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IO::Handle::Prototype::Fallback(3pm) User Contributed Perl Documentation IO::Handle::Prototype::Fallback(3pm)


IO::Handle::Prototype::Fallback - Create IO::Handle like objects using a set of callbacks.


    my $fh = IO::Handle::Prototype::Fallback->new(
        getline => sub {
            my $fh = shift;


This class provides a way to define a filehandle based on callbacks.

Fallback implementations are provided to the extent possible based on the provided callbacks, for both writing and reading.


This class provides two additional methods on top of IO::Handle, designed to let you implement things with a minimal amount of baggage.

The fallback methods are all best implemented using these, though these can be implemented in terms of Perl's standard methods too.

However, to provide the most consistent semantics, it's better to do this:

        __read => sub {
            shift @array;

Than this:

        getline => sub {
            shift @array;

Because the fallback implementation of "getline" implements all of the extra crap you'd need to handle to have a fully featured implementation.

Return a chunk of data of any size (could use $/ or not, it depends on you, unlike "getline" which probably should respect the value of $/).

This avoids the annoying "substr" stuff you need to do with "read".

__write $string
Write out a string.

This is like a simplified "print", which can disregard $, and "$\" as well as multiple argument forms, and does not have the extra "substr" annoyance of "write" or "syswrite".


If you provide a single reading related callback ("__read", "getline" or "read") then your callback will be used to implement all of the other reading primitives using a string buffer.

These implementations handle $/ in all forms ("undef", ref to number and string), all the funny calling conventions for "read", etc.


Any callback that can be defined purely in terms of other callbacks in a way will be added. For instance "getc" can be implemented in terms of "read", "say" can be implemented in terms of "print", "print" can be implemented in terms of "write", "write" can be implemented in terms of "print", etc.

None of these require special wrapping and will always be added if their dependencies are present.


When overloaded as a glob a tied handle will be returned. This allows you to use the handle in Perl's IO builtins. For instance:

    my $line = <$fh>

will not call the "getline" method natively, but the tied interface arranges for that to happen.

2021-01-03 perl v5.32.0