IO::YAML - read and write YAML streams incrementally

        use IO::YAML;
        $io = IO::YAML->new($path_or_filehandle);
        $io = IO::YAML->new(
            'path'      => '/path/to/a/file',
            'auto_load' => $bool,
        $io = IO::YAML->new(
            'handle' => $fh,
            'mode'   => '>',  # or 'w'; '<' or 'r'; '>>' or 'a'
        $io = IO::YAML->new;
        $io->open($path, '>')  or die $!;  # Open a stream for writing
        $io->open($path, '>>') or die $!;  # Open a stream for appending
        # Automatically add "...\n" at end of each document written
        print $io $mystring;
        print $io @myvalues;
        print $io \@myarray;
        print $io \%myhash;
        print $io $myobj;
        $io = IO::YAML->new;
        $io->open($path, '<')  or die $!;  # Open a stream for reading
        while (<$io>) {
            $data = YAML::Load($_);
        $io = IO::YAML->new;
        $io->open($path) or die $!;  # Default mode is reading
        while (not $io->eof) {
            $data = <$io>;
        $io = IO::YAML->new($path_or_handle);
        my @values = <$io>;  # Roughly equivalent to YAML::LoadFile(...)

    IO::YAML may be used to read and write YAML streams one "document"
    (i.e., one value) at a time.

    A YAML stream is a file consisting of a sequence of YAML documents, each
    of which may (optionally) be terminated by a line consisting solely of
    three periods ("...").

    The first line of each document must begin with the three-byte sequence

    Here's a simple YAML file consisting of three documents; their values
    are the string 'foo', an empty array, and a hash with three elements:

        --- #YAML:1.0 foo
        --- #YAML:1.0 []
        --- #YAML:1.0
        title: Testing 1, 2, 3
        author: nkuitse
        date: 2004-03-05

    (Here, "^D" indicates the end of the file.)

    In this next example, the stream consists of a single YAML document
    whose value is "undef":

        --- ~

    As this example shows, the first line in each document need not contain
    the full YAML 1.0 header.

  Reading from a YAML stream
    To read from a YAML stream, you may use the angle-brackets operator
    (e.g., <$fh>) or the equivalent methods "getline" or "read". Rather than
    reading a single line, this will read an entire YAML document.

        while(defined(my $yaml = <$io>)) {
            my $value = YAML::Load($yaml);

    The "YAML::Load" step may be omitted by setting the IO::YAML object's
    "auto_load" property to a true value:

        while(defined(my $value = <$io>)) {

    However, this example is complicated by the fact that the value of a
    YAML document may be undef; the loop as written will terminate when the
    end of the stream is reached *or* when an undef value is read.

    To avoid this problem while still taking advantage of the "auto_load"
    property, use "$io->eof" to test for the end of the stream:

        while(not $io->eof) {
            my $value = <$io>;

    IO::YAML properly recognizes the document terminator ("..."). Some
    versions of YAML do not recognize it, however; in order to prevent
    problems when reading YAML streams with auto-loading off, IO::YAML
    strips the document terminator line if it is present.

  Writing to a YAML stream
    To print to a YAML stream, call "print" just as you would with a regular
    file handle; the value(s) you're printing will be converted to YAML
    format before being written:

        $io = IO::YAML->new;
        $io->open('>file') or die "Couldn't open 'file'";
        print $io $anything;

    You can `print' anything that YAML is capable of serializing; an
    exception will be raised if you attempt to print something that can't be
    serialized (e.g., a reference to a subroutine).

    The complication with undef values that affects the reading of a YAML
    stream is not an issue when writing to a YAML stream.

  Terminating YAML documents
    Documents in a YAML stream may be terminated by a line consisting solely
    of the string "...". You can use the "terminate" method to add an
    explicit document terminator to a YAML stream that you have open for
    writing (or appending):

        $io = IO::YAML->new($file_or_handle, '>');
        foreach my $value (@data_values) {
            print $io $value;

    It's generally safer to have YAML documents terminated automatically:

        # 1. Set auto_terminate to a true value
        #    a) When creating the object
        $io = IO::YAML->new(
            'handle' => $fh,
            'mode' => '>>',
            'auto_terminate' => 1,
        # or b) At any point thereafter
        $io = IO::YAML->new(...);
        # 2. Documents are now auto-terminated
        foreach my $value (@data_values) {
            print $io $value;
            # $io->terminate called implicitly

    Note that it's not the YAML *stream* that's terminated; it's the YAML
    document that was previously written.

  Low-level access
    Sometimes it is helpful to be able to access a YAML stream at a lower
    level. For example, you may wish to read and write a file consisting of
    a YAML document (here, serving as a header of sorts) followed by
    arbitrary text. The "handle" method may be used to obtain the underlying
    file handle so that it can be used for this low-level access:

        # Read header + body
        $header = <$io>;
        $fh = $io->handle;
        while (<$fh>) {
            # Process each line after the YAML document
        # Write header + body
        print $io $header;
        $fh = $io->handle;
        for (@other_stuff_to_write) {
            print $fh $_;

            $io = IO::YAML->new;
            # Concise forms
            $io = IO::YAML->new("$file");     # Default is read-only
            $io = IO::YAML->new("<$file");    # Read-only made explicit
            $io = IO::YAML->new(">$file");    # Read-write (empty header & body)
            $io = IO::YAML->new($file, '<');  # Or '>', '+<', 'r', etc.
            $io = IO::YAML->new(\*STDIN);
            $io = IO::YAML->new(\*STDOUT, '>');
            $io = IO::YAML->new($anything_that_isa_GLOB);
            # Full-fledged forms
            $io = IO::YAML->new(
                'path' => $file,        # File will be opened read-only
                'auto_load' => 1,       # Default is 0
            $io = IO::YAML->new(
                'path' => $file,        # File will be opened or created
                'mode' => '>',          # Default is '<'; '>>' is also allowed

        Instantiate an IO::YAML object. An exception is thrown if anything
        goes wrong.

        If a path is specified, the file at that path will be opened.
        Otherwise, you'll have to open it yourself using the "open()"

        If a path has been specified and the file doesn't already exist, it
        will be created -- but only if you've specified a mode that permits
        writing; if you haven't, an exception will be thrown.

        The following arguments may be specified in the constructor:

            Path to a file to create (if it doesn't already exist) and open.

            Read/write/append mode for the new file. This must be specified
            in one of the following forms:

            a   Modes that allow for both reading and writing are not
                allowed, since YAML documents are variable in size.

            NOTE: Numeric modes are not yet implemented.

            Indicates whether YAML document values should be auto-loaded
            after being read (see above). The default is not to auto-load

            Indicates whether YAML documents should be auto-terminated when
            they are written (see above). The default is not to
            auto-terminate documents.

            $io = IO::YAML->new;
            $io->open("<$file") or die $!;
            $io->open($file, $mode) or die $!;

        Open a file with the specified name and mode. You must use this
        method if the instance was created without a "path" element (and one
        has not been assigned using the "path()" method).

        Upon failure, sets $! to a meaningful message and returns a false

        The possible modes are as described for new.

        The "open()" method may be called repeatedly on the same instance,
        without having to close it.

            $io->close or die $!;

        Close the filehandle.

            $io->print($data) or die $!;

            $io->seek($pos, $whence);

        Set the IO::YAML file handle's position *in bytes* within the YAML
        stream. This will fail unless it moves the position to the beginning
        of a YAML document or the end of the whole file handle.

            $pos = $io->tell;

        Return the the IO::YAML file handle's position *in bytes*.


        Truncates the IO::YAML file to the specified length. As illustrated
        here, this must be either 0 or equal to the filehandle's current

            if ($io->eof) { ... }

        Return 1 if the IO::YAML filehandle is at the end of the YAML

    Autoflush might not be working.

    Seeking to the first position beyond the end of the YAML stream should
    be possible but doesn't currently work.

    Normalize modes passed in the constructor.

    Implement numeric modes.

    Add tests for seek and tell methods.

    Enable seeking to the first byte beyond the end of the YAML stream.

    Figure out how to allow read-write access and truncate().


    Paul Hoffman (nkuitse AT cpan DOT org)

    Copyright 2004-2007, 2009 Paul M. Hoffman.

    This is free software, and is made available under the same terms as
    Perl itself.