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    if ( !ref $xform ) {
        my $module = $xform;
        $obj = use_module( $module )->new( @args );
    }
    elsif ( ref $xform eq 'CODE' ) {
        $obj = ETL::Yertl::Transform->new(
            @args,
            transform_doc => $xform,
        );
    }
    loop->add( $obj );
    return $obj;
}

#pod =sub file
#pod
#pod     my $stream = file( $mode, $path, %args );
#pod
#pod Create a L<ETL::Yertl::FormatStream> object for the given C<$path>.
#pod C<$mode> should be one of C<< < >> for reading or C<< > >> for writing.
#pod C<%args> are additional arguments to pass to the
#pod L<ETL::Yertl::FormatStream> constructor. Useful keys are:
#pod
#pod =over
#pod
#pod =item format
#pod
#pod Specify the format that standard input is. Defaults to C<yaml> or the value
#pod of C<YERTL_FORMAT> (see L<ETL::Yertl::Format/get_default>.
#pod
#pod =back
#pod
#pod =cut

sub file( $$;% ) {
    my ( $mode, $name, %args ) = @_;
    # Detect whether to read_handle/write_handle via '<', '>'
    open my $fh, $mode, $name
        or croak sprintf q{Can't open file "%s": %s}, $name, $!;
    if ( $mode =~ /^</ ) {
        $args{read_handle} = $fh;
    }
    elsif ( $mode =~ /^>/ ) {
        $args{write_handle} = $fh;
    }
    else {
        croak sprintf q{Can't determine if mode "%s" is read or write}, $mode;
    }
    return stream( %args );
}

#pod =sub yq
#pod
#pod     my $xform = yq( $filter );
#pod
#pod Create a L<ETL::Yertl::Transform::Yq> object with the given filter. See
#pod L<yq/SYNTAX> for full filter syntax.
#pod
#pod =cut

sub yq( $ ) {
    my ( $filter ) = @_;
    return transform(
        'ETL::Yertl::Transform::Yq',
        filter => $filter,
    );
}

#pod =sub loop
#pod
#pod     my $loop = loop();
#pod
#pod Get the L<IO::Async::Loop> singleton. Use this to add other L<IO::Async> objects
#pod to a larger program. This is not needed for simple Yertl streams, and is mostly
#pod used internally.
#pod
#pod This is not exported by default. You can import it using C<< use ETL::Yertl 'loop' >>.
#pod
#pod =cut

sub loop() {
    state $loop = IO::Async::Loop->new;
    return $loop;
}

1;

__END__

=pod

=head1 NAME

ETL::Yertl - ETL with a Shell

=head1 VERSION

version 0.044

=head1 SYNOPSIS

    ### On a shell...
    # Convert file to Yertl's format
    $ yfrom csv file.csv >work.yml
    $ yfrom json file.json >work.yml

    # Convert file to output format
    $ yto csv work.yml
    $ yto json work.yml

    # Parse HTTP logs into documents
    $ ygrok '%{LOG.HTTP_COMMON}' httpd.log

    # Read data from a database
    $ ysql db_name 'SELECT * FROM employee'

    # Write data to a database
    $ ysql db_name 'INSERT INTO employee ( id, name ) VALUES ( $.id, $.name )'

    ### In Perl...
    use ETL::Yertl;