like a single-quoted string. In fact, this kind of here document is even stricter. Because the single quote is not a delimiter, there is never a need to escape a single quote with a backslash. And because the backslash is never needed as an escape character, there is never a need to escape the backslash itself. In this kind of here document, therefore, backslashes are simply part of the string literal.

    You may also use a double-quoted string literal as the delimiter for a here document. This is the same as using a single identifier, except that it allows spaces within the delimiter:

    document = <<-"# # #"    # This is the only place we can put a comment
<html><head><title>#{title}</title></head>
<body>
<h1>#{title}</h1>
#{body}
</body>
</html>
               # # #

    Note that there is no way to include a comment within a here document except on the first line after the << token and before the start of the literal. Of all the # characters in this code, one introduces a comment, three interpolate expressions into the literal, and the rest are the delimiter

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  • 遥遥无期
    2020-12-03 06:10

    This post will tell you everything you need to know about the "heredoc" string syntax. In addition, you can view the rubydoc page for string syntax.

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  • 小鲜肉
    2020-12-03 06:15

    http://www.ruby-doc.org/docs/ruby-doc-bundle/Manual/man-1.4/syntax.html#here_doc

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  • 轻奢々
    2020-12-03 06:26

    This is the Ruby "here document" or heredoc syntax. The addition of the - indicates the indent.

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  • 伪装坚强ぢ
    2020-12-03 06:26

    The reason why you cannot find any documentation on the <<- operator is because it isn't an operator. It's literal syntax, like ' or ".

    Specifically, it's the here document syntax which is one of the many syntactic forms of string literals in Ruby. Ruby here documents are similar to POSIX sh here documents, but handling of whitespace removal is different: in POSIX sh here documents delimited by <<-, only leading tabs are removed, but they are removed from the contents of the string, whereas in Ruby all leading whitespace is removed, but only from the delimiter.

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