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Raku port of Perl's tie() built-in


use P5tie; # exports tie(), tied() and untie()

tie my $s, Tie::AsScalar;
tie my @a, Tie::AsArray;
tie my %h, Tie::AsHash;

$object = tied $s;
untie $s;


This module tries to mimic the behaviour of Perl's tie and related built-ins as closely as possible in the Raku Programming Language.


        This function binds a variable to a package class that will
        provide the implementation for the variable. VARIABLE is the name
        of the variable to be enchanted. CLASSNAME is the name of a class
        implementing objects of correct type. Any additional arguments are
        passed to the appropriate constructor method of the class (meaning
        "TIESCALAR", "TIEHANDLE", "TIEARRAY", or "TIEHASH"). Typically
        these are arguments such as might be passed to the "dbm_open()"
        function of C. The object returned by the constructor is also
        returned by the "tie" function, which would be useful if you want
        to access other methods in CLASSNAME.

        Note that functions such as "keys" and "values" may return huge
        lists when used on large objects, like DBM files. You may prefer
        to use the "each" function to iterate over such. Example:

            # print out history file offsets
            use NDBM_File;
            tie(%HIST, 'NDBM_File', '/usr/lib/news/history', 1, 0);
            while (($key,$val) = each %HIST) {
                print $key, ' = ', unpack('L',$val), "\n";

        A class implementing a hash should have the following methods:

            TIEHASH classname, LIST
            FETCH this, key
            STORE this, key, value
            DELETE this, key
            CLEAR this
            EXISTS this, key
            FIRSTKEY this
            NEXTKEY this, lastkey
            SCALAR this
            DESTROY this
            UNTIE this

        A class implementing an ordinary array should have the following

            TIEARRAY classname, LIST
            FETCH this, key
            STORE this, key, value
            FETCHSIZE this
            STORESIZE this, count
            CLEAR this
            PUSH this, LIST
            POP this
            SHIFT this
            UNSHIFT this, LIST
            SPLICE this, offset, length, LIST
            EXTEND this, count
            DELETE this, key
            EXISTS this, key
            DESTROY this
            UNTIE this

        A class implementing a filehandle should have the following

            TIEHANDLE classname, LIST
            READ this, scalar, length, offset
            READLINE this
            GETC this
            WRITE this, scalar, length, offset
            PRINT this, LIST
            PRINTF this, format, LIST
            BINMODE this
            EOF this
            FILENO this
            SEEK this, position, whence
            TELL this
            OPEN this, mode, LIST
            CLOSE this
            DESTROY this
            UNTIE this

        A class implementing a scalar should have the following methods:

            TIESCALAR classname, LIST
            FETCH this,
            STORE this, value
            DESTROY this
            UNTIE this

        Not all methods indicated above need be implemented. See perltie,
        Tie::Hash, Tie::Array, Tie::Scalar, and Tie::Handle.

        Unlike "dbmopen", the "tie" function will not "use" or "require" a
        module for you; you need to do that explicitly yourself. See
        DB_File or the Config module for interesting "tie"

        For further details see perltie, "tied VARIABLE".

        Returns a reference to the object underlying VARIABLE (the same
        value that was originally returned by the "tie" call that bound
        the variable to a package.) Returns the undefined value if
        VARIABLE isn't tied to a package.

        Breaks the binding between a variable and a package. (See tie.)
        Has no effect if the variable is not tied.


Please note that there are usually better ways attaching special functionality to arrays, hashes and scalars in Raku than using tie. Please see the documentation on Custom Types for more information to handling the needs that Perl's tie fulfills in a more efficient way in Raku.

Subs versus Methods

In Raku, the special methods of the tieing class, can be implemented as Raku methods, or they can be implemented as our subs, both are perfectly acceptable. They can even be mixed, if necessary. But note that if you're depending on subclassing, that you must change the package to a class to make things work.


Because Raku does not have the concept of magic that can be added or removed, it is not possible to untie a variable. Note that the associated UNTIE sub/method will be called, so that any resources can be freed.

Potentially it would be possible to actually have any subsequent accesses to the tied variable throw an exception: perhaps it will at some point.

Scalar variable tying versus Proxy

Because tying a scalar in Raku must be implemented using a Proxy, and it is currently not possible to mix in any additional behaviour into a Proxy, it is alas impossible to implement UNTIE and DESTROY for tied scalars at this point in time. Please note that UNTIE and DESTROY are supported for tied arrays and hashes.

Tieing a file handle

Tieing a file handle is not yet implemented at this time. Mainly because I don't grok yet how to do that. As usual, patches and Pull Requests are welcome!


Elizabeth Mattijsen liz@raku.rocks

Source can be located at: https://github.com/lizmat/P5tie . Comments and Pull Requests are welcome.


Copyright 2018, 2019, 2020, 2021 Elizabeth Mattijsen

Re-imagined from Perl as part of the CPAN Butterfly Plan.

This library is free software; you can redistribute it and/or modify it under the Artistic License 2.0.